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Radiant Floor Heating

Some basic rules to observe when considering efficiency:

Combustion and compression efficiency is a function of return fluid temperature - in heating the lower the better and in cooling the higher the return temperature the better.
Everything else being equal, small surface heat exchangers (baseboards, free standing radiators, fan/coils) or un-plated radiant systems need higher temperatures in heating; large surface heat exchangers (radiant floors, walls and ceilings using heat transfer plates ) need lower temperatures.
Firing a mid efficiency boiler with a programmable thermostat does not change the fact that the boiler is still mid efficient.
Firing a high efficiency boiler to high temperatures with a programmable thermostats does not change the fact that the boiler is behaving like a mid efficient appliance.

Last but not least...

The skills of an indoor climate engineer in designing a system for effectiveness and energy efficiency should never be confused with picking HVAC equipment out of catalogue nor the contracting skills it takes to assemble components on site.

In our opinion, the HVAC system influences:
the health of the environment and by association the health of the occupants;
the operating costs for utilities due to the efficiency of the systems and by association the occupants cash flow;
the temperatures, humidity and pressures in the enclosure and interstitial spaces and by association the integrity of the occupants assets and the building itself.

For these three reasons an indoor climate engineer (building scientist with HVAC) should be the first persons on the design team followed by the interior designer and then the architect.

Once an energy efficient  indoor environment has been defined with the enclosure and interior design features then the architect can create the building along with the various systems for developing the tender documents for suppliers and trades people.

The unspoken story on setback thermostats

Dew point temperatures in a building are a function of the absolute moisture content in the space, and the relative humidity and temperature at a surface.

When setback thermostats are used, it is possible to drop the temperature low enough such that condensation can occur on some surfaces.

You can see a demonstration of this principle with some windows and humidity in cold climates. To prevent window condensation one has to raise the window surface temperature (choice in performance) or lower the humidity (a function of the enclosure performance, the HVAC system and the occupants).

If one does not lower the humidity and allows for a setback in temperature using a programmable thermostat, one sets up the potential for condensation. This can occur on the window and/ or inside the enclosure. Click thumbnail below for illustration of window performance and condensation.

The concern also applies to summertime conditions where dehumidification is reduced (humidity rises) during the extended off cycles of the cooling system.

In both extreme winter and summer periods, it is also possible for right or slightly undersized equipment to recover inadequately after being being setback in heating or set-up in cooling.

Are we saying don't use setback thermostats? No, on the contrary, they have their application in the right projects and when operated correctly they work.

We are saying, like many aspects of construction and space controls there can be unintended consequences unless all aspects of the indoor environment are considered and managed.

Radiant Research

Thermographic images (below) of various radiant installation options from the Kansas State University  laboratory (ref.: ASHRAE research project 1036) photo credit: Bob Rohr

If you've not interpreted thermographic images before, be advised what you are seeing are low temperatures, less than 80 deg F (26 deg C).

The images also do not necessarily represent steady state conditions and the image will change between heating season start up, summer shut down and winter steady state conditions; and will change depending on floor coverings.

Fig. 6 Suspended tube without heat transfer plates. These types of system will run the hottest out of all options due to lack of contact between the pipe and the floor. As such, when compared to other options, they can be the least efficient of all systems. They should only be considered for very conductive / low resistance floors  and when the building load is very low.

Fig. 7 Staple up without heat transfer plates. These system will run lower than suspend tube system (Fig. 6) but hotter than plate type system (Fig. 8 and Fig. 9) due to less conductive surface. As such they too operate at a lower efficiency than other options. In un-plated sub floor system above, users are cautioned about the long term performance integrity of reflective barriers used below the tubes. This uncertainty is based on the unknown accumulation rate of dust and other particulate which can vary from project to project.

Fig. 8 Staple up with heat transfer plates. These system will run at a lower temperature than Fig. 6 and Fig. 7 due to the addition of aluminum fins which extend the contact surface area. Again a caution to the untrained eye, the temperature is low and the perception of striping is inconsequential due to diffusion under operating conditions.

Fig. 9 Grooved panels with heat transfer plates. Out of all options for sub floor radiant system these types typically offer the best performance at the lowest temperature for the highest efficiency and provide the easiest installation next to a poured floor type system (Fig. 10 and Fig. 11).

Visit our comprehensive bibliography on radiant cooling and heating.

Thick extruded heat transfer plates versus thin stamped heat transfer plates: An FEA Study

Boiler efficiency
Copyright (c) 2012, Robert Bean, R.E.T., P.L.(Eng.), and content contributors

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As noted in Part I, for energy conservation it is not enough to just install programmable thermostats and upgrade ones old mid efficient boiler to a new high efficiency unit unless the boiler can actually operate at the conditions necessary to deliver its high efficiency rating.

