The relief valve on my furnace is discharging. At first I thought it was a faulty valve, so I replaced it. New valve does the same thing. I took a closer look and discovered that the pressure on the gauge reads 30psi, well above the 12-17psi operating pressure.

I checked the accumulator tank, it is cool on the bottom, warm on the top, so I don't believe the bladder is ruptured.

Any ideas on what to check next? I don't know what regulates the internal pressure of the boiler.

Lob

There should be a pressure regulating valve on the water make up line near the expansion tank, probably bad, Good luck, Mike

I checked the cold water feed to the furnace, there's a shutoff, a backflow preventer, and a regulator, then the copper feed goes into the inlet side of the heating system.

Is that it? Is that "feed" the water make up line?

Thanks for your help,

Lob

That sounds like the critter to me. It is probably stuck.Should replace the regulator.

Thanks... it's about the only thing left that I haven't swapped.

I see it has a release on it... not sure what that does? There is no hose fitting or outlet, it's just an inline valve.

Lob

It should have a tag on it saying the pressure limits,

Yes, it does. Says 12-17psi.
Not sure that's it though. I flipped the spring lever a few times.

There is a pipe that comes out of the boiler that has a gray relay box on the end of it.
I know there are relays inside, but not sure what the relays do?
Is there anything in there that could be causing this symptom?

Lob

If that gray box is attached to the water piping it may be a low water cutout switch, if it is attached to the gas piping it will be the gas control valve neither will cause the increased pressure.

Another question to you... do you have an indirect water heater piped off the boiler, that heats your dom. Hot water for your faucets??