verne said:
I was told I needed larger expansion but not how large
Okay, it's been a while since I've done this (I typically deal with non-pressurized storage tanks, so my expansion tank sizing is usually for the heating loop itself)...
If I'm using the chart correctly, you have an expansion factor of 0.0351 (assumed for a worst-case 40-200 degree reheat).
So, if you have 1000 gallons, that means you need a tank that can accept 35.1 gallons. Of course, you want to size this for the whole system, not just the tank, so add the fluid volume of each boiler, and a few percent more for piping. It never huts to over-size an expansion tank, except when you have to write a check for it... (let's call it 40 gallons, for the sake of discussion)
I'm assuming that you'll go with a non-bladder tank, due to the cost involved with a bladder tank of the size you'd need...
You typically want to size your expansion tank to never fill more than 2/3 with water. And you're going to have some water in the bottom under "cold" conditions, to make sure that the air doesn't leak down into the main tank. So I'd suggest at least doubling the acceptance volume when sizing the tank. Find an 80-gallon or larger tank, and use that. I think a used water heater would likely be a good choice. You can mount it above your propane tank, connecting to the "drain" tapping at the bottom. Do yourself a favor, and have a valve immediately after the connection to the propane tank, so you can isolate the expansion tank if needed for service.
You'll want to use a tee fitting there, add a valve and then a barb adapter facing up, and the same at the top, but with the barb facing down. Connect a high quality (don't skimp on this) nylon-reinforced clear hose between these two barbs, and double-hose-clamp each connection. This is your "sight glass" which will let you monitor the tank. The valves let you isolate it, for periodic replacement, if it hazes or leaks.
You'll have to pre-charge it with compressed air, so that's what the extra tapping on the top tee is for - add a valve, then a tee with a pressure gauge, then another valve and an air adapter of some sort (that's up to you). When you are filling, wait until you see water entering the bottom of the sight glass, then start adding pressure. You want to keep at least 10 gallons of water in there, so make a mark roughly 1/8 of the way up. Add pressure as you fill, making sure not to add enough pressure to push the water below that line. Make another mark 1/4 of the way up, and make sure the water ends up below that (this is the cold fill - it's going to go up as it heats). Record the final air pressure (do it on scrap paper, then when you've run the systen a while and are confident that things are set right, mark it right on the tank with a Sharpie).
Since it is a non-bladder tank, the air will slowly dissolve into the water, so keep an eye on that sight glass. Get used to how the water level raises as the system heats, and mark the initial "high" water line, the first time you fully heat the system from cold. As the air is absorbed, that's going to creep up. Keep a decent safety margin, and periodically drain some water and add some more air. This is what that valve on top of the propane tank was for - close it, then you can drain the expansion tank and re-adjust it without fighting the pressure in the main system. Drain it to that 1/4 mark, and re-pressurize the air to the previous pressure. Then re-open the valve on the propane tank and you're good to go.
Joe Brown
Brownian Heating Technology
www.brownianheating.com
