Unpressurized propane tank possible?

@Eureka, that is basically what I am going to do only my tank will be a 1000 gal NH3 tank, my boiler is mild and has never had any treatment, has been in service for 7 or 8 years and when I was in adding the firetube HX I could see into the water jacket and all it had was a very light greyish film on it, those tanks will last a long time if you get the air out and keep it out

You could just use your OWB to get the storage via a plate heat exchanger that way you could pressurize the tank and the rest of the system while leaving the boiler open to atmosphere.

But to answer your question yes you could leave it open.

I'm not sure how bad it would actually be - maybe not bad at all. But if I did that I would be picturing the LP tank in a constant state of slow rust inside - and constantly sending rust residue type dirt through the rest of whatever else it was hooked to. That's a lot of surface area for rust to work on. So I wouldn't be worried about the tank failing but more so exhangers & circulators etc. all dirtying up.

Not sure what all you have in place now & how it's all arranged, but pressurizing the tank would only mean adding one more HX between it & the boiler. Well, you'd also then need expansion tank considerations too - that would need worked in also.

If all your system is hooked on to the boiler now and your thermal storage can be placed anywhere near the boiler it would be easy to plumb everything to the tank. A FPHX between the tank and boiler and you're off to the races again.

That's how I did my system. First year I had it, I ran a everything open system. The next year I added 750 gallons of storage and put in the plate HX between boiler and tank. Now only the boiler is open. The transition was easy being the fact I put my storage in my garage next to where the boiler lines came through the floor. Hooked them to the HX and hooked everything else to my tank everything ran great. I put my expansion tank in the rafters of my garage which made that easy as well.