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Radiant Mythology: How myths about low temperature radiant heating and high temperature radiant cooling get spread. Copyright 2004-2016, Robert Bean, R.E.T., P.L.(Eng.) All Rights Reserved

Notice: We're running it again, Integrated HVAC Engineering: Mastering Comfort, Health, and Efficiency. 

2014 was the 10th Year Anniversary for, Radiant Mythology: 22 Myths About Radiant Under Floor Heating and Cooling. Since 2004 this single article has caused more tightly wound up individuals to explode/implode than any other thing I've written. It has brought me countless hours of entertainment and wonderful nastygrams and I hope to wring out the last bit of joy right up until each myth is vaporized.

For additional support visit our visitor services page.

Below is the current list of actual internet radiant excerpts from various "expert sources" of information ...thanks to our readers for sending these myths in for us to set straight.

Clearing the air on radiant - (see our comprehensive list of research citations on radiant cooling and heating.)


Radiant Myth



Radiant heat is dry heat

Ok sometimes people use wet heat vs dry heat to compare systems using water, aka “hydronics” or without water, aka “forced air” or "electric" but in this case they are referring to the absence of moisture.

The thing is - heat is a form of energy, moisture and air are not - they are mass. Describing “radiant heat as dry” would be like saying energy is dry. There are no units in science to describe radiant as dry. Yes we know it's done all the time but that doesn't make it correct.

Look at it this way; a characteristic of heat is temperature, it and moisture are measured with completely different instruments respectively a thermometer and a hygrometer. The thermometer indicates temperature as being between subjective terms of hot and cold using units of °C, °F or K. The hygrometer describes the moisture as being between wet and dry using units of kgwv/kgDA or lbwv/lbDA, or grainswv/lbDA.

If you still don’t see the difference - consider the warm and wet rain forest of Hawaii and the hot and dry conditions of the Sahara desert. Both locations are heated with radiant from the sun so using the incorrect logic of "radiant heat is dry heat" - how can this be when clearly conditions exists in Hawaii where it is wet? So now do you see why radiant heat in of itself cannot be described as dry? When people state, "Radiant heat is dry heat" they are describing the coincident condition of moisture and temperature. The way to measure the coincident condition is with both a black bulb thermometer (mean radiant) and a wet bulb thermometer (aka a sling psychrometer). All of this is to point out that the moisture is independent of the heating system be it radiant or forced air – neither of these is wet or dry just hot or cold until you add or remove moisture. Now, further on you'll learn that there is no heat in radiant just electromagnetic energy.


Radiant Myth



With radiant you’ll have stale air because nothing is moving the air around, or you need to have a furnace to move air around, or you need to move air to provide thermal comfort.

I’ve never understood these comments since ventilation is required regardless of what type of HVAC system you have. Just because you have radiant doesn’t make you exempt from building codes – you still need to ventilate…and that takes air. So what radiant systems do is enable the use of balanced 100% dedicated outdoor air systems such as HRV/ERV’s. Such systems have the highest efficacy of all ventilation systems. When designers use exclusively a furnace to move ventilation air, the ventilation air efficacy is a function of the on/off cycles of the blower. When the blower is ‘on’ it is good – when it is ‘off’ it is less good – translation: if the indoor air quality system in your home is rendered useless by controlling thermal comfort it is IMHO dysfunctional. Also you don’t need to move air for thermal comfort. Is it a factor? Yes. Is it a prerequisite? No. Air movement it is not a prerequisite for thermal comfort. If air movement were a prerequisite for thermal comfort you would feel cold while sitting in the sun on a calm day...does that makes sense to you? Of course not..


Radiant Myth




Oy...there were so many unbelievable statements at this eHow article it's easier for us to advise readers to avoid any article that mentions 'polyurethane' pipes and 'floor furnaces' in the same breath. Pipes aren't made of polyurethane nor are floor furnaces even close to underfloor radiant systems. This is just bad information all around. Keep reading below for other amusing statements and claims.


Radiant Myth



"On average, radiant floor heating systems deliver only about a maximum of 60% of their heat by radiation; the rest is mostly by convection."

If 60% is the majority - it is the "mostly" i.e. the heat transfer is mostly by radiation. We find the syntax in the internet statement interesting...see this link for heat transfer coefficients (HTC)...this difference between HTC values is based on the differences in convection, and the amount of convection is a function of the enclosure performance, room geometry and surface temperatures and orientation of the radiant panel (wall, floor, or ceiling).

Here's what the ASHRAE Systems Handbook says, "A temperature controlled surface is called a radiant panel if 50% or more of the design heat transfer on the temperature-controlled surface takes place by thermal radiation."

There are several "authoritative" sites on the internet which claim that radiant systems such as floors are not radiant systems at all, further they go on to state that these systems should be called convective systems. We suspect the authors have confused "baseboard" systems with radiant systems where the former is predominately convection and the latter by ASHRAE definition a radiant system.

In the absence of forced drafts (i.e., in space fans), radiant heating ceiling systems and radiant cooling floor systems provide marginal convective transfer ( <10%+/-) due to reduced influence on air density, ergo they transfer most of their energy by radiation.

