Some Active Solar Heating
Design Considerations, Part II
Radiant floor combined with solar heating is a marriage made in heaven! When backed up by a high efficiency boiler, the marriage becomes even stronger! Here’s why: With more commonly used heat sources, such as radiators and baseboard heaters, the temperature requirement is high; say 160-180 degrees F. With radiant floor, the delivery temperature (through PEX tubing) is in the range of, say, 85 – 120 degrees. This lower temperature range is a perfect match for the temperatures available from the solar storage tank, whose usable temperature range will be from about 90 – 170 degrees. If the solar storage tank heat were to be sent to, say, a baseboard unit, it would work fine for a short time while the solar tank was at 170 degrees. As soon as you start sending heat to a baseboard, however, the tank temperature starts dropping immediately. Once the tank temperature dropped below about 160 degrees, the house would start to cool uncomfortably. Unlike a boiler, the solar tank can’t replace the lost heat when the sun isn’t shining.
The next natural question is: What happens when you try to put 170 degree storage tank water into the radiant floor? The answer: you don’t! The 170 degree water is first “mixed down” by the hydronic (hot water) plumbing system to the “appropriate” temperature for the radiant floor panel. This temperature depends on the outdoor temperature; it’s mixed hotter on the coldest midwinter days, and it’s mixed cooler on cool spring days, so as to match the “heating load” of the house. The heating system controller accomplishes this temperature adjusting function using a feature known to heating nerds as “outdoor reset”. The outdoor reset function is set up in the controller by the heating contractor, and the homeowner need not worry about it (but it’s good to know about it). No modern heating system should be installed without “outdoor reset”!
Earlier, I mentioned High Efficiency (HE) boilers. HE boilers, much like solar, work at lower temperatures than traditional cast iron boilers. This makes the HE boiler a good match for the solar and radiant subsystems. The two significant features of the HE boiler are MODulation and CONdensation (MOD/CON). Modulation refers to the heat system controller using the reset function to modulate the burner flame to match the current heating load. If its -10 degrees outside, the reset function tells the boiler to crank up the burner all of the way; if its 50 degrees outside, the burner only comes on with a small flame. Burner modulation saves a great deal of energy that would otherwise go out the exhaust stack.
Condensation refers to the fact that the heated water going to the floor, and consequently, the cooler water returning to the boiler, are cooled to the point that flue gases (the exhaust gases resulting from combustion of the NG or propane, which are rich in H2O vapor) condense on the heat exchanger inside the boiler. If you watch a condensing boiler in action, you can see a constant trickle of this water exiting the boiler and going into a drain. Why is this so great, you ask? The boiler has just extracted the “latent heat of vaporization” from the flue gases, and transferred that heat into the water going to the radiant floor, instead of sending it up the exhaust stack, as happens with a cast iron boiler. The “condensing” function of the boiler adds about %10 to the efficiency of the boiler.
In the future (starting now!), the use of solar thermal and high efficiency mod/con boilers with radiant heating will cut building energy usage drastically, thus increasing our national energy security!