Anyone here know if - in normal operation, does a expansion tank in a heating situation even see water? In other words, can a cheap bastard use an appropriate size LP tank?
At 4'x4' 1000 gallons would be a 8.4' tall column of water. At 16' you would even use less floor space. A square tank needs stay bolts.
I need to measure some propane tanks for sure. Those are commonly used for storage in hydronic heating. They used to be easy to come by, not anymore.
I didn't have too much trouble finding scrap tanks, the only problem being they removed the valves and sat outside for a while. One had quite a bit of surface rust inside that was not fun to clean out. I plasma cut an access port and had to crawl inside to properly clean and then weld back on a radiused plate to seal it back up. Lots of leak checking to get it sealed again. If there is a next time I think I'd buy them new. Folks are afraid of doing this but keep in mind the finished product is full of water and only pressurized to 10-14 psi in a tank that was designed for 300 psi. If my weld does fail it's only going to leak not go shooting through the roof.
Yes some people use a propane or even a air compressor tank for expansion . I'm not sure all the details but they tend to get too much water and not enough air in them after some period of usage and the proper water to air ratio must be maintained so it's not as install and forget like a bladder type expansion tank.