What do we mean by this?

It means a boiler rated at say 98% only achieves 98% when it sees less than 80 deg F (27 deg C) return water temperatures (Fig. 2) regardless of the heaters Energy Star rating or use of programmable setback thermostats. This is not a trivial matter as the cost to upgrade equipment and controls can be significant and all will be for not from an energy perspective if it doesn't lower fuel consumption.

If it is a new project, one way to obtain low return temperatures is the use of large conductive heating surfaces such as radiant floors, walls and ceilings; and/or have very low loads typical of transitional and terrific performing homes. With these housing categories there will be no need for the high temperatures typically associated with terrible and traditional homes using small heating surfaces such as baseboards and free standing radiators.

For retrofits it becomes a little more trickier if you plan on upgrading your old boiler with a new and improved version - things you have to think about:

  1. If the enclosure has not been upgraded (a bad thing) and the original heating system was right sized (a good thing), then lowering the new condensing boilers fluid temperature under some operating conditions may result in under heating and thus discomfort (a bad thing). 

  2. If the enclosure has not been upgraded (a bad thing) and the original heating system was undersized sized (a bad thing), then lowering the new condensing boilers fluid temperature under most operating conditions may result in under heating and thus discomfort (a bad thing). 

  3. If the enclosure has not been upgraded (a bad thing) and the original heating system was oversized (a bad thing), then lowering the new condensing boilers fluid temperature could likely provide acceptable results (a good thing). A rare case of two wrongs make a right...go figure.

  4. If the old boiler was upgraded followed later by an enclosure upgrade, then the new boiler was just made oversized by the new enclosure (a bad thing).

  5. If the enclosure was first upgraded (a good thing), and the original heating system was oversized ( a bad thing) then the old system is now really oversized ( a really bad thing); but now lowering the fluid temperature of the old boiler could likely provide acceptable results with greater efficiency (a good thing). However, not all old boilers were designed to run at low temperatures and making them do so has led to several carbon monoxide deaths (a very bad thing and very difficult to recover from...). Meaning you will need to replace the boiler even though the building is better. As they say, no good deed ever goes unpunished.

  6. If the enclosure was first upgraded (the best thing), and then a right sized new condensing boiler installed (the next best thing) then it's likely the new building and boiler will provide acceptable results (a good thing) - so long as the boiler controls allow it to shift to the lowest temperature necessary - but no lower (Fig. 3).

So what does all this mean?

It means if you don't get the sequence and conditions right, it might be necessary in some cases to upsize the surface area of some or all of the radiators, add extra radiators, or increase the area of the coil in an an air handler, or increase the footage of baseboard in your system. If you do get the sequence right, buildings before mechanical systems, you may have to do nothing; or if you do replace the boiler it will have the opportunity to operate as the boiler design engineers intended. A qualified building and mechanical technician can assist you in this regard.

boiler efficiency is a function of temperature and heat exchanger sizes

Fig. 1 illustrates the relationship of heat exchanger surface area and the associated temperatures where the lowest return temperatures provide for the highest efficiencies (see Fig. 2 below).

Fig. 2 (above) is typical for natural gas boilers and illustrates the efficiency of the boiler based on the return fluid temperature. You can see that to obtain anywhere near 98% efficiency one would have to have return temperatures less than 80 deg F. Likewise one can see that a typical cast iron radiator (or fin tube) system designed to operate with returns in excess of 170 deg F might achieve a nominal 86% efficiency. You can also see that in order to improve an existing system with a new high efficiency model, the radiator surface area would typically have to be increased by a factor of 3 or 4 to put it into the 92% range.

boiler weather compensation reset ratios

Fig. 3 (above) represents the fluid temperature options (reset ratios) typically required for various systems at different outdoor conditions. Controls on boilers using this logic are essential for ensuring fluid temperature are adjusted up or down as the weather gets warmer or colder for obtaining the highest efficiency.

reset ratios and boiler efficiency
Fig. 4 (above) represents the potential return fluid temperatures based on the use of a weather compensator and three sample reset ratios.  It illustrates how various systems can be operated down from their maximum temperatures as the outdoor temperature warms up and by doing so improves the efficiency of the boiler.

A word of caution: some boilers are not designed to operate at low return temperatures. These appliances are typically called mid-efficient non-condensing boilers. Operating such a device below a nominal 140 deg F (60 deg C) for sustained periods could result in flue blockage (due to accumulation of precipitates) leading to roll out and potential for carbon monoxide poisoning of the occupants.

Effectiveness coefficient for temperatures in various countries

Table 1 (above) Effectiveness coefficient for temperatures in various countries. North America and the UK rank lowest of the nations evaluated based on the use of unnecessarily high temperatures.

Fig. 5 (above) The importance of evaluating return temperatures in a system. In the example above, all other design parameters being equal, the difference between a non plated radiant system and a plated radiant system could result in as much as a 10% difference in boiler efficiency. This difference becomes less with building performance increases and/or improvements in the heat exchanger design such as adding more pipe in a radiant system or a larger coil in an air handler.

Message: To obtain the rated performance from a high efficiency boiler you have to operate it at a low return temperature which can be achieved with a weather compensator and a heating system that has been designed for low temperatures.

Adding programmable setback thermostats can provide some additional conservation measures in some systems but more so in low mass air based or baseboard systems and less so in radiant floors, walls or ceilings.

Make note that when programmable stats are not properly used due to their complexity they can actually increase energy consumption rather than reduce it and finally, if moisture is not controlled in the space, setting back the space temperature can cause condensation on cooler surfaces.

Maximizing efficiency: Computer modelling of different radiant floors using finite element analysis (FEA).

Computer programs such as FEA allow engineers to optimize designs for efficiency and effectiveness. As per the thermographic images, it is important to not interpret results as representative of all system during all operating conditions as the model only represents the input for a specific application and condition. They do however illustrate the thermal diffusion and surface efficacy and give an indication of the operating conditions which impact the system efficiency. All other conditions being equal, plated system such as Fig. 8 and Fig 9. plus poured floors Fig. 10 and Fig. 11 when used with conductive flooring such as tile operate at the lowest fluid temperature and offer the highest efficiency.

Fig. 10 Poured slab on grade floor

Fig. 11 Poured sub floor using a gypsum based mix.

Fig. 12. Sub floor system using heat transfer plates with air space assigned an effective resistance to model the radiant component.

Suggested reading:

  1. Kobayashi, A., Kohri, K., A Study on the Thermal Response Characteristics of the Floor of Hydronic Floor Heating Systems, Eighth International IBPSA Conference, Eindhoven, Netherlands, August 11-14, 2003

  2. Chapman, K.S, Shultz, J.D., ASHRAE Research Project 1036 Develop Simplified Methodology to Determine Heat Transfer Design Impacts Associated with Common Installation Alternatives for Radiant Conduit, 2002

  3. Siegenthaler, J., Lowered Expectations, Plumbing and Mechanical, BNP Media, June 1, 2011

  4. Butcher, T., Hydronic baseboard thermal distribution system with outdoor reset control to enable the use of a condensing boiler, Brookhaven National Laboratory, (for) Office of Buildings Technology U.S. Department of Energy, October, 2004

  5. Arena, L.B., Butcher, T.A., Zoeller, W.A., Shapiro, C.R., In-Field Performance of Condensing Boilers, ASHRAE Transactions CH-12-C046, Volume 118, Part 1, 2012

  6. AHRI; (2008). I=B=R Ratings for Boilers, Baseboard Radiation, Finned Tube (Commercial) Radiation, and Indirect-Fired Water Heaters, The Hydronics Institute Division of AHRI.

  7. AHRI; (2009). Certified Product Directory – Boilers, Baseboard Radiation, Finned Tube (Commercial Radiation), Indirect-Fired Water Heaters, Air-Conditioning, Heating, and Refrigeration Institute (AHRI).

  8. AHRI; “Residential Hydronic Heating: Installation & Design Guide”; Air Conditioning, Heating and Refrigeration Institute,Arlington, VA, 2009.

  9. Arena, L.; (2010). “In-Field Performance of Condensing Boilers in Cold Climate Region.” National Energy Technology Laboratory, Morgantown, WV; US Department of Energy, Building Technologies Program.

  10. ASHRAE; “ASHRAE Handbook of Fundamentals”; American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA, 2009.

  11. Butcher, T. “Condensing boilers and baseboard hydronic distribution systems.”; ASHRAE Transactions; Vol. 112, part 1, 2006.

  12. Butcher, T; (2009). “Optimal Design and Operating Parameters of the Condensing Boiler/Hot Water- Baseboard Combination:Task Report.” NYSERDA Agreement, No. 10927. Albany, NY; NYSERDA.

  13. D&R International, Ltd; (2011). “2010 Building Energy Data Book.” Pacific Northwest National Laboratory; U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE).

  14. Kreider, J.F., Rabl, A.; “Heating and Cooling of Buildings”; McGraw-Hill, 1994.

  15. Siegenthaler, J.; (2004). “Modern Hydronic Heating for Residential and Light Commercial Buildings, 2nd Edition.” Mohawk Valley Community College, Utica, NY.

  16. Zoeller, W.; (2011). “Condensing Boilers and Low-Temperature Baseboard Convection.” NYSERDA Agreement, No. 10927. Albany, NY; NYSERDA

Related pages:
Programmable Thermostats Part I
Radiant design guide
Effectiveness coefficient, (Φ ) for temperatures in various countries 

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