Radiant floor heating and radiant ceiling cooling exchange between 50% and 60% of the energy via radiation ergo they, by ASHRAE definition are radiant systems.


2012 ASHRAE Handbook—HVAC Systems and Equipment, Section 6, Figures 1 and 2

Pedersen, C.O., Fisher, D.E., Lindstrom, P.C. (1997) ASHRAE RP876 Impact of Surface Characteristics on Radiant Panel Output
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Radiant Myth



"The floor material then radiates off the heat into the cooler surrounding air"

"The air is warmed because of the radiating heat"

"Radiant heat is effective because it heats the air rather than blowing warm air throughout your home."

"heated wires in the floor that warm the air in the room above."

"With a radiant floor heating system, heat is even and warms the part of the room most used by people."

"it heats a room from the bottom up".

 "It is efficient because it does not feature a fan blowing heat around the room; it heats from the bottom up, which warms the feet and body first."

"The real advantage of in-floor heating is it radiates heat back to your body."

"The air around you will be slightly cooler, but your own body will absorb the warmth generated through the heating system in the floor."

"they warm up the bottom half of the body making it feeling more comfortable and reducing the feelings of lethargy." 

"the uniform heat distribution over the entire surface of a floor heats the lower half of the room,..."

"The warm or cool surfaces then radiate heat to occupants"

For all practical discussions outside of academia, the radiant energy exchange within a building does not directly heat the 'air' molecules - it heats the surfaces in the room which in turn are in contact with the air...ergo the air becomes warm only because of the contact with warm surfaces vis-à-vis the conduction and convection process.

The aggressiveness of natural convection and stratification is a function of the building enclosure, radiant panel orientation (wall, ceiling, or floor) and whether it is heated (energy emitted) or cooled (energy absorbed). For further discussions on this topic visit our page on heat transfer coefficients. 

Furthermore, many people think radiant floor heating heats the body but (sans discussion on conduction heating the feet) this is not exactly the case; radiant heating systems provide comfort by warming the interior surfaces which reduces the temperature difference between your clothing and skin and the interior surfaces which in turn reduces the loss of body heat via radiation. You see it's not necessarily the radiant energy you are absorbing - it is the heat you are not losing which results in perceptions of comfort.

The amount of heat released via radiation for a clothed person at various metabolic rates is covered in Table 1. Representative Rates at Which Heat and Moisture Are Given Off by Human Beings in Different States of Activity.

Some people compare the warmth from the sun to the warmth of a heated floor but the sun and floor operate at two completely different electromagnetic wave lengths. Both provide thermal comfort but in different ways.

Radiant cooling works in the opposite direction of heating by encouraging the loss of body heat via is the loss of heat from your clothing and skin via radiation which provides the cool sensation. Learn more from our pages on thermal comfort.

With regards to just heating the lower half of the room...all we can say is that has to be some very smart radiant energy because the energy we know doesn't know a lower half from an upper half - all it knows is hot goes to cold and because of this there is no "lower and upper half of the room" as it pertains to radiant energy.
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Radiant Myth



" water is circulated in the pipes and warms up the slab and slowly radiates heat into the space."

"With other thermostats, underfloor radiant systems also overshoot your target temperature by an average of 2.2ºF/1.2ºC since radiant pipes stay hot even after the boiler has stopped heating."

First, it isn't hot water -  in fact in a high performance home with heating loads less than 10 Btu/hr/sf (32 W/m2)with low VOC conductive floors (tile, slate, concrete) and tight tube spacing of 6" to 8" o.c.(150mm to 200mm), the average water temperature is a nominal 85°F (39°C) or very close to skin temperature and cooler than your core body temperature - tepid to warm water - but definitely not hot water.

The cooler the fluid in heating and warmer in cooling the better the system efficiency through first and second laws of thermodynamics (also see our comments further down on exergy yes "e-x-e-r-g-y" it is not a spelling mistake.).

Second, the speed at which the slab warms up is a function  of the heating load, building mass characteristics, enclosures performance, tube spacing and depth, back losses to grade, flooring conductivity and control strategy. With proper designs and fabrication including passive and active controls there is marginal differences in thermal comfort between air and radiant based systems. For the range and rate of temperature changes acceptable to occupants see ASHRAE Standard 55 - Thermal Environmental Conditions for Human Occupancy.

Third, radiant energy travels at the speed of light - there is nothing slow about it.

Discussion: when it comes to "slow" to heat up...for some bizarre reason people must imagine letting their thermostat to drop to say 50°F (10°C)before turning it on and then see themselves standing there with a time clock to count how many hours it takes to heat the room up to comfort conditions. This is so far from reality it's almost silly. With one run of the mill strategy, as soon as the outdoor temperature drops below some preselected outdoor ambient temperature, say 65°F (18°C), an outdoor reset control wakes up from it's sleep mode and starts to monitor the interior and exterior conditions and wakes up the boiler controls...if it continues to get cold outside and it shows up inside as drop in temperature - then it starts to manage the system fluid and heater by turning it on and ramping up the fluid temperature.

