Leaking Pressure Relief Vavle

[Anthony S]

I have a pressure relief valve on top of my furnace that will not stop leaking. I have replaced the exspansion tank because the bladder had went bad. I released some pressure from the new tank to 24.5 psi. I replaced the valve itself. I turned down the temp to 160 deg. I also checked the main pressure coming into the system and it is well below any level that would cause the valve to continue to leak. Still when my furnace kicks on to heat hot water the pressure builds to over 100 psi and the valve continues to leak.
Note, that this started to happen after splicing into hot and cold lines a couple weeks ago for a new bathroom. Any help would be great, Thanks!

[jadnashua]

If you have a boiler rather than a furnace, it will have an expansion tank separate from one (you might have) for your potable water tank.

In a boiler, the pressure should not get to much more than (typically) 20# or so, and the expansion tank is normally set to the normal pressure of the system. The T&P valve for a boiler will trip if the pressure gets much above the normal pressure. This is a different valve than one designed for a WH, each has its own settings for both pressure and temperature where they start to vent.

If you install a T&P meant for a boiler on the WH, it will dump water. If you install one made for a WH on a boiler, you may blow seals before it trips.

If you get 100# in the boiler, you likely have either a leaking boiler make-up valve or the new bladder tank is shot, or the pressure is too low, or it is not the right size, and is filling up with water.

Maybe a picture of what you have would shed some light.

[nhmaster]

The pressure in the boiler should only be between 12 and 15 lbs.

If you drain it down to there and it goes back up, either your feed valve is bad or if you have a domestic hot water coil, it has a hole in it.