Discharging boiler pressure relief valve

garbid

The pressure relief valve on my Burnham boiler (for a gas-fired hot water home heating system) has begun to discharge slightly - not full force by any means, but a heavy drip - each time the boiler temp and pressure reach 200 degrees/35psi, shortly before the furnace cuts off, at which point the discharge stops until the next cycle.  I drained and recharged the expansion tank routinely two months ago as I do once each heating year (there was no discharging from the PRV at that time) and the Watts pressure relief valve was new about 5 years ago.  No modifications have been made to any settings on the heating system or boiler and I've never experienced this discharge before; it only started about 3 weeks ago.  Any suggestions as to what might be the cause?  Thanks.

lchmb

p.r.v

Could be your expansion tank has actually started loosing air, or your Pressure reducing valve is over feeding the system. two other  thoughts is your indirect is leaking through the coil over feeding the system or your P.R.V. is failing...Did you check what pressure your boiler filled to after you drained it to test your tank? 

garbid

System pressure after last tank drain

Thanks for your comments.  I did not check the pressure after I drained the tank a couple of months ago but I will drain the tank again and check the system pressure then as a first step to further diagnosis.  I doubt that the PRV is faulty because it's fairly new, but the feedstock water here is fairly corrosive so it's a possibility. The pressure reducing valve was replaced at the same time as the PRV. How should I check if the reducing valve is overfeeding the system