my storage tank plan

[free75degrees]

I am in the midst of building my unpressurized storage for a Tarm 40. Any feedback on my plan (especially any potential problems) would be greatly appreciated..

The tank will be about 4 feet by 6 feet by 5 feet tall which will be about 900 gallons. It will be half buried in my basement slab in a hole that has a concrete slab floor and cinder block walls. For the top half I will use pressure treated 2x4s every 8 inches and they will be inserted down into the cinder block holes for support. Then there will be 3/4 inch plywood inside of the 2x4s (maybe 2 sheets or whatever it takes to get flush with the inside of the cinder blocks. Then the entire thing gets lined with 2 layers of 2 inch think polyicocyanurate insulation board to give an R value of 26. Finally, an EPDM liner goes inside of the insulation. On the top half I will also add extra insulation between the studs to get up to about R-40. For the lid I am thinking about using plywood with a couple layers of the polyiso board and a sheet of acrylic on the bottom that will make a seal with the rest of the tank (to keep moisture in). For the heat exchanger I am making 4 coils out of 1/2 inch hard copper tube. Each will be about 60 feet to give a total of about 240 feet and they will be plumbed in parallel.

Ok, so does anybody think there will be any prolems with this plan?

 

[solo40]

You storage idea sounds alot like the storage tanks I would like to add.
I only have 4 feet in my crawl space so I was thinking of digging down
a couple feed and doing concrete blocks on the bottom and wood on the
top.

Ethan

 

[free75degrees]

There are definitely some nice aspects to half below grade. One of the difficulties of building a tank is making it strong enough to withstand the high pressure at the bottom. Having the bottom of the tank in a hole basically makes this a non-issue.

 

[sparke]

Are the existing cinder blocks at floor grade?

 

[free75degrees]

I am in the process of putting in the cinder blocks. I have two choices: either the top row will be about 4 inches below floor grade and I will pour a concrete concrete cap on the top, or I'll put on an extra row and the top row will be about 4 inches above grade. I haven't decided yet.

 

[sparke]

The reason I asked was because I am trying to visualize how structurally sound the 2x4 walls will be if they are simply inserted in cinder blocks. If you are building from scratch you may consider making the tank an extra 8.5" bigger all the way around and build your walls inside the cinder block. Utilizing the 2' cinder-block wall to brace the bottom of the tank. With only 2 or 3' of wall above grade it wont take much to brace it. You will not regret taking the extra time/cost for the end result. Also, it would allow you a bit more insulation below grade (r 12 or 13 in lower half of 2x4 wall). The more insulation barrier you have from the earth the better. Currently I am building 1 fire every 3-4 days with approx 700 gallons of storage with a family of 5. I have said goodbye to the oil truck. That feeling is priceless.

 

[free75degrees]

I think I am accomplishing what you are saying by making the 2x4s extend 2 feet down through the inside of the cinder blocks (not the interior side of the blocks like you suggested, but actually inside of them). By going two feet down there should be plenty of bracing, similar to if they were inside the tank. I am using 6x8x16 cinder blocks and a 2x4 just barely fits down into the holes in the blocks, and may even require ripping about 1/32" from the 2x4s. So they will be very snug. I have already layed the first row of blocks and even if I hadn't, the space in my basement doesn't allow for a bigger hole.

 

[sparke]

As long as your confident with the structural integrity, the rest of your plan sounds solid. The only other suggestion I have is to make the cover strong enough to handle the weight of a full grown man. Like Nofossil said in another thread if someone were to fall through the cover with 180* - 200* water. They would either die or be severely injured. Maybe someone else can do the math and figure out if your Hx is sized properly. What is your BTU output of the Tarm?
Also, have you thought of using a Plate Hx instead of coils? Jbatty has a bunch of threads on his set up... Requires an extra pump and I dont know if you can reverse the flow. Jbatty says his works great. Oh one more thing Don't forget to integrate temp sensors for bottom and top of tank. Preferably bottom, middle, and top.

 

[headrc]

Out of curiosity ...what do you estimate the cost to be for building this type of storaage tank? Thx RH

 

[free75degrees]

Out of curiosity ...what do you estimate the cost to be for building this type of storage tank? Thx RH

Stuff I have bought already:
~30 80lb bags of concrete - $100
72 cinder blocks - $80
4 bags mortar - $20
16 10' PT 2x4s - $80
Copper tube for coil HX - $350
15'x20' EPDM liner - $120

Stuff I haven't bought yet so costs are approximate:
15 2" thick 4'x8' sheets of polyisocyanurate insulation - (I forget the price, but I think it is $15 each) - $225
remaining PT lumber for top and extra bracing - $40
4 sheets PT Plywood (I think these are $30 each) - $120
Misc copper fittings, wood fasteners, etc. - $100
Inside of lid seal (maybe PVC shower liner or acrylic sheet?) - $65

Grand Total = $1300

 

[free75degrees]

The only other suggestion I have is to make the cover strong enough to handle the weight of a full grown man.

Yes, I definitely will do this, especially since it will only be 3 feet above floor level. I also plan to put a lock on the lid so kids won't try to open it.

Maybe someone else can do the math and figure out if your Hx is sized properly. What is your BTU output of the Tarm?

This came up in another thread and somebody said I would need 210 feet. The Tarm 40 has 140k btu max output.

Also, have you thought of using a Plate Hx instead of coils?

I did think about it but decided to go with coils. I realize they are a bit more expensive, I like that but they don't require an extra pump and I also was influenced by the fact that STSS uses coils for their tanks.

Don't forget to integrate temp sensors for bottom and top of tank. Preferably bottom, middle, and top.

Definitely. I just ordered 25 heat sensors and I plan to scatter them around liberally.

 

[sparke]

Sounds like you have it well under control. Please post some pics if you have the time.