With a well designed system with proper controls you can maintain comfort all the way from 65°F (18°C) outdoor conditions down to design conditions without having to "wait" for the system...that is unless of course you actually enjoy starting up your system at polar temperatures.

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Radiant Myth



"radiant heating is expensive"

"radiant cost a lot more than other systems"

"the most comfortable system is radiant but it's a lot more money".

"The more robust the enclosure gets, the more costly the radiant system — the hundreds of feet of tubing, the circulator pumps in every zone, the computer components to monitor things…"

"Radiant heat can be installed in an existing home, but it can be very expensive."

"It's also expensive."

" Typical costs for a hydronic system within a concrete slab are $6 to $10 dollars a square foot, and that’s just for the tubing."

Show us a big inefficient home with whacked out geometry in an extreme  climate with wicked temperature swings; a mechanical contractor who wants to do a custom systems and a builder who doesn't want to do the system, and we'll show you an expensive radiant system. That's it - that's as complicated as the economies get...price is always proportional to contractor intentions, system complexity, building performance, climate and builder flexibility and confidence (which comes with skill which comes from practice).

Give the experienced designer and skilled contractor a high performance enclosure (deals with the climate) with some normallacy in design (simple geometries) and we'll show you a low cost affordable radiant based HVAC system.

The exception is when over zealous designers and contractors use peoples homes as a blank canvas to express their mechanical artistry ... in this case - owners buy a 'one of a kind'  system for the 'one of a kind' with us at the forum on how to prevent this from happening to you.

Look, rather than listening to the internet rif raff - do your own homework...assume you have a 1400 sf basement slab and call up a plumbing retailer and get a price for 1400 ft of 3/8" air barriered PEX pipe, a single 1/24 hp circulator and a dual purpose water heater with internal is obviously not the whole list of equipment nor does it include labour etc...but it'll quickly put pricing of components into perspective. If you don't have the time let me say one retailer list its 3/8" PEXa for 0.65/ft...that would be 0.65 for each square foot at a 12" spacing...not sure where $6 to $10 a square foot comes from but it must be made of gold.

People who claim you need lots of pipe and lots of pumps and lots complicated computer stuff to support the statement, "radiant is expensive" are not living in the present or not working with the right vendors or local circumstances force them to deal with service providers who have grossly inflated the system costs.

Lastly, the term 'expensive' is relative, circumstantial and subjective - expensive to one is affordable to another. Consider millions of people stand in line at Starbucks day after day to pay $5.00 for a boutique cup of coffee...others spend thousands of dollars each year on jewellery, clothing and accessories for their pets...and some splurge significant cash on outdoor decks, patio's and furniture trying to be more comfortable outdoors rather than investing in their own HVAC systems to be comfortable indoors where they spend 90% of their time...expensive's all about ones priorities.

One final note...consumer psychologist will tell you that when one persons says to another an item is expensive what they are saying is they personally can't afford doesn't mean that you can't afford it.

Suggested reading:
Where will your indoor climate system score?
How to "ball park" your budget for indoor climate control.
Indoor environments: Self assessment
Built to code: What does it mean for thermal comfort?
The Total Comfort System - The "Un-minimum" System

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Radiant Myth


  radiant can't heat the whole home

Eight of the top ten 2016 winning projects from the American Institute of Architects (AIA) Committee on the Environment (COTE) used radiant based HVAC systems. What types of big project use radiant? Museums, aircraft hangers, high-rise office towers, educational buildings at many universities around the world etc. etc. We're not sure what cracker box this myth crawled out of but heating a home - the entire home - is child's play for radiant systems. For a small list of large scale radiant systems around the world see the CBE data base of radiant systems.


Radiant Myth



radiant..."is not effective in energy efficient homes with really small heating loads because even a small amount floor heating can cause overheating due to the thermal mass effect."


We often see this statement associated with "popular science" experiments involving conventional construction practices rather than on low load energy efficient projects... never-the-less, using proper controls and systems typical of well designed projects mitigates overheating in all types of buildings in all types of climates.

General discussion: overheating in all buildings occurs with various combinations of enclosure performance, building mass, solar control, control over internal loads and control over the heating systems (and all types of systems not just radiant). Poor control over one or more of these elements can prevent the occupants from shedding their internal body heat at a rate fast enough to feel comfortable.

Your options:  control the building by site orientation, aspect ratio; and choosing the proper combination and location of windows, insulation and mass; control the solar gain and internal gains; control the heating system or preferably - all of the above.

Since the finger is pointed at radiant heating lets discuss the radiant by first talking about controls:

In high performance single zone homes with less than 10 Btu/hr/ft2 (32 W/m2) loads, and low to medium mass floor options; with proper exterior shading and glazing and with less than 20% window to wall ratios, there is little demonstrated benefit for having sophisticated radiant heating controls. A simple non electric thermostatic valve and analogue control on the heater or even a simple pulse width modulated (PWM) thermostat controlling the heat source is a suitable and effective solution.  In conventional housing with multiple zones and dynamic loads it is useful to modulate the temperature of the system fluid with in-space room feedback. Modulating the fluid temperature with what is known as weather compensators or indoor/outdoor controllers or reset controls is a tried and true technology having been in existence for  decades - so this is not a "new" solution nor is it "high tech" nor "sophisticated" - as far as expensive, see previous comment...put it this way,,,,weather compensators are like the cruise control on your car...they regulate your speed, control your fuel consumption and no proper car would be without one...likewise with radiant systems in some applications.

