STSS Rebuild - any suggestions?

[Highroad281]

I'm just about ready to start rebuilding an STSS tank that came with the Tarm that I recently bought. I'm going with one of Tom's AST liners (just came off the truck) so that I can run it hotter than EPDM allows, but before I get that far I'm wondering if anyone has had any luck increasing the R value of these tanks by using alternative insulation, or if there are any alternatives that might be easier to use or longer lived than the original 1" polyiso foam.

Here's my reasoning. The tank had five heating seasons on it, and when it came apart for travel there were some issues. The aluminum top hangers were obviously reacting with the nails holding them in place, and most had dissolved. Also, the top foam and even the sides were partly saturated (certainly not good for R value lol). Not sure if the tank was run over temp, not sealed completely when assembled, or it's just common but even the back foil facing had reacted with the tank skin and was largely gone, thus allowing condensation full access to the pores of the foam. It may not matter as much in my case bc I plan on using the setup year round, and will always be hot (not condensing as much?). Not sure, just a thought.

Anyway, I'm wondering if anyone has experimented with a layer of something like Roxul panel or similar between the tank skin and the Polyiso? I know it's porus, (and $$$) but would likely dry better with heat if it got dampmand says it has good compression resistance. And I know XPS is no good next to the heat, but how about if it was between the polyiso and the tank skin to prevent that from reacting again? It comes in 1/4 inch accordion sheets at the box stores that look like they would go in nice and quick. Finally, I guess the most likely option is to just use polyiso like OEM and add extra insulation on the outside of the tank to bump up the R over 12, but that doesn't address the vapor absorption issues and maybe compounds things. Lots of foil tape on the exposed edges wouldn't hurt, but I'd love to find something better.

If you've had similar experience, or have any suggestions for products that aren't on my radar I'd love to hear about it.

Thanks!

 

[tom in maine]

Roxul is not going to transfer the structural load to the outer shell of the tank very well.
You can use Styrofoam but you will have to cut it in pieces and try to get it to stay in place while assembling the rest of the tank.
I would more polyiso foam. I suspect the corrosion was from a moisture leak.
If you want to protect against the corrosion, just use a sheet of 6-10 mil poly plastic between the insulation and the aluminum skin.
You are correct in that it is a good idea to bump up the insulation value.
Sven seemed to think 2" was enough. IMHO, it is not.

 

[pbvermont]

I think you are stuck with using polyiso. Not sure why alum. foil skin on original is reacting with aluminum tank skin. Even the nails supplied should have been alum.-- that hold the top hangers in place. You are doing well by getting one of Tom's AST liners. I suspect that your problem is overheating, and excess vapor escape. Get some new 1" polyiso board (I used double layer of 1"--for 2" total, when I built my "STSS-style" tank. Make sure you don't OVERFILL with too high of a water level. It should not be ABOVE any tank sidewall penetrations. Make sure any of those penetrations are well sealed with high-temp silicone caulk. Tom's tanks have no penetrations in the liner, I believe. And you need to seal the membrane lid liner to the lower tank liner top--well, to prevent vapor escape. I'm not sure putting more insul. outside the tank skin is the best solution. Doesn't really solve PROTECTING the somewhat "tender" polyiso board. Good luck.

 

[Highroad281]

Thanks guys, I appreciate your input.

After studying the install sheet that came with the liner I think there are definitely some benefit in trying to reduce the number of side/liner penetrations (currently 4 for HX and 4 for the top support braces). I'm thinking there's a simple way to make lid supports without making holes in the liner. I do like the idea of the HX tubes exiting over the top of the sidewall though and not cutting 4 holes for that. I guess if I doubled or tripled up the 2" foam top and ran the upper pieces out past the outside edge it might allow me to cut some channels in the foam for the tubes. That would remove 4 of the 8 penetrations... I wish I could think up a way to get rid of those cross support braces too, but I don't think that's possible without some sort of external support structure. But, that might give me a nice box to insulate, right?

To be clear, it's not my intention to bash the system. I know there are thousands of these things out there and folks are happy with them. I just happen to be in the situation of starting over with a tank skin and some heat exchangers that I bought, and if there are opportunities to make small changes for increased performance and reliability now is my chance.

Thanks again

 

[Bad Wolf]

I made my own STSS knockoff out of swimming pool sides and EDPM. Going on my sixth season and no degradation that I can see. 8' dia. and I set it on 4" of pink foam from HD and wrapped it in yellow 2" insulation that I got at a surplus sale. 48" high and I was able to go around the tank five times so I have 10" on the sides. I finished it off with 10 mil plastic. The cover is 2" foam board again and 4" of yellow insulation. I lose very little heat through all of this.

 

[Highroad281]

Interesting, just re-read the tank manual and the only stated purpose of those cross braces is to support the cover, not for tank structure. I was thinking they were structural as well, and was hesitant to not use them. Makes perfect sense I guess, 3 tons of water pushing out in all directions is going to make that tank much more stable than any copper pipe would ever do. So, unless I'm missing something it looks like I *should* be able to get by without making any holes in the liners, which seems like a pretty good thing!

Greg, glad to hear you're having good luck with your tank. I like the simplicity of the design overall and may be over thinking much of this - but that's just how I'm wired. I think I'm definitely going to wrap the outside of the tank after it is operational like you mentioned. Would love to get to R40 or so, but I don't want to add too many layers on the inside, need to leave room for some water after all.