How much insulation on my storage tank

[emesine]

I have a cement cistern I am using for a storage tank 9' by 9' by 9'. The tank is 10" concrete wall. I am planning on laying down 3 inches of sytrofoam insulation (total R15) on the inside of the tank, then lining the tank with a plastic liner to hold 3500 gallons of water. Two questions:

1. Is this enough insulation?

2. Do I need to use a liner? The cement tank will hold water- Can I just seal the foam insulation well and pour in water?

Tanks! I mean Thanks!

Andrew

 

[WoodNotOil]

Couple things come to mind:

1. Use polyiso foam because it gets 6.2 per inch R value and use a minimum of 3", but more is better.
2. Yes you will have to line the tank because even if the cistern still holds water, you don't want the insulation submerged or it can't do its job and will break down. EPDM liners are commenly used. Firestone makes them and calls it a pond liner.
3. A 9' column of water will exert enormous pressure at the bottom of your walls and the tank might not stand up to it. I am no engineer, but this seems too tall for concrete wall to withstand. (The engineers here can correct on that if I am wrong)
4. If you are planning on using copper coils as a heat exchanger, 3500 gallons is going to require a LOT of it. It is common for a 1000 gallon tank to have around 300' of 3/4" copper coil to exchange heat. That would mean your would need like over 1000' of coil to transfer the heat in and out.

Therefore, I would suggest not coming up the full height of the cistern and reducing the overall amount of gallons. Pressurized storage is a little easier to size this big because there is no need for a heat exchanger.

 

[emesine]

The tank will be filled to about 6.5 to 7 feet. I am using a large plate HX to heat it, which, if my calculations are correct, should do the job (I oversized it X3)

My foam insulation (1.5 inch sheets) is waterproof, so my installer suggested we might just let it sit in the tank without a liner. Sounds like trouble to me, but I want to ask if anyone here has ever done that.

I am also worried about my R value, which is a little low at 15 total (for a 3 inch layer).

Andrew

 

[tom in maine]

You cannot use Styrofoam by itself, it will melt when the temps go over 140F.

Use a liner. The foam will waterlog and will also want to float!

A layer of polyisocyanurate foam (1" minimum, more is better) on top of the Styrofoam directly underneath the liner will protect the strorofoam.

 

[leaddog]

The tank will be filled to about 6.5 to 7 feet. I am using a large plate HX to heat it, which, if my calculations are correct, should do the job (I oversized it X3)

My foam insulation (1.5 inch sheets) is waterproof, so my installer suggested we might just let it sit in the tank without a liner. Sounds like trouble to me, but I want to ask if anyone here has ever done that.

I am also worried about my R value, which is a little low at 15 total (for a 3 inch layer).

Andrew

waterproof and water resistant are two different things. remember that you are going to have 190* water and even spray foam might absorb a slight amount of water submerged in hot water for a long time. ANY water absorbed will take away R value. I would use a liner. I would also use as much insulation as You can afford as btu's lost are gone. You said that this is a cistern, does that mean inground or in the building as that might let you use less insulation if you are using the heat loss as part of the building heat. That would also make a difference on the side load from the wt. of water. I had a tank that I built and had to keep adding bracing as I didn't relize just how much the tank wanted to go round and not stay sq. concrete doesn't take side loads well. When I built my house I didn't get the basement covered and we got a HEAVY rain. water came up about 4ft on the outside where it wasn't back filled. 8in poured reinforced concrete caved in. Had to do one whole side. Contractor covered it but wasn't pretty.
leaddog

 

[leaddog]

I had given some thought of making a concrete tank and painting it with a water proof coating or using ceramic like in a swimming pool but after thinking about the temp differentials that the tank would go through I would be afraid that because of thermo stress that it might crack and a crack lets water go where you don't want it. It's not like just holding water it's about holding water that goes from 55* to 200* to 150* all the time. A liner is alot safer.
leaddog

 

[emesine]

The cistern is built into my basement walls, so theoretically I will get some of the lost heat back..... However, the house is insulated INSIDE the walls, so I expect most of the lost heat will be sucked into the 50+ thousand pounds of concrete that make up my basement walls and very efficiently radiated into the earth. I might get some heat back, but I'm expecting that most of the BTUs I loose through the foam I'm not going to get back.

I will check on the "styrofoam" insulation I have in the cistern now. I don't actually know what it is made out of.

The insulation will be on the inside of the concrete walls, so the walls themselves will not see huge swings in temperature. I also considered having hot water in contact with the concrete, but I decided against it partially on concerns it would stress the concrete.

I will put in a liner.

Thanks again for all your input.

Andrew