'Weekend engineers' who experiment without doing the research or by those who don't want to invest the money in controls and then blame the system for over and under heating tend to be the same folks who spend the rest of their life on the internet telling everyone about how bad radiant is. When you read this stuff on line what you need to read is, "I'm a weekend experimental engineer and didn't do a proper job of researching building science and radiant transfer nor did I pay for a proper system design with suitable controls".

Secondly,...if you don't want to mess with solar gains then (hold on - wait for it) ...don't give the solar energy a place to store itself... this is not that complicated...keep the short and long wave energy off and out of the building with external shading, reduced glazing areas (20% or less window to wall ratio's) with an appropriate solar shading coefficient, improved enclosure performance and use readily available off the shelf low mass flooring.

Consider this: 100% of all over heating and under heating complaints in homes heated exclusively with furnaces did not have radiant floor heating to blame.

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Radiant Myth



"The heated pipes radiate energy into the slab which then heats the air"

"Radiant heat from the Pex hot water tubes in the concrete slab is sent out in a 360 degree pattern."

"This fluid picks up radiant heat coming from the ground."

Embedded or buried pipes or cables use conduction (not radiation) to heat the in the form of electromagnetic waves are radiated (emitted) from or absorbed at surfaces. There is no temperature in the electromagnetic waves - only energy. In order for these electromagnetic energy waves to be converted to heat (measured by temperature) they must travel through a transparent space until they hit an opaque surface of a cooler temperature where they are absorbed. The absorption of the energy increase the molecular action in the surface which raises the temperature of the surface and through conduction heats the underlying mass.


Radiant Myth



"to prevent overheating with very small loads, the slab has to be no more than a few degrees above room temperature which means the floor won't be warm to the touch in fact it may feel cool to the feet."

You have to differentiate between "comfort" and "warm to touch" or "cool to touch"...again decades ago, research discovered that people wearing normal foot wear prefer floor temperatures in and around 75°F (24°C). In fact ASHRAE Standard 55 and ISO Standard 7730 both state for comfort, floors should be regulated between 66°F (19°C) for cooling and 84°F (29°C) for heating with 75°F (24°C) being the optimum temperature.

For those in bare feet or feet with light socks the limitations of the floor are also a function of the flooring itself.

If 74°F (23°C) is necessary for comfort than this is GOOD ENGINEERING PRACTICE.

 For further details see this link on floor temperatures  and see this discussion on having warm floors in high performance homes.
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Radiant Myth



with..."solar gain it may be possible with floor heating to be delivering excess heat causing overheating."

This is a twist on what we have already addressed but lets explore it further.

Have you ever placed an ordinary thermometer in the sunlight inside your home on a summer or winter day? What does it read and what does this tell you?

Here's one example: south facing double pane glazing, February 25th, 2011, Calgary, Ab., Canada - outdoor temperature -8°F(-22°C), floor temp = 88.5°F (31.4°C) WITHOUT floor heating.

Sans any discussion on window performances, it should tell you even without radiant floor heating the floors will warm up - so long as the floors see the sunshine the surface temperature will rise due primarily to the short wave solar radiation travelling through the windows. Throw in the long wave energy emitted from interior surfaces warmed by the short wave energy and you have a room heated with radiant energy without having embedded heating pipes.

Discussion: in a properly controlled high performance home under maximum load (no solar gain...middle of the night...wind howling...nasty cold), the floor temperature would be operated at a few degrees above a room temperature of 72°F (22°C), so lets say the calculation shows the floor to be between 74°F (23°C) and 76°F (24°C) and if designed correctly maybe an average 80°F(27°C) fluid temperature...cooler than core body temperature and cooler than skin temperature but still comfortable ...recall you have your own internal heat source that generates at rest a nominal 360 Btu/hr - so in fact under normal conditions - you need to get rid of heat to stay comfortable...and by the way feet are a great place to shed heat due to the blood flow and conductive transfer with the floor...anyways the million dollar question is,  will you have that ooh ahh sensation of a really warm floor? No but it will be comfortable and warmer than the floor in the same space heated with a furnace.

Anyways...morning comes along and as soon as the sun hits the 74°F (23°C) to 76°F (24°C)  floor, it starts to raise the floor temperature - but only those surface areas that "see" (absorb) the shortwave morning sun radiation - as such the room operative temperature of  72°F (22°C) begins to rise as what happens next? Well the thermostats stops the flow of tepid water to the floor and after some period of time the floor surface is at or warmer than the fluid in the floor due to solar gains. Since hot travels to cold or in this case warmth to coolth (yes it's a real word) ...the floor warming system becomes a solar absorber and can cool the floor by sending the solar gain ( a temperature rise in the fluid)  to a colder part of the house...or you could turn the system off completely and let the sun take over the comfort in your space. All you have to do is remember warmth travels to coolth and if the sun makes the floor warmer than the fluid in the pipes...the floor system is not contributing to the overheating.

