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Old 08-11-12, 12:21 PM   #7
opiesche
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Join Date: Jul 2012
Location: Rohnert Park, CA
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All good questions! I'll need to collate some of the info and add it to my blog

Our house was built in 1984 on a crawlspace with posts in concrete. I'ts a two story with four betrooms and two baths upstairs. Underneath is packed dirt. When I said the house is pretty well insulated, I meant 6" exterior walls with fiberglass roll insulation. I didn't do it myself, but have opened some of the walls (to run new power outlets), and that's what I've found in two of the exterior walls. The windows are relatively new (late 90s or so), and are double pane vinyl. The attic is insulated with about 6" of loose fill fiberglass (something I plan to augment with additional batts in the future).

We didn't do a full heating load analysis - our estimate came from the fact that my parents' house sees temperatures at least 35 degrees below what we experience in winter. They've got double the density of heating loops, and similar insulation, and about the same water temperature (they have in-slab loops over a basement though). So we figured that half the density would work fine for us with the relatively mild temperatures we're seeing.
Your information about average temperatures seems roughly correct. There's a couple of weeks or so when the temps occasionally drop below 30 for a few hours (between 3 and 5am), but that's the worst we see.

We've never done a blower test, but the house seems reasonably infiltration proof from the fact that it's the front door doesn't easily slam if all windows are closed.

We haven't yet done any underfloor insulation, which is still on our list of things to do (I'll probably hire a few students from the nearby colleges to help because I don't fancy the idea of scooting around on my back in a 15" crawl space for several hours :P). It'll likely be fiberglass batts with tension wire to hold them in. The foam underlayment of course insulates a little, so I plan on insulating very well in the crawlspace to offset that.

As for the water (this is still upcoming, as I haven't actually hooked it up yet), I'm going back and forth between a dedicated 30 or 40gal natural gas water heater (the total water in the system is about 25 gallons for the 1/2" PEX) and an air source heat pump. The water heater is going to be much cheaper and should be pretty efficient, although not as efficient as a heat pump of course. I think it'll strike a pretty good balance between upfront and runtime costs.

Since I've got only 9 or 10mm of the bamboo flooring and the thin underlayment on between feet and heating, I figure 80 to 85 degree water temperature should be sufficient, but that and the flow rate is something I can play with and fine tune to the needs of the rooms. Regulating the temperature is likely going to be done just with the water heater itself - I'll set it so that the temp ends up about 85 degrees (a thermometer in the feed line to the manifold should do the trick) and the water will be pumped from there directly to the loops. That way I don't have to mess with a second cold water line and a mixing valve.

Edit: I haven't thought too much about the thermostat yet. I figure I'll regulate the system by outside temperature - if it drops below a certain level, it turns on the pump. It'll take some experimenting to find the right setting, but should work. Since I don't have the tubing in concrete or other large thermal mass, there should be relatively little delay between turning on the pump and the floors warming up.


Do you have any input on the above setup? I'd appreciate additional ideas!

Last edited by opiesche; 08-11-12 at 12:40 PM.. Reason: Added some info
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