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Think Of Comfort When Installing Radiant Systems

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Too often building professionals, architects, general contractors, and even HVAC contractors, seem to not consider the comfort of the people living in the home. We see many beautiful homes with terrific views and uncomfortable occupants.

Consider this: Humans are living radiators. Our bodies want to radiate heat at a specific rate. If we radiate heat too fast, we’re cold. Too slowly and we’re hot. Think back to a summer grocery store trip wearing shorts and a short sleeve shirt. You are comfortable in the canned goods aisle, but get goose bumps in the freezer aisle. Why is that when the air temperature is the same?

It happens because the temperature of the canned goods is close to room temperature; on the other hand, those glass freezer doors in the frozen food aisle are much colder than those canned goods, causing our bodies to radiate heat faster and making us feel chilly.

The same thing happens in homes. Those beautiful wood framed, double pane picture windows with the great view will make you feel chilly sitting next to them on a cold winter night. This is because we have two main factors at work. The “faster than our bodies want” radiant loss mentioned above, and we also have a cold air “waterfall” flowing off the glass that can make our feet cold as well.

Radiant floors will help keep our feet warmer and us more comfortable, but it doesn’t really address the radiant loss a person feels if they’re within a few feet of a cold surface like a window.

The best solution we’ve found is the use of European style panel radiators located under cold surfaces like windows. This allows both a convective “curtain” in front of the cold surface and direct radiation to help keep people comfortable. The best implementation is panel radiators coupled with a low temperature modulating boiler, constant circulation, and an outdoor reset control.

The only concern we have now is how to prevent “thermal overshoot.” On occasions when the room temperature exceeds the temperature we’d like it to be. Think a sunny, but cold winter day with a south facing living room and lots of glass for those great views. Heat is being added to the home based on the outdoor temperature, but we’re also getting a lot of solar gain in areas as well.

To counter this, the panel radiator’s thermostatic valves will sense the increase in room temperature and reduce the volume of water flowing through the panels, thus reducing that radiator’s heat output. The valves are adjustable and in effect give room by room temperature control. Have a seldom used guest bedroom? Dial the radiator valve down. Want a room a little warmer? Dial the valve up a bit.

These types of systems are ideal for the implementation of variable speed circulators that run off delta-P or pressure differential. Set the pumps for the maximum flow rate required by the system and they’ll often run at 1/3-1/2 of the wattage that a traditional multi-speed circulator requires. The real beauty of these circulators comes into play as the thermostatic radiator valves start reducing the flow through the panels in rooms that require less heat because of solar gain, cooking in the home, etc. As the flow requirements drop, so does the energy used. We recently installed a panel radiator system using variable speed circulators and found it only required about 30 watts to move the water throughout the system for an 1,800 square foot home.

As most of you are probably aware, modulating condensing boilers maximize their high efficiency with low return water temperature by extracting the latent heat of vaporization from the water vapor in the exhaust gasses. The cooler the return water is the greater amount of heat we can remove from the water vapor in the exhaust gasses. Hot water baseboard is generally considered a high temperature distribution system. Most of the heat delivered by baseboard is convective, not radiant, so higher water temperatures are required. Unlike baseboard you can design a panel radiator system to operate at much lower temperatures while still maintaining great comfort. Want a lower return water temperature? Use a little larger radiator. Since panel radiators are available in a great variety of heights, widths, and thicknesses; it’s usually fairly easy to find a good match for the space.

The home mentioned above was designed to operate with 140º water supply on a below 0º design day. Return water will be around 120º on the coldest days of the year allowing us to extract lots of heat from the exhaust.

The next time you have a prospect with a beautiful home with lots of glass think of the people who will be living there and consider offering panel radiators as an option.

Steve Scott
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Posted In: ACCA Now, Hydronics

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