If you want to shed solar heat gain in up the didn't cost you anything to get it in the first place so it won't cost you to lose it...the greenhouse people figured this out centuries ago...if you don't want to manually manage it...reduce your window area, reduce the windows solar heat gain coefficient, use exterior shading and low mass floors...but don't blame the radiant floor ...unless of course you didn't control the incoming solar heat in which case it's still not the fault of the floor.

Conclusion (once again): 100% of all over heating and under heating complaints in homes heated exclusively with furnaces did not have radiant floor heating to blame.

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Radiant Myth



"...radiant heating systems transforms cold floors and furniture into heat-omitting objects."

Heat is not "omitted" from warm objects it is "emitted" and can be absorbed, reflected or transmitted. There is no thermal effect of consequence from radiant transfer between two bodies of the same temperature.


Radiant Myth



"...the heat travels slowly upwards from the floor to the ceiling."

Radiant energy is not heat energy and it does not 'travel slowly' it travels at the 'speed of light'. The energy is only converted to heat when it hits a solid object of a cooler temperature. Again, radiant floor heating operates in almost identical long wave infrared frequency as the human body meaning its not heat absorption but heat retention which provides the warmth perception.


Radiant Myth



"The best place for radiant heating is on the floor because heat rises."

"Will the heat rise"? It's like gravity - it's physics."

"Heat rises so the whole room from floor to ceiling will heat up..."

"In-floor puts the heat where you are and not on the ceiling."

Heat doesn't air does relative to colder denser air serving as the motive force...radiant energy travels from hot to cold and in all directions...and heats the entire room not just the first 6 feet (2m)...if you don't get this basic principle try standing under a heat lamp...heat rising? No...if heat rose we would have to stand on top of the sun and put heat lamps on the just exactly where is the top of the sun? Radiant heating can be done from any surface, floor, ceilings and walls.

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Radiant Myth



"The best floor covering is tile or concrete because of its heat-conducting qualities. Wood floors or thick carpeting are insulators..."

This is one of the more predominate myths due to peoples confusion with insulation, resistance which are conductive elements affecting the heat transfer within the radiant panel and emissivity and absorptance which are radiant elements affecting heat transfer from the panel surface.

The fact is the effectiveness of a surface as a radiator has nothing to do with its insulative qualities but rather with its emissivity. 99.99% of typical floor coverings have very high emissivity's ergo they all make good radiators.

The insulating qualities are important but they affect only the fluid temperatures and pipe spacing and not the effectiveness of the floor covering as a radiator.


Radiant Myth



"...radiant heat can crack or otherwise damage fine wood flooring."

"Radiant heat will cause solid hardwood flooring to dry out too much and cause it to crack and shrink apart."

Beans first law of radiant heated hardwood: 100% of all hardwood flooring complaints in buildings heated exclusively with forced air did not have radiant floor heating to blame.

The primary cause of dimensional changes in wood is changes in moisture content - not heat. If this messes with your head, ask a furniture craftsman or piano tuner schooled in the ways from the old country. See below for the difference between heat and temperature.

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Radiant Myth



"...a flexible plastic tubing called "PEX" -- a polyurethane.."

PEX is actually PE-X, an acronym for cross (X) linked polyethylene (PE). There are different types of PE-X designated by a letter which is an indicator of the method used to cross link the molecules, for example, PE-Xa, PE-Xb etc...


Radiant Myth



"Comfort, in fact, is the No. 1 advantage (with radiant floor heating) with its warm-feet, cool-head environment."

While it is generally true, in heating people prefer a warmer lower body and a cooler upper body it should be clarified there is no single ideal floor surface temperature for everyone and in all situations - as it depends on the heat loss of the room, clothing including foot wear, metabolic rate and the conductivity of the flooring and the occupants subjective view of comfort.

Generally speaking, when wearing normal footwear -  floor surface temperatures are optimal around 75°F (24°C) to 77°F (25°C) ...however keep in mind - air temperatures are not the same as surface temperatures - when we talk air - the smaller the air temperature difference between the ankles and head the more comfortable people will be (favouring warm below to cool above)...See our pages on skin and comfort and temperature stratification.


Radiant Myth



"A wet system ..."

Refers to poured floors like concrete or gypsum based topping systems - no one we know wants to live with a "wet system"....we suggest people use poured floor system instead of wet.


Radiant Myth



"A dry system..."

Refers to sub floor systems installed under, in or on top of the sub floor, - no one we know wants to live with a "dry system"....we suggest people use subfloor system instead of dry.

Confusing the matter further - some literature will use a "wet" system in reference to a hydronic system and "dry" in reference to an electrical system.


Radiant Myth



"Heat and temperature are essentially the same thing..."

No they are not - heat is a form of energy - temperature is not.

Substances can have the same temperature but hold different amounts of heat. Temperature is an indicator of the speed of the molecules. You can apply more heat to speed up the molecules i.e. raise the temperature of the substance.

