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Old 04-14-13, 09:41 PM   #31
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Originally Posted by opiesche View Post
FWIW, I'm running my radiant floors at a constant supply temp of ~115F which makes the floors feel quite comfortable. The controller only turns on and off the pump.

Did you arrive at 115F by trial & error?

Have you logged the duration of your pump "ON" times during the day? In other words, to roughly calculate the duty cycles and how they expand and contract?

Have you compared that info above with the outside temperatures as they rise & fall?

I'll probably use the very same kind of setup as you are using, with the exception that I'll be using heat pump(s).

I had figured that I'd use a water storage tank (electric water heater) with a sensor on it and drive the heat pump(s) until the set temperature was reached. Then, if the heat pump(s) couldn't pull the grade, I'd rely on electric resistance for backup.

It all seems the simplest way for DIY.

BTW, what is the capacity of your water heater? Do you think you have it correctly sized?

I seem to recall that you were trying to create a thermal 'model', mathematically, of your house & heating system. Am I getting that right? If so, what did you come up with?

Very interesting project you are doing, and thanks for your documentation & photographs...



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Last edited by AC_Hacker; 04-14-13 at 09:43 PM..
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Old 04-14-13, 11:01 PM   #32
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Yeah, getting to that temperature was pretty much a matter of trying what felt comfortable and gave the desired heat output. Once crawlspace insulation is finished, I'll probably end up lowering the temperature a little more.

The exterior temperatures don't play too big of a role. The colder it gets outside, the more the pump is on I'm logging pump on and off times - here's a plot from March 19th:

X axis is time (24 hours roughly from midnight to midnight), y axis is HH:MM of pump on time. So, without insulation in the crawlspace that's 13 hours total the pump was on, which is still much more than I'd like. Insulation should make a significant difference here - we can definitely feel the difference between the rooms that have the floors insulated and ones that don't!

The coldest during that time of the year are the hours between 4am and 10am (between ~26 and ~35 degrees), which nicely shows in the chart (the pump was on for 7 hours after 5:28am, yikes!).
The exterior temperature usually drops significantly between 10pm and midnight, after which the pump was turned on for about 2 hours. That fed enough heat into the house to then only have it on for a few minutes at a time every 15-20 minutes or so.
We had an average of 54 degrees in March this year, with daytime highs in the high 50s to low 60s.

I haven't gotten around to hooking up the exterior sensor yet (I've been incredibly busy at work), but hope to do that soon to be able to correlate the data with that. I anticipate the exterior temperature drops to nicely line up with the pump on times we see here
The water heater is 35gal - it seems big enough, but I haven't logged duty cycles (thinking about a photo diode at the bottom where it can 'see' the burner, hooked up to my controller). Empirically, it should be less than 30% seeing how the heater is firing much less than a quarter of the time I go out to the garage - but then I don't go to the garage often in the middle of the night A bigger water heater would potentially reduce this a little. In terms of practical savings, switching to radiant managed to reduce gas usage enough to get the PG&E winter energy savings credit this year

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