My son in law just called and said that his pressure relief valve just blew on his boiler. We've been out of town for 4 days and he set the temp down to 60. When he got home, he fired up the boiler, then he said about ten minutes later, the pressure valve blew and he couldn't shut it off. I tried asking him over the phone if there was a shut off valve for the main water supply to the boiler and he could not find one. Water kept spewing out the valve so he shut off the main water supply from the well. There could be a number of questions here, but mainly, what controls the boiler pressure. Is it the expansion tank's pressure or something else. And are there boilers that would not have a shut off valve. He said this is a Stewart Warner system and probably fairly old.
If it is a really old system, it may not use a bladder type expansion tank that is more common on newer systems. Older ones may not have an auto-fill valve, so controlling the pressure is dependent on opening the main water feed valve while watching the pressure gauge.
If it has a old water tank for expansion, if there's a leak, it could fill entirely with water, and then wouldn't function as an expansion tank anymore. To restore it, you'd need to shut the water off (and the boiler!), drain the tank, then close the valves and turn on the fill valve until you get the proper pressure (probably somewhere in the 12-18# range).
The pressure relief valve could be shot - the piston on them can get corroded or covered with deposits, and once they open, they won't close again...you'd need to replace it.
Boilers with compromised safety devices can be very dangerous. It may be time to call in a pro.
Your problem is definitely due to thermal expansion. This is cause by a failed expansion tank. Because it wasn't blowing off with cold water in the system.
You need a new expansion tank and relief valve for your boiler.
I think the problem has already been identified, the relief valve definitely sounds likes it is corroded inside which is why it did not close and/or there is no automatic fill valve so water just kept pumpin' away.
It sounds like he had a fair amount of water pouring into the basement. I hope nothing was damaged. My suggestion is when your son in-law has it fixed the contractor should give him a "tour" of the system. This way he knows where the correct valves are to turn the water off, and safety switches to cut power to the boiler, if this every happen again or if any other problems occur.
Just a quick walk through to gain a little knowledge about his own system.
The expansion tank and the auto fill valve control system pressure.
Originally Posted by frank in ohio
Replace all 3 play it safe water feed relief valve and expansion tank and after that bleed the system out :)