Relief valve - system over-pressure?

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Posted by Gustin Kiffney on March 15, 2001 at 10:44:42:

I have an older gravity-feed oil-fired hot water system that is pretty regularly releasing water through the relief valve - about a bucket-full every few days. Everything else seems OK, and I don't think the relief valve is bad because the pressure IS high when it happens - 180 degrees + and 25+ pounds of pressure, if the guages on the front are right (and they seem OK). The high-limit thermostat that is wired to cut off the burner is set as low as it will go (160 degrees), but I think it's a little off because it doesn't actually shut off the burner until the boiler temp is 180 or over.

There is a oldish horizontally-mounted steel cylindrical tank about 2.5 feet long with concave ends mounted up between the floor joists above the boiler that looks like a compression tank, but I don't see any valves or anything I can reach on it to check if it's full, empty, or what, or to pump any air in there. There is a spigot at the base of boiler connected to the smallish pipe that goes up there which I suppose could be used to drain down some water and perhaps get some air in that tank (I think it's probably full of water), but it looks like it will put air in the whole system if I do that.

There are not many people who even do service work where I am, and the ones who know what they're doing just aren't taking any new customers (Portland, ME, if you're looking for a place with more work!) Anybody here know how I can get some air back in this compression tank, and would sticking a circulator on this system make the over-temperature happen less often? (there are two return pipes and one supply pipe, so it would be cheaper to put one circulator on the supply side - is this OK?) I'm pretty good with mechanics - I've rebuilt some car engines, replumbed my house, etc - assuming I can find a circulator, is putting this on myself something that's totally ridiculous?

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