This is also a good place to point out that in reality there is no such thing as 'radiant heat'. I know it's used all the time but in reality, radiant and heat are just forms of energy...radiant is electromagnetic energy measured by its wave length..heat is energy measured by its temperature and comes from the stimulation of molecules in a substance absorbing the radiant energy.


Radiant Myth



"The Romans invented radiant floor heating..."

Wrong wrong wrong...inhabitants of Asia (Korea and China) were the first by several thousands of years. Currently there are archaeological excavations which have also uncovered very old radiant system in the Aleutian Islands of Alaska carbon dated to the same period as earlier finds in Korea. Per building, Korea has the most radiant heating anywhere in the world. Northern Europe, Germany, Sweden, Denmark etc... come in second.

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Radiant Myth



"...the thicker the slab, the more it will heat up and radiate..."

Slab thickness has nothing to do with how well a concrete surface will radiate - this is a function of its emissivity (see Figure 16). A thicker slab (yes) will hold more energy in the form heat but if the surface temperature of the slab is the same as all the surface temperatures in the room there will be no radiant transfer regardless of how thick the slab is or how much energy it holds.


Radiant Myth



"So, a lot of people will install a foil double-poly reflective insulation. They use a barrier insulation under the slab and help keep the radiant heat moving up, not down."

"the portion of radiant heat being lost into the ground below."

What bubble foil S & M folks are good at is creating a problem where none exists so they can try to solve it with their products. They have mastered the art of a solution looking for a problem.

Fact: there is no radiant heat transfer from the slab to the ground its conductive ergo there is no need for a reflective barrier. There is a long history of reflective insulation scams dating back over 30 years and a list of successful lawsuits brought by the U.S. Federal Trade Commission against manufacturers and distributors of these types of products.

See our pages on reflective barriers, paints and under slab insulation.


Radiant Myth



The piping can be wrought iron, steel, copper or plastic, including polybutylene and polyethylene. The steel and wrought iron are typically only used for floors. They are too heavy for walls and ceilings.

Steel, iron, copper...these haven't been used in radiant systems for a long time and polybutylene hasn't been readily available in North America for well over a decade.


Radiant Myth



"Warm air rises and much is lost, making forced air heating less energy efficient. Radiant heating works by transferring heat to objects, so when placed beneath wood flooring it heats the floor. Since less heat is lost through the air, radiant heating is more energy efficient than forced air heating systems."

All things being equal, radiant is more effective at conditioning people and spaces but it is not necessarily more efficient. Efficiency is a complex discussion because it involves other external systems and a few other terms such a exergy and efficacy- something most people are not familiar with...also there is an inverse relationship between building performance and the need for mechanical efficiency. The undeniable and undisputable fact is - is radiant is an enabler. It enables higher combustion and compression efficiency. But a lousy building with high efficiency boilers and heat pumps can be less efficient than a great building with mid efficiency systems...ergo any statement about radiant being more efficient should be questioned for clarity - in most heating case it is difficult to scientifically support the statement unless there are mitigating circumstances. However, there can be significant electrical power conservation with radiant cooling systems. See below for efficiency claims and see this overview of energy, exergy, efficiency, entropy and efficacy or what we call the E5 factors.

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Radiant Myth



"Electric radiant floor heating is widely considered to be the most energy efficient heating process currently available."

Electricity generated by combustion processes can have as much as 70% losses between the point of generation and the home...i.e. of the 100 units of energy to produce electricity you get between 20 and 30 units at the plug - far from being the most energy efficient process. Hydro, geo or wind generated power - now that's a different story. See below for more efficiency claims.


Radiant Myth



"brand x insulation forces the heat from the radiant heat system up to the floor's surface."

"brand x insulations help to retain the heat generated by the radiant heating system blocking it from escaping into the ground."

"Reflects cold back into the ground."

Under slab insulation does not force heat up to the floor surface nor block it from going down nor does it reflect cold nor is the heat within a concrete slab being transferred by radiation...insulation is slows or impedes the flow of energy in the form of heat travelling from hot to cold via conduction...radiation only comes to play at the uppermost surface of the floor. Learn more about reflective insulation scams.


Radiant Myth



"...savings associated with the use of radiant floor heating range from 10% - 40% on utility bills."

"it saves 30 percent in energy costs"

"Radiant heat can save up to 50% in total heating cost "

"can save up to 15% on heating bills ..."

"...saves 20 to 40 % on monthly heating ..."

"you can save 10-30 percent on your monthly heating bills"

"can save 40% off your heating bill"

" can save consumers as much as 50% in energy usage"

" radiant floor heating systems that should be up to 80 percent more energy efficient."

"They are typically 40 percent more efficient"

"Experience savings of 30% to 50% over hydronic radiant heating systems and even greater savings over conventional forced air heating systems."

...and this next one is our favourite claim of all time...

"All radiant heat systems are 100 percent efficient at any capacity."

There is so much anecdotal claims floating around you can pretty much find whatever you want for savings depending on what source you want to believe...the reality is  - there is no formal independent scientific residential research that we are aware of that conclusively supports any of these here again are the facts...radiant is an enabler...and by that we mean that radiant enables condensing boilers, chillers or heat pumps to operate at their maximum factory rated efficiency - no one can dispute that statement.

