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Old 03-09-2010, 12:52 PM   #1
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Concrete tank for solar water preheating?


I am looking at various panels for solar water heating and have decided on a closed loop pre-heating system for use in the home that we plan to build eventually. The principal is very simple. The panels collect heat energy from the sun and a pump circulates water through them in order to carry the heat to a buffering tank in the basement. The system is closed loop-the water in the tank which passes through the panels is strictly used as a heat transfer fluid-it is not the DHW for the house. The domestic cold water feed for the hot water passes through a large coil immersed in the tank where it picks up heat (it should get to 140 degrees F) before getting to the hot water heater, the point being so that the hot water heater only has to work during high demand (clothes washing, multiple showers at once, etc...) periods. The tank should be very well insulated so that it can store heat so that two or three cloudy days will not affect the system very much. I've seen designs that utilize tanks made from wood framing, foam insulation, and an EPDM liner like those used in ponds. Since we want to incorporate this into new construction, I thought that designing a tank as a part of a poured concrete foundation would be a good idea. Structurally of course it would be hard to get anything more robust, and it would be easy to add multiple layers of insulation.

Basically the idea would involve adding three walls along one of the outside walls of the home, or a corner could be utilized so that only two additional walls would be needed. I would want an open top tank 4' wide x 4' high x 12' long, so allowing for an 8" concrete outer wall, a 2 x 6 framed interior wall (to create a void for fiberglass batts of insulation), and 3" Polyisocyanurate rigid foam insulation, the outside dimensions of the tank would be about 6' wide x 6' high x 15' long. I would also of course need to construct a lid from plywood, dimensional lumber, fiberglass batts and polyiso foam of the same thickness giving the tank a height of 8' total (the ceilings in the basement will be 9'). The amount of insulation would provide about an R-value of 30 and the tank capacity will be almost 1500 gallons. That should give me at least two to three days of storage depending on usage of course. The basics of constructing such a tank are pretty simple, but I was wondering if anyone else had any ideas or things to watch out for.

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Old 03-09-2010, 03:56 PM   #2
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Concrete tank for solar water preheating?


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but I was wondering if anyone else had any ideas or things to watch out for.
a leak


you might want to have the unsupported walls approved or even designed by an engineer. That is a lot of weight to retain.

I don't know where you are and how it will affect you but even with insulation, you are going to have a huge heat gain in the basement. You need to figure out what to do with the heat lost into the area around the tank.

You also would want to insulate under the tank to prevent heat loss into the ground as well.

Personally, I also believe using any wall common with the basement is not a good idea. The physical attachment will allow the entire basement to be a heat sink and cause heat loss (from the water) and heat gain (in the basement) as the concrete throws off the heat.

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Old 03-09-2010, 04:58 PM   #3
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Concrete tank for solar water preheating?


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you might want to have the unsupported walls approved or even designed by an engineer. That is a lot of weight to retain.
That's definitely on the list

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I don't know where you are and how it will affect you but even with insulation, you are going to have a huge heat gain in the basement. You need to figure out what to do with the heat lost into the area around the tank.
That's the big question I'm still looking for answers on. I figured out that if I used 9" of polyiso foam board (Three 3" layers) I could achieve an R-60 rating. The million dollar question is with 1500 gallons of water @ 140 degrees F inside the tank completely surrounded by a layer of R-60 insulation, how fast will the heat escape to the surrounding concrete walls, floor, and airspace above?
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Old 03-09-2010, 09:05 PM   #4
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Concrete tank for solar water preheating?


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That's the big question I'm still looking for answers on. I figured out that if I used 9" of polyiso foam board (Three 3" layers) I could achieve an R-60 rating. The million dollar question is with 1500 gallons of water @ 140 degrees F inside the tank completely surrounded by a layer of R-60 insulation, how fast will the heat escape to the surrounding concrete walls, floor, and airspace above?
that calculation is beyond my knowledge.
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Old 03-09-2010, 09:15 PM   #5
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Concrete tank for solar water preheating?


I'd consider building it outside against the house
1500g is over 12,500 lbs
Your floor may not hold it
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Old 03-09-2010, 09:20 PM   #6
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Concrete tank for solar water preheating?


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I'd consider building it outside against the house
1500g is over 12,500 lbs
Your floor may not hold it
why would the ground be any stronger outside the house than in the basement?

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