I've been having continued problems with the pressure in my home oil boiler rising to 30psi over time (and then leaking out the pressure relief valve). Technicians have examined it multiple times but have not addressed the issue.

The last technician indicated there were only two ways the pressure in the boiler could increase: through the water intake valve or a faulty expansion tank, and through backpressure from the external hot water heater (heated via a boiler zone).

To validate it wasn't the water intake valve/expansion tank, I shut down the system, closed all incoming and circulating water, then lowered the pressure to zero manually off the pressure release valve. Once it was at 0, I opened the water intake to restore pressure (it climbed to 15), then shut that valve before turning the system back on.

After a few days, the pressure has again climbed to 30psi. So, my questions are: was the technician correct that the leak must be coming from the water heater? Is the only solution replacing the water heater in that case? Or is there anything else that can cause a pressure build-up?

  • @Tester101 - the T&P valve on most oil boilers goes at 30 PSI, and BenG has already said that it then leaks out the T&P valve, so it can't go above 30...and he's shut off the valve before the "automatic intake valve" and still had a pressure rise.
    – Ecnerwal
    Apr 14, 2015 at 12:58
  • @tester101 climbs to 30 and holds because yes, it leaks out the relief valve at that point.
    – BenG
    Apr 14, 2015 at 13:04
  • Pressure before the pressure reducing valve would be whatever street water pressure is (80psi-ish is what I've heard, I don't know for certain). The reducing valve is set to ~15 (the default level the boiler pressure gauge climbs to from zero when the water intake is opened).
    – BenG
    Apr 14, 2015 at 13:12
  • @Tester101 correct me if I'm wrong, but after temp is stabilized (heated to ~170) wouldn't the pressure also stabilize? It might rise to ~20-25, but I wouldn't expect it to rise to 30, particularly only after a couple of days.
    – BenG
    Apr 14, 2015 at 13:14

1 Answer 1


Your diagnostics so far point to the indirect hot water heater coil leaking potable water into the furnace loop as the most likely issue. You could perhaps verify if you can shut your hot water down for a few days (not convenient, I know.)

Depending on the overall condition of the indirect water heater tank, the coil can often be replaced as a part, if the indirect hot water tank is otherwise in good shape. Some are not designed this way, however.

While your test might leave room for a failed expansion tank, if you did not cool the boiler down too much (so the cool water then expands as it's heated; which with a failed expansion tank would raise the pressure significantly, and with a functioning one will raise the pressure slightly) it's probably valid, especially if the pressure rise took days, rather than shooting up the first time the boiler fired to temperature (a sure sign of expansion tank troubles.)

For a typical "modern" bladder-type expansion tank, there's an easy, quick test of the tank integrity - drop the pressure to zero on the system again and check the pressure on the air fill valve for the expansion tank. If it's also zero, the tank (diaphragm/bladder) has failed. The even quicker (if crude) test is to "burp" the air fill valve very slightly (don't want to release significant air or you need to drop the system pressure and refill it) to see if air or water comes out - if it's water, the tank has failed. I would expect your service technicians to have already done one or both of these tests, given your symptoms.

  • If the leaking heater coil was really the problem, wouldn't you run the risk of water from the boiler loop intruding into the potable hot water supply if you dropped the potable hot water pressure?
    – kgutwin
    Apr 14, 2015 at 15:45
  • Only if you were sloppy about dealing with it. You suspect the coil. You turn off and drain the hot water supply. You await events - presumably boiler pressure will now drop, not rise, if the input is cut off. You replace the coil or whole indirect heater. In the replacement, you remove whatever leaked from the boiler into the heater. If, for some reason, you cannot manage to replace the heater immediately, you flush the heater several times before refilling, and then run all the hot water taps to further flush the system.
    – Ecnerwal
    Apr 14, 2015 at 16:22
  • 1
    Expansion tank on the boiler checks out; it has air pressure, no burst diaphragm. Looking more and more likely there's a pinhole leak somewhere in the water heater. I'll try to test that out sometime soon, though as you noted turning off the hot water won't be very popular.
    – BenG
    Apr 15, 2015 at 0:34

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