If you have a lousy house nothing is going to save you from high energy bills but what you will get is maximum performance from your radiant system while the building loses lots of heat.

If you want to reduce your heating bills - first build a high performance building and then get the maximum efficiency from your hydronic heating system by using low temperature radiant heating. cooling - scientific research and practical applications have  demonstrated significant electrical power conservation when radiant cooling is used with dedicated outdoor air systems.

If anyone can provide us a side by side identical comparisons in peer reviewed scientific papers  that can support any of these efficiency statements we'll be elated to revise our statement.

Please don't send us anecdotal statement made in sales literature and from product distributors websites nor send us reports that fail to demonstrate side by side identical comparisons. Nor are we interested in the setting the thermostat lower argument (as valid as it is) it is a subjective operator based argument.

Just so you don't go chasing your tail, here's a list of radiant based HVAC research papers in our library...I've not found anything in these papers that conclusively supports the plethora of efficiency claims found in sales and marketing literature.

Like I said, elated to revise our statement if anyone can show me the proof to support the internet efficiency claims.
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Radiant Myth



"...anything that can insulate the floor also reduces or slows the heat entering the space from the floor system. This in turn increases fuel consumption."

"Radiant surfaces should not be covered with carpet..."

This is only partially true. Fuel consumption is a function of many things including but not limited to building performance, boiler over and under sizing, types of controls, tube spacing and fluid temperatures. Floor coverings and their corresponding R-value are compensated for by tube spacing which influences fluid temperatures (in heating, more tube  =  lower temperature = better efficiency, in cooling more tube = higher temperatures = better efficiency). A correctly designed carpeted floor can operate at the same effectiveness and efficiency as a poorly designed wood or tiled floor.


Radiant Myth



"However, in humid climates, problems with over-cooling the floor could lead to wet slippery surfaces and fungus growth."

Regardless of the HVAC system, moisture must be controlled for biological concerns (bacteria, viruses, mites and molds), for hydrolysis and the control of VOC emissions (formaldehydes, phthalates, terpenes etc.), for the dimensional stability of hygroscopic materials (woods) and for occupant respiratory and thermal comfort; moisture must be regulated for these greater priorities making the condensation on radiant cooling panels a moot point.

Naysayers and sceptics conveniently forget, 100% of all condensation problems in buildings conditioned exclusively with air did not have radiant cooling panels to blame (go figure). See our pages on radiant cooling.

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Radiant Myth



"The radiant floor heating will save the most energy."

"it saves energy"

As pointed out by the International Energy Association, "This claim (saves energy)...conflicts with the first law of thermodynamics stating that the total amount of energy is conserved even though forms of energy may change from one to another." In layman's term you can't create nor destroy energy ergo it cannot be made and saved like pennies in a bank - not even by radiant floor heating systems.

At the end of the day, energy can be preserved and conserved but not saved. For a thorough study read about "exergy, entropy, energy and conservation ."


Radiant Myth



"It can lower the allergens in your home by eighty percent!"

"Radiant heating will also keep your home free of air-contaminants, bacteria and dust."

"There is no hot air being blown around as with traditional systems, so no dusty sneezes."

While there are some environmental benefits to using radiant, lowering allergens by 80% or keeping your home, "free of air-contaminants, bacteria and dust" are not ones that we can find supported by research. We love to know where these came get the facts read our featured article on the health effects of low temperature radiant heating systems.

Also, every properly designed and built building will need to have moving air for ventilation purposes. Yes the volume of air moved is considerably less with radiant systems but it isn't zero as told by some.


Radiant Myth



"radiant heating is only for new construction."

"retrofitting radiant is too difficult..."

" you have to rip up floors to have radiant heating."

One of the lowest cost, easiest and most effective ways of retrofitting radiant heat is to use the walls. If that messes with your head ask yourself why your cat or dog finds sleeping in front of a window (a wall you can see through) so comfortable...hmmm it might just be the radiant heat...who knew?  Integrating radiant into wainscoting is just one creative way of adding the beauty of woodwork and the comfort of radiant. See our pages on retrofitting radiant into existing buildings.


Radiant Myth



"Floor heating comes in three varieties: electric, hydraulic (liquid), and air-based systems."

Hydraulic? Try hydronic...semantics yes but this page is about how myths get started and spread...people read hydraulic from an expert and they think brake fluid. Is "hydronic" any better - no...but it's the correct term.


Radiant Myth



"glowing floor heating systems."

Glowing? Lets put this into perspective, the surface temperatures of a wood stove is over 400°F (204°C)  with interior temperatures over 1000°F (538°C)  and still these temperatures are too low to glow...(pun intended)...floor heating operates below 85°F (29°C) surface temperatures...far below glowing temperatures.  We get what the writer is trying to do - it's actually nice to think of warm floors as glowing - but if the floor were glowing - your house would be on fire.


Radiant Myth



"Heating the entire slab may result in a condition known as "hot foot".  Your feet and ankles are not accustomed to standing on a warm surface."  

