This is a wierd one. I have a residential oil-fired boiler, with a
backflow preventer and PRV set at 12PSI, expansion tank charged to 12
psi, city water at around 80 psi, and typical temp-pressure relief
valve that relieves at 30 psi. Normally, everything works fine.

When I (the owner) leave home for more than a couple of days, I like
to turn off the main city water shutoff valve to the house to lessen
the risk of having a flooded basement. Last winter when I did this, I
returned home to find the boiler TP relieving water all over the
floor, and boiler pressure gauge reading 40 psi. I checked the
expansion tank, found it had been charged to too high a pressure, and
reduced it to 12 psig. The expansion tank was checked out later by a
local boiler serviceman, and found to be OK (half full of water).

I suspected a faulty PRV valve, and closed the makeup water valve to
the boiler, and kept an eye on the gauge for a few days. The boiler
pressure krept up and the boiler TP valve started to drip.

I suspected that I had a small leak in the system (I've found one that
I haven't fixed yet), and that when I close off the source of makeup
water, eventually I loose pressure and get air into the system, which
expands the next time the boiler fires and overpressurized things.

To test this, last weekend, I closed the city water main valve, and
turned off the boiler burner circuit when I left. I returned home to
find the boiler cold (as it should be), but the boiler TP was once
again flooding water all over the basement, and the boiler pressure
gauge read 40 psig.

I should add that I have no problem for months at a time with the
boiler TP valve as long as I leave the city water main valve and the
boiler makeup water valves open. Once one of them is shut, whether
the boiler is on or not, the pressure in the boiler system starts to
climb.

I'm confused!! I'm a heating systems design engineer, but that ain't
helping me figure this out. Any ideas?

Thank you very much!

Linc Vannah
Stratton, Maine