plumbing addiction: slab water tempering techniques
Monday, December 5 2005
The focus of my plumbing addiction remains the problem of how best to deal with the circulation of boiler-heated water in the basement slab. For those just joining me in the discussion of this fascinating topic, the existing oil-based slab heating system uses generic hydronic water, roughly 200 degrees Fahrenheit, to heat the slab underlying the house. This slab is the only heating zone in the basement, and heating it is such a massive undertaking that Gretchen and I usually abandon the basement during the winter months, even though Gretchen's library (her analog to my laboratory) is down there. On sunny days I've been able to circulate solar-heated water through the slab, but the effects have been modest, usually only raising the temperature of the slab by a couple degrees over the course of an entire day. Several weeks ago the slab was at about 60 degrees Fahrenheit, but there have been so few suitably sunny days that by yesterday morning it had fallen to fifty five degrees. Yesterday I was able to circulate enough hot water to raise it back up to 57 degrees, a temperature the slab maintained throughout today despite a complete absence of sun. This confirms its resilience as a thermal mass, and it retroactively blesses my idea to provide a plumbing loop allowing the slab to be heated by the sun. Still, in an emergency (such as the imminent arrival of a guest - our two guest rooms are both in the basement), there would be a need to bypass the sluggish pace of solar heating and immediately jack the slab's temperature up to something on the order of 75 degrees. In such a case we'd be forced to rely on fossil fuels, the kind that now cost several dollars per gallon. At that point the efficiency of the present system breaks down. Everything I've read about hydronic slab systems tells me that the circulated water shouldn't be more than 100 degrees Fahrenheit. To circulate 200 degree water doesn't just use excessive boiler resources; it also applies unnecessary stresses to the slab plumbing and the slab itself. Mind you, this system as described has been in operation since 1994 and had been used continuously through every winter until the Spring of 2002. But I view it as broken and I have the nerve to try to fix it.
The conventional solution to the slab's current design problems would be to install a pair of thermostatic three-way mixers, allowing water from the slab to mix with water going to the slab, tempering it down to a reasonable temperature. But three-way thermostatic mixers are expensive, at least at the one inch size I need. (Half inch models, on the other hand, are available for wanton shoplifters at your local Home Depot but not Lowes.) This gave me the idea of maybe achieving the same effect with electrically-controlled valves hooked up to a hydrostat. I'd only need one such valve (in addition to the one already present) to simulate the behavior of a single thermostatic mixing valve. Then I could probably get by with a little half-inch mixing valve for the modest requirements at the cold end of the slab loop (since most of the water going through the slab at any one time would simply be recirculating independent of the boiler, the amount going back into the boiler could probably be handled by a "free" half-inch mixer valve).
But then I thought about how those two electric valves (one for hot the other for cold) would constantly be switching on and off to keep the water at a certain temperature, and it seemed like the mechanisms would wear out in the course of a single day. Obviously I'm going to need to hatch a better idea.
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