Another twist on the above "glowing floor heating system"...once again the control of floor heating temperatures are specified in ASHRAE Standard 55 - Thermal Environmental Conditions for Human Occupancy. There is no "glowing floor" or "hot foot" unless the system has been designed and installed by amateurs. See this article on do it yourself HVAC.


Radiant Myth



"Among the advantages of (radiant) are the absence of any air flow from the outdoors that has to be heated, as is the case with many forced air systems."

We're not sure if the writer was referring to exfiltration/infiltration (leakage) or ventilation (to replace exhaust air from fans) regardless both are heating loads which are independent of the type of comfort system. See dedicated outdoor air systems.


Radiant Myth



"...uses a lower thermostat setting as compared to forced air system thereby lowering your energy bills."  

All things being equal, it is possible in standard construction, to theoretically achieve comfort levels with lower thermostats settings in spaces conditioned with radiant, however there is a report by CHMC stating that even though people could use a lower setting few occupants from the study actually do. Now I know some people with radiant who do use a lower setting but I also know people who also use a lower setting with air based system. These are subjective choices and hardly the basis for carte blanche statements. Furthermore, in higher performing homes this argument becomes a moot point anyways as the building enclosure solves the energy use and minimizes the cold surface effect on the operative temperature.


Radiant Myth



"radiant costly to install, and maintaining the system can be difficult."

The cost and maintenance of any mechanical system is inversely related to the complexity and efficiency of the architecture. A high performance home built to R2000 (Canada) or Passivhaus (German) standards have very simple and affordable systems that can use robust tried and true components. If you want a difficult system build a difficult building and use new stuff designed and installed by inexperienced people.


Radiant Myth



"..if you set your thermostat to about 68 degrees, the room will likely heat up to about 72 degrees. This is because the heat is sustained through the mass, so the higher the surface is heated, the lower you need to keep the boiler temperature."

Surface and boiler temperatures are established from the tube spacing, floor conductivity, fin efficiency and heat loss - it has nothing to do heat being "sustained through the mass"..  A high performance home with conductive floors and tight tube spacing can operate at very low surface and boiler temperatures resulting in very high combustion efficiencies.


Radiant Myth



"Hydronic radiant floor heating systems use boiling water in the pipes to generate heat..."

Water is not boiled in a hydronic system. A high performance home with conductive floors with tight tube spacing the temperature of the water is similar to the core temperatures found in your own body, i.e. 85°F (29°C) to 100°F (38°C).


Radiant Myth



Hydronic radiant floor heating systems are also environmentally friendly as they have an option of recycling your used water. In fact, it is also a part of purifying your water where the water is brought to a boil.

That statement is by far the most bizarre statement we have come across....Fact: hydronic radiant floor heating systems do not recycle your used water and are not part of purifying water for domestic use. Read here about using domestic water heaters for space heating.


Radiant Myth



"The heat energy that is created by warm water, flows through pipes..."

Heat 'is' energy and its not created by warm water flow - energy in the form of heat is transferred into the water through the heat exchanger of the boiler or heat pump.


Radiant Myth



"TLC points out that while radiant heat is definitely more efficient in smaller, snug homes with lower roofs, it might not always be the greenest solution in homes with bigger rooms"

Huh? Not sure what this myth is based upon, but again all things being equal, the larger the space the greater the potential for increased system efficacy and efficiency. This is due to a lower flux from a larger floor surface area which affords the use of a lower fluid temperature which increases the boiler efficiency. There are countless examples of large buildings with very big rooms using radiant heating including aircraft hangars, manufacturing facilities, warehouses, office towers, museums etc.


Radiant Myth



"Of course, pairing a radiant heating system with an energy efficient EnergySTAR-approved programmable thermostat can indeed save households hundreds of dollars a year on home heating bills "

The #1 mistake made in the industry is trying to treat radiant systems like air based systems including how to operate them with so called, "EnergySTAR-approved programmable thermostat". The optimum way to control a radiant systems is use an indoor/outdoor controller to adjust system water temperatures but set the room thermostat to a comfortable position and leave it alone.


Radiant Myth



"Radiant heat is also very different in that the operative temperature of the system (the actual average temperature of the radiating surface) is generally significantly cooler than a hot air source."

The operative temperature is not the, “the actual average temperature of the radiating surface”. The author is mistaken with mean radiant temperature or MRT. In its simplest form “operative temperature” is the mean between the dry bulb and MRT. For a greater explanation see our pages on MRT and Operative Temperature.

more to come...if you want us to look at a questionable statement let us know at the forum.

Related reading:

Do I need an engineer? A Guide to HVAC/Indoor Climate Design Service Providers
Where will your indoor climate system score?
How to "ball park" your budget for indoor climate control.
Indoor environments: Self assessment
Built to code: What does it mean for consumer thermal comfort?
The Total Comfort System - The "Un-minimum" System
Thermal Comfort: A 40 grit perspective for consumers
Thermal Comfort: A Condition of Mind

Do-It-Yourself HVAC - Should you do it?
The Cost of HVAC Systems - Are You Paying Too Much for Downgrades?
Radiant Installations - The Good, Bad and Ugly
Thermal Comfort Surveys - Post Occupancy, Part I
Thermal Comfort Surveys - Post Occupancy, Part II

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