Yesterday, Chris and Justin from CRS Mechanical started stapling down the 1/2″ diameter PEX tubing that is a key element of the hydronic heating system. The tubing will be embedded in 3″ of concrete, which in my case will form the finished floor. The tubing is stapled down first; then #2 rebar is laid over it; then the concrete is poured. The staple-down phase goes pretty quickly with the right tools (a spindle for uncoiling the tubing so it doesn’t twist, and a pneumatic stapler that has a special nose that centers the staple over the tube.

This kind of heating system is often called “radiant heat.” My college heat transfer professor would roll over in his grave to hear that description. This would better be called “convective heat.” The warm water/glycol solution heats the concrete via forced convection (pumping warm fluid from the boiler) and then the concrete heats the air via natural convection. Very little of the heat transfer is via radiation. Radiation is the transfer of heat via infrared waves, as in what you feel if you put your hand under a heat lamp, or for that matter, the way you are warmed by the sun. The transfer of heat via radiation is minimal at the temperature of the floor in a typical hydronic heating system.

Update after spending two weeks in the house in December/January:

The heating system works very well. last night (12/31/2010) the temperature dropped to around -10F. The house remained at a perfectly constant temperature with no drafts or cold spots (those benefits due largely to the spray foam insulation). There are two unexpected issues. First, the house is so well insulated and with so much floor area heated that the floors are not very warm even during periods of extreme cold. They are perfectly comfortable even in bare feet, but I bet they aren’t warmer than 80F. My house in Pennsylvania is so drafty and poorly insulated that our heated concrete floors are quite warm to the touch in the winter (maybe 95-100F), which is very nice. When more of the floor area is covered with rugs perhaps the floor temperature will come up a bit.

Second, we do get some condensation at the corners of the windows. Last night with the outside temperature at -10F, we got a little ice at the corner of the windows. These windows are quite high performance (U=0.29), but there’s just no way in an affordable window to keep the edges of the glass above the dew point. I suspect this will be an issue 10 days a year. I suppose we could run the heat recovery ventilator more frequently to bring down the indoor humidity, but that would reduce indoor comfort.