Open Expansion Tank Questions

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Bob Rohr

Minister of Fire
Jan 9, 2008
SW Missouri
He is the calc I came up with at Remember a huge difference in expansion tank sizing between a bladder and non-bladder tank. they have a calculator for both at westanks website.

Also the pre-charge needs to be adjusted to the cold fill pressure, 5 psi in your case? Any pre-charge over 5 psi limits the tanks expansion ability. Hydronic tanks are factory pre-charged with dried air to 12 psi typically. DHW type expansion tanks may be 45- 60 pre-charge. so you need to back down the pre-charge when you install the tank.

Max. pressure should be just under relief valve setting.


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Minister of Fire
Sep 15, 2011
Nova Scotia
Couple more points from things said above.

If there is 'stuff' in the boiler that you are worried about contaminating the rest of the system with, it would likely cause issues with your heat exchanger too. The spaces inside those are pretty tiny.

Also - you won't get much useable heat out of a storage tank that small. Not sure what you are hoping for, but I'd guess in the ballpark of an hours worth. Maybe. Depending how cold it is out.

ED in CO

New Member
Oct 23, 2013
Thanks for the extra calc at westank. The 8 gallons is probably closer to ideal. I can get a N-40V tank for about $150. It is a non ASME that operates under 100 PSI and is 20 gallons. 10 gallon tanks looked like they were about $180. Kind of pricey. The Watts is a diaphram tank and so is the Amtrol, both are heat system tanks and pre-pressurized to 12 PSI.

Maple1 I'm not expecting big heat from the system, nor do we want it. While the stove is operating, it should be putting out near 30,000 BTU under moderate fire. (It is rated for 68-85,000 BTU) That should keep the 50 gallon tank warm, about 120 degrees and up to 180 or so while the stove is running. Then when the fire dies down for some time afterwards. Maybe an hour or so from storage alone. I think it will be similar to a hot water tank that is turned off when the water is being used, except the incoming water should only be about 20 degrees cooler than the outgoing. I was going to have the boiler water circulate through the heat exchanger whether the water from storage was hot or not but I'm starting to think I might put in another zone valve controlled by the aquastat so that the water doesn't go through unless it is picking up heat from the storage tank. Still thinking about it.

I would rather replace the heat exchanger than the boiler if it gets gunked up. I don't know how much gunk will come out of a steel boiler. Probably like anything the more we use it the less it will sit around corroding. I might drain it and clean it in summer. That is at least an option.

Great info, thanks for your time.
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