diy solar

diy solar

DIY Large insulated water drain back tank?


New Member
Mar 4, 2022
I have seen a lot of builds for DIY storage tanks as well as indoor koi ponds that seem to use wood, typically 2x6 on edge 6" OC bolted together and tied together with plywood on either side, and I was thinking of using scrap C Channel I have left over from building.

The C Channel is 16ga, 2x4x4 and I have it in lengths at least 11ft long, at most 12ft long. With a ~7.5x7.5x4ft tank I think I can get about 900 gallons insulated with 4" polyiso in between the channels and another 3/4" polyiso inside the tank to protect the liner from the screws. I am thinking 16" OC horizontally and screwing the sides where the channel overlaps with a couple tek screws (top/bottom) and maybe even putting a piece of sheet metal inside the C Channel to better protect the foam and provide more rigidity (could be overkill) ... I could add more sheet metal on corners to help tie the channels together as well.

I got a quote on a 1000gal insulated tank from a good manufacture of soft tanks, with two coils it was like 8k 0__0 I already own a bunch of 4" polyiso and this left over C Channel so figured I would DIY it.

I tried to do load calculations for this but I am not a structural engineer :/

Anyone ever do something like this with steel?

While steel isn't a good conductor, it is better than wood. Depending on how you design and insulate, it might be great, but the steel could also conduct the heat out of the water.
What gauge studs?

My experience with steel stud construction is limited but even with the common two gauges of steel studs, the heavier gauge has limited strength in a structural arrangement where- if vertical- lateral loads will be applied; and it has limits when oriented in a span like a ceiling application.

And I’m not an engineer.

But 1000 gallons is ~8000lbs and is like 4’x4’x8’
The bottom of the ‘tank’ would be at ~20psi so one could think that the bottom vertical edges at 1”depth would see a bending moment force of ~1900lbs cumulative - but not really because while liquid applies pressure in every direction equally only being constrained by adjacent water or walls of the vessel, engineers know how to properly calculate the cumulative pressure at various depths from the top where the pressure is small to the bottom where pressure is at its highest.

So not actually knowing what I’m talking about I’m thinking that the wood will support the psi if supported such that it cannot deflect horizontally- you’ll have to tie the sides together I think, or perhaps use two steel studs thoroughly connected back to back to prevent deflection.

Hopefully someone with real knowledge will be stopping by here to weigh in.
id try it... as long as it’s not in your dwelling..
Oh it will be lol ... our original "plan" was a 2 story 10k tank you would use to store rain water but built inside, it has since sized down to this.

Tons of people build tanks of this size for indoor koi ponds, we do have in floor drains near this area.

While steel isn't a good conductor, it is better than wood. Depending on how you design and insulate, it might be great, but the steel could also conduct the heat out of the water.
The 3/4 internal poly iso, R5? , was going to be used to avoid the fin affect for sure. I am not sure it will loose too much though, and if it will then I can bump this up to 2" polyiso which is R12+ and take the slight loss in capacity. With this much water I think I can store a good 450kBtu which could get us through a couple rainy/cloudy days of heating in winter. We have a backup wood boiler which will be used to heat this tank as well if required.

General design, not exactly "original" as I know a guy who use to build tanks similar to this but IDK the detail of the steel he used. I was originally going to buy one of his tanks but his business burned down :*(. The 16ga we have here is crazy thick so it is likely "ok" to use it. A 12' stick is like 20lbs if I recall correctly. I also have a lot of extra 20ga stud and track material left over from our build.
What gauge studs?

***This is not an original concept*** 12 panels build a tank, each panel attached top/bottom to the adjacent panel. A stack of 4 panels go on top the previous, sheet metal corners with 2 screws per C Channel intersection min, likely might do 4.

If I don't go this route I might build it in layers, so attach bottom C Channel together, stack on the next layer of C Channel and attach, then stack on the next layer of C Channel and attach both the layers together to form I Beam style construction, rinse and repeat to the top.

Dumb question: have you considered a different design?

A circular tank with proper reinforcing rod inside the pour adds thermal mass as well as ensuring a strong construction assembly. I would be unlikely to build a wood tank but would rather a concrete tank or a circular steel containment with a liner or ‘glass it.
Dumb question: have you considered a different design?
Never dumb questions, just dumb answers. Lots.

The original was going to be a 10k, two story, corrugated steel tank with 1ft of internal insulation which would have bumped the total gallons to 5k

I have looked at the all wood tanks and while I think they would "work" I don't already have the material.

I had extra ICF blocks but we poured some with concrete to use as a retaining wall later ... so don't have enough for that. We already have 155cuyd of concrete in our building though ... and it is all insulated from the ground up with 55cuyd of it in the radiant slab.

We also looked at commissioning a guy to build a legit boiler tank, but it was 6k. Would have been round, smaller footprint, and 5-6ft tall if I recall. We would have to add bat insulation to the exterior to insulate the tank from the environment.

Also looked into thermo plastic prefab tanks but the cost is pretty high for that.

Also considered used 1200 gallon propane tanks BUT those are super long, and we really don't have any dairies around here where I can find a used SS 1k round tank to use.

I looked into the round soft tanks, but was quoted 8k for a 1k size one 0__0

I found the square design above years ago but the guy is out of business who sold them so I figured I would make them since I accidentally had leftover material from our steel building we erected this year because the manufacture gave us product which didn't meet the spec and later had to send us properly bent material in its place. So I own a good 38 of those C Channel above at 11 / 12ft and I also have a ton of polyiso sitting around as our building is externally insulated with a good 4-6" of foam.

As far as pouring a concrete tank inside the house that is likely a no go. Our floor is a burnished concrete floor and it is our finished floor ... we don't want to damage it by pouring again and getting the concrete on the slab ... having poured concrete for our building 5 times now I know it will happen :/

I am always open to other ideas though. I have likely looked into many other options than the ones listed above but I can't recall them.
What gauge studs?
forgot to address this as well ... these are not "studs" these are the 11ft C Channel we are using at the top and bottom of our windows which span 88ft in total (24 windows in each floor which are ~79" high each and weigh a good 155 each). These are attached on to our steel columns only on the ends with two screws BUT the load is sitting on the middle 4" segment. They are dimensionally 2"x4"x4" and are beefy .... carrying three at a time is my max and no one else on our build site can do it.