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
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.The
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".
By 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.
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.
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 -
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