# Concrete Septic tank for thermal storage



## Jersey Bill (May 1, 2008)

As I mentioned on previous posts, I am helping out a friend with his system. The design calls for 1000 gallons of water storage.
I called a company that does precast concrete and asked for a septic tank. The woman on the phone gave me the size and price. She asked what it was for, and I told her thermal storage. She said "oh"

When the owner called to order it, someone at the company took notice because our town has all city sewer. They got into a conversation and a heavy, 6" wall tank with rebar and an interior plastic coating was recomended instead of the thin wall tank usually used.
The OD of the tank is about 7'x7'x7', the cost is about $3,000 delivered. He checked and told me that 200 deg f max temp is OK. The total volume is around 1200 gallons. 

We plan on insulating all sides w/ 4" extruded polystyrene.

What do you guys think? Don't beat me up too bad because its ordered already.


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## free75degrees (May 1, 2008)

Is the polystyrene going to be inside or outside of the cement?  I am asking because polystyrene is only rated to 160*F, which is why I chose to use polyisocyanurate instead (rated to 300*F).  Plus polyiso has a slightly higher R-value and the boards I am going to buy have foil facing for heat reflection.

$3000 sounds like a lot for just the structural part of the tank.  I think the STSS tanks cost about that much (I think that may even be with the HX).  Are you looking into this because you need it to be in the ground?


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## Jersey Bill (May 1, 2008)

The polystyrene will go outside the tank, all sides.
Thats a good point about the temp limit. I could figure out what the tank wall exterior temp will be. If I recall, it will be in proportion to the R values and delta T.
6" concrete=R6, 4" foam=R28, total= R34, interior=200F, exterior ground=50f, delta=150f
conc. delta= 150*6/34=25
foam delta= 150*28/34=123
conc. exterior= 200-25=175

I think that the foam will be OK. I wonder if the polyisocyanurate is cheaper? 
What is the load rating of poly-iso? 
I think that the extruded polystyrene is rated @ 60 PSI

The STSS tank is more than $3k W/O the heat exchangers. (they are $700 ea X 3)
Also, we have to bury the tank for lack of space.


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## free75degrees (May 1, 2008)

Jersey Bill said:
			
		

> The polystyrene will go outside the tank, all sides.
> Thats a good point about the temp limit. I could figure out what the tank wall exterior temp will be. If I recall, it will be in proportion to the R values and delta T.
> 6" concrete=R6, 4" foam=R28, total= R34, interior=200F, exterior ground=50f, delta=150f
> conc. delta= 150*6/34=25
> ...



I forget the exact numbers , but the the polyiso is around 20psi or so, whereas a 5 foot deep tank only produces a few psi at the bottom.

The polyiso I will be getting is $28 per 4' x 8' x 2" sheet, which is R12.  At Home Depot, that is cheaper than polystyrene.


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## Nofossil (May 1, 2008)

If you can get more insulation, it would probably help, especially on the top. I don't think 18" would be too much there. Houses are supposed to have 15" in the attic, and the delta-T there is MUCH less than your storage tank. I'd also spand a little time and effort to make sur that water drains away from it, above and below the ground. Water is not your friend when it comes to insulation of any type.


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## free75degrees (May 1, 2008)

I just checked Dow's specs for the polyiso board that I was talking about using.  The max use temp is actually only 190*F (not 300*F as I previously stated, I think that was another mfg's spec).  The compressive strength is 25psi.


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## Reggie Dunlap (May 1, 2008)

I think the septic tank is a good idea and I almost did it myself. Seems like the thicker tank with rebar is overkill, but I'm no engineer. I would put the insulation on the outside. That way you don't lose any capacity, and the concrete acts as a thermal mass.

My concrete tank is insulated with 4 inches of foil faced Celotex, and it's plenty. I stopped using my Tarm 10  days ago and the tank has only lost a couple of degrees per day. The outside of the insulation is never warm to the touch, even when the tank is at 180 degrees.


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## Willman (May 1, 2008)

> My concrete tank is insulated with 4 inches of foil faced Celotex, and it’s plenty.


Reggie, 
Do you have the foam underneath the  tank, if so is it set on sand or a slab ? I was wondering how it would hold up with all the weight.
Willman


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## Reggie Dunlap (May 2, 2008)

My tank is concrete block built on a slab. The bottom insulation is above the slab and under the EDPM liner. So the tank does not actually sit on the insulation.


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## Vtgent49 (May 2, 2008)

Has anybody used ICF, Insulated concrete forms? They seem like an easy way to buils a tank in an existing cellar.


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## BrownianHeatingTech (May 2, 2008)

Vtgent49 said:
			
		

> Has anybody used ICF, Insulated concrete forms? They seem like an easy way to buils a tank in an existing cellar.



I haven't used them, but I spoke to a distributor/installer of ICF systems a few weeks ago, and he thought it was a very good application, as long as some extra insulation was used (polyisocyanurate) on the inside, to protect the ICF insulation from the high temperatures.

He says they've done it before, for customers who were building new with the intent of having a storage tank for a wood boiler.

Joe


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## Como (May 4, 2008)

My Septic Tank supplier makes concrete tanks specifically designed for storage.

http://www.valleyprecast.com/


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## Jersey Bill (May 11, 2008)

Check out these images.

I think that it was 12,000 lbs of tank

The tank is sloped 1/8"/ft toward the access opening.


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## Chris S (May 11, 2008)

We build with ICFs,  I posted in a previous thread my idease for a buried ICF storage tank.  I have abandonded that idea though as I beleive my site is too wet for it.  There was a post about someone building a concret block tank in his basement. IMO this would be an ideal ICF application.  The advantage being that you can install a rebar ring, or rings as needed, and you can make your tank any size you like ( even round) with insulation built in.  We have even done some small jobs where we mixed on site with sakrete- definite HO possibility!
I'll be going pressurized though so all this is just talk
Chris


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## Willman (May 12, 2008)

> I think that it was 12,000 lbs of tank


Jersey Bill, How thick is the foam under the tank ? What is the compressive strength of the foam? Will you be foaming the inside also ? How about lining the inside walls.Will tank be pressurized or open with HX's. Please keep the pic's coming. Really awesome set up.
Will


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## Jersey Bill (May 12, 2008)

The foam is 4" thick. The comprerssive stregnth was 20 PSI. The weight of the tank puts about 2 PSI on the foam bottom, and the water, when its filled will put another 2 PSI on it.

For under the tank, we used 4" extruded polystyrene for R20. The sides will get 4" polyisocyanurate (at home depot sold as supertuff) for R24. We thought that the polu-iso.. was too fragile for underneath.

We do not plan on lining the inside. 

It will be an open tank with heat-x coils.


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## Willman (May 12, 2008)

Jersey Bill, Thanks for the info. Keep us updated.
Will


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## JrCRXHF (May 12, 2008)

Jersey Bill said:
			
		

> The foam is 4" thick. The comprerssive stregnth was 20 PSI. The weight of the tank puts about 2 PSI on the foam bottom, and the water, when its filled will put another 2 PSI on it.
> 
> For under the tank, we used 4" extruded polystyrene for R20. The sides will get 4" polyisocyanurate (at home depot sold as supertuff) for R24. We thought that the polu-iso.. was too fragile for underneath.
> 
> ...



Could you use a car radiator would it be any cheaper?


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## Jersey Bill (May 13, 2008)

thats something to think about. I assume you are thinking about immersing it in the water.  the only issue that I can think of is the hose fittings. car radiators should be rated to 30 psi. 
It may be cheap, but it cant be ideal. A car radiator is built for air to water with forced circulation on both sides.
I was thinking about a used shell & tube heat exchanger off ebay. I could remove the shell and lower the tube bundle into the tank. But then the tube bundle was designed for forced circulation as well. I don't think that the tank will stratify as nice as the (proven) copper coil method.


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## JrCRXHF (May 13, 2008)

I was thinking about setting up a water path in my tank when i do it so that the water wants to go through the radiator. You can get them really cheap from the junk yard or even brand new. I think even with the coil what you are after is surface area for heat transfer. Water is going to move anyway when it goes from hot to cold so i really don't see why it would not work.

In cars we make water to air intercoolers all the time from air to air intercoolers It works well.

Any way just an idea i was going to try.


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## pybyr (Jun 8, 2008)

Jersey Bill- you mention building an ICF tank and then adding the polyiso insulation, but not lining it.  Sounds interesting (as in something I might want to consider), but I am missing the boat on what you're planning to do, in that I am not understanding how the ICFs alone will be sufficient to hold the water in?
Thanks
Trevor, in VT


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## Chris S (Jun 8, 2008)

An ICF tank with a water seal to the footing would be just like a septic tank- in that it would hold water,  but that seal is critical.
I built a catch basin that way-  poured  the base ( bottom)  with some rebar & wire, then set up the ICFs & poured them (slowly) so it was in effet a one piece pour.
Very easily doable with sakrete ( a lot of it)  or the concrete to go buggy carts.

I thought at it, but decided to go pressurized instead.

Chris


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