Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
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radiant cooling for thermal comfort

A note on residential radiant cooling...unlike commercial spaces where it is possible to regulate the opening of windows and doors with automatic devices, there are parts of the residential human race that can't be regulated to close doors and windows...they are called kids and pets (and some spouses).

Leaving a window or a door open unintentionally can contribute to a sequence of events which no one wants to deal with after the fact...so as radiant cooling evolves for residential systems I predict some very smart people will develop low cost open window/door sensors to regulate the flow of chilled fluid to radiant surfaces.

In the meantime, radiant cooling is for educated responsible adults that have the maturity to  grasp the principles required to regulate radiant based cooling systems.

I.e. if you don't want to child proof your radiant cooled home - then common sense says use something else.

Your Five Senses

Five Senses, Boilly
Source: National Library of Medicine


The simplest areas for radiant cooling are those regions with the highest solar loads, the hottest temperatures and the lowest humidity.

Solar Intensities Canada
(Credit: Thermomax)


Solar Intensities U.S.
(Credit: Thermomax)

Note: The above graphs were generated by separate programs using different units. Visit Thermomax for details.


Learn about Solar Loads
 


More About Humidity

  

Radiant based Cooling Systems and Equipment - Part II

Controlling the Relative Humidity

To prevent condensation from accumulating on a cooled surface we have to reduce and maintain the humidity to less than 50% RH+/- and keep our surfaces above 66 deg F.

Lets get the 66 deg F surface temperature out of the way first since its the easiest. Just like in heating, we can control the fluid temperature going to the floor by adjusting the ratio of chilled water from the cooling plant with the return temperature from the floor or we can control the chiller. If this makes no sense to you don't worry these control packages come prefabricated just like your freezer or refrigerator. By having a sensor in the floor and one in the fluid we can tell a controller to lower or raise the surface temperature and just like in radiant heating the resistance to heat flow is based on the surface R value.  For radiant floor cooling use highly conductive solid surfaces like tile, concrete, or slate. Do not use any kind of thick carpet or similar soft rolled blanket type surfaces.

So how do we keep the relative humidity below 50%?

My technically correct colleagues cringe when people are told to think of air as a sponge for moisture. But even thought it is not technically correct, it's an easy way (yes...at the expense of science) to help communicate to some that the air has to be delivered to the space dry enough to anticipated moisture accumulating from the occupants and their activities plus the moisture coming in through building leakage and ventilation air. If the moisture content in the air continues to build up in a space at a fixed temperature eventually it will reach its saturation point (dew point) and condense out. We don't want that to happen on radiant cooling surfaces.Controlling HumidityThe most common way to prevent condensation is to remove moisture from the incoming air by wringing it out, i.e. by condensing the moisture with cooling. Condensing with cooling is like deliberately placing a great big chilled drink in the path of the incoming air stream of the home. Can you image all the water dripping down the sides of a monstrous sized cold drink? The ideal place for placing this cold surface would be in a continuous flow of air which happens in the ventilation ductwork. The name for the cold surface is called a "chilled water coil" or "evaporator coil".

Relative HumidityBy recirculating air over a chilled water coil or refrigerant based evaporator coil we can  continually wring out the moisture in the air.  Just like the drink, some of the heat goes into the coil which is why we have to keep cooling it and some of it is drained away instead dripping onto your shirt or blouse. In dedicated outdoor systems we only need to dehumidify the fresh incoming air. There are other ways such as using desiccants which work well but at the time of this writing, we've not seen small residential desiccants as yet from the manufacturers...but it's only a matter of time so stay tuned.

So how much "dehumidifying" does the air in your home need?

Well these two images below, courtesy of Building Science Corporation will give you a basic idea. The first map is an illustration of typical precipitation patterns in North America.  The second map is a generalization of temperatures and will tell you if your HVAC system is predominantly used for cooling or for heating.

North American Humidity Zones

A competent HVAC designer will look at your geographic region and based on regional weather data can determine how much de-humidification is required and how to prioritize your investment in building efficiency and HVAC equipment.

North American Temperature Zones

To see what type of HVAC system you will need for radiant cooling go to part III.

go to part I - click here, Introduction
go to part III - click here, Heating and Cooling Comfort Systems
go to part IV - click here, Radiant Cooling Educational Movie Clips
go to part V - click here, Featured Project, Manitoba Hydro Office Building

 


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