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Old 09-02-09, 02:51 AM   #1
Christ
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Default Solar hot water to heat a house?

Let's say the house is supposed to be 50x80, and will be at least partially buried in a hill, built from cinder block, directly on top of a concrete slab with water pipes running through it.

How do I figure out how many gallons of bulk water/antifreeze mix do I need to store to keep the system operating and able to heat the home through the day/night?

Assuming for a minute that I need about 600 gallons of antifreeze mix in bulk tanks, plus the line capacity and the collector capacity, will the system work strictly using thermal siphoning?

I've seen solar water setups that reach 150-180 degF, so I'm relatively sure I should be able to do this, but no matter what I search, I haven't been able to find a good example of anyone else having done it, so I have nothing to go on when mentally mulling over the majority of options and variables.

The setup I dream of is a system with a bypass loop and each room of the house on it's own thermal circuit, which is temperature controlled by a series of valves that redirect antifreeze mix into the bypass loop rather than under that particular part of the house. In this way, each room/area can have a thermostat that controls the temperature via a valve which controls flow to that room's heating pipes.

Is there such a thing as "too much" with a system like this? I mean, even if I create way too much heat to actually use in the home, I can always just bypass the home w/ the antifreeze, allowing the water to then cool into atmosphere with a radiator.

Side question: Could that thermal siphoning effect be strong enough to generate a small amount of electricity if a generator were installed in the cold side? The actual heater will be about 200 feet away, and about 30 feet down hill from the house, if that matters. I don't think a sealed system will develop head, will it?

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Old 09-02-09, 08:29 AM   #2
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Excellent question and the answer is it can absolutely be done. In fact, that is my same dream (but different house obviously).

First, you'll need to figure out about how much heat you'll need. Or you can calculate the heat loss of the house. Previous utility bills would be a great indicator of how many BTUs you'd need to have to heat in winter. From there, you can look at a solar map like this one to see how much panel area you'd need to collect that much heat. Also, you'd probably want to oversize the system so you can sustain heat over a few cloudy days.

I highly recommend looking over BuildItSolar.com. It has tons of DIY info on exactly this subject. The owner of the site has the same setup as you describe and gives great detail on how he made his system.
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Old 09-02-09, 03:20 PM   #3
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Quote:
Originally Posted by Daox View Post
Excellent question and the answer is it can absolutely be done. In fact, that is my same dream (but different house obviously).

First, you'll need to figure out about how much heat you'll need. Or you can calculate the heat loss of the house. Previous utility bills would be a great indicator of how many BTUs you'd need to have to heat in winter. From there, you can look at a solar map like this one to see how much panel area you'd need to collect that much heat. Also, you'd probably want to oversize the system so you can sustain heat over a few cloudy days.

I highly recommend looking over BuildItSolar.com. It has tons of DIY info on exactly this subject. The owner of the site has the same setup as you describe and gives great detail on how he made his system.

Thanks for the link, I'll check it out later tonite, when I have more me-time.

As far as previous utility bills, there aren't any, since the house is new construction (I don't even have the permit yet.. LOL)

Once the floor plan is laid out and finalized, I'll be digging the giant hole in the hill that I'll need, and laying the foundation slab, upon which will be laid the floor slab, with some form of thermal/moisture barrier between them. The heating pipes will be run in the floor slab, based on the floor plan, with all the valves being external, if the individual climate control is indeed the way I go with it.

I'm not really sure how to calculate heat loss values for homes... do you have a program or a link that will give me some idea of how to do so with cinderblock construction and a cinderblock/living roof, with at least 50% of the surface area of the walls buried in the hill?

Ideally, I'd like to remain off-grid with this house, even though it will be connected to the grid for "emergency" sake.

The only utilities in the area are electricity and phone... water is well-drawn, and septic systems are in use, although I'm looking into ways to not have a septic tank buried in the ground.
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Old 09-03-09, 01:04 AM   #4
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Looks like I'm in a 3-4 KwH/m2/Day area...

I found a few resources which say that a good average or starting point for new construction is to use the CF of the home and multiply by 4.5.

50x80x10=40000CF

40000x4.5 = 180kBTU.

This seems very high... I won't be losing as much heat as the normal house, due to the berm construction and living roof, as well as packed walls and cinder block/stone construction... I'll still use it for the estimate, though, since too much heat isn't an issue.

1 kWh/m2/day = 317.1 Btu/ft2/day, so 3kWh/m2/day = 951.3 BTU/ft2/day.

180k/951 = 189.275ft2 to produce enough heat to keep me going all day.

So this just raises more questions... how much thermal store do I need to keep going once the sun goes down?

Is this 200 ft2 of surface area, or area exposed directly to the sun? Is it area of heated piping?

I mean, if I put 200 feet of 1 ft2 piping coiled in a tank that only has 18 ft2 (3ftx6ft) of exposed surface for the sun to enter, but has 54 ft3 of interior space, will that work out correctly?

At least I have one part of this system figured out... I don't have to worry about it cooling the house at night, because it will technically keep heating as long as the temp of the house is lower than the temp of the liquid, and once the house gets hotter than the liquid, the siphon effect stops, because the heated area is lower than the house. Once the outside temp gets higher than the house again, the siphon effect picks up, starting to heat the house (or bypass loop) again.

Last edited by Christ; 09-03-09 at 01:07 AM..
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Old 09-04-09, 01:02 PM   #5
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Builditsolar actually has a nice heat loss calculator.

Home Heat Loss Calculator

Once you figure out your heat loss you can figure out how much storage you'll need/want.

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