presure relief valve leaking

Great site. I'm learning a lot from you folks.

I have a gas furnace (about 20 years old) in the basement, radiators on the first and second floors. When I start it up, the pressure goes up high pretty fast and the water starts coming out the relief valve. I got a water pressure meter from Lowes hooked up to the laundry sink. It says 90 psi. The expansion tank (Extrol) seems ok from the "knock" test (which I learned on this site! thanks!), the bottom sounds hollow and the top thuds. Any idea what the problem is?

Karl_northwind said:

check the pressure in the expansion tank with a decent digital tire gauge. and see if any water comes out of the valve at the bottom of the tank. assuming that the tank has the pipe connection at the top and the tire valve at the bottom.
if the pressure of your system is typical, you'll see the cold pressure of 10-15 PSI, and the air pressure in the tank at the same pressure.
if it is grossly different something is up.

Click to expand...

Don't have a guage, but the tank is just like you said, pipe on top, valve on the bottom. All I have is a dial-type car tire gauge.

I see a pressure reducing valve going into the furnace intake. Can't make out the manufacturer, it's in a tight spot.

Thanks!