Too much pressure?

I fired up the coal boiler this afternoon. Everything was going fine till about 9:00 PM. The pressure relief valve opened on the oil boiler and wouldn't re seat ( water temp about 190 ). I shut down the coal boiler, shut down the oil boiler, and removed the relief valve after the system cooled. It had a rust- sludge build up in it so I thought this was the problem. Went to Home Cheapo, got a new one and installed it. When I fired up the oil boiler the new one relaesed at 140 deg. and won't reseat itself. Any thoughts? I've havn't had any problems with this system in the four years I've been using it.

Sounds like you have a lot going on...fired up coal boiler, pressure valve opened on the oil boiler, shut down both boilers. So I would have to ask are these systems connected? Also is this only a pressure valve or is it a temp / pressure relief valve? What was the valve's original P/T rating?

So then you installed a new valve and fired up the oil boiler - only? and it poped at 140F - again curious if this is a TPR valve or just pressure and what it's ratings are. Then you say it won't reseat? Did it ingest more rust sludge when it opened? is it just dripping or stuck wide open?

Sounds like your expansin tank on your oil fired boiler is water logged . Try tapping on the side of the tank if it dose not sound hollow , try to replace it with a bigger size . When you unscrew it watch out it could be very heavy.

Hopefully you can isolate the tank. Otherwise, you'll have to drain everything down below that point.

As cozy heat alluded to, make sure you have a regular (30 psi) pressure relief valve on there, not a hot water heater valve, which is designed to open at temps around 190. They look similar and have the same 3/4-inch threads. Boiler pressure gauges don't have a temperature trigger, however. You definitely don't want to use a water heater valve because they operate under much higher pressure.

Pressure relief valves open with pressure and close with a spring. Unless there's a major obstruction, tt pretty much has to reset when the pressure drops.

Thanks Eric very important make sure pressure is out of system and boiler has cooled down .

Thanks guys. I had the right valve. As it urned out when I revisited the situation this morning the expansion tank was flooded. both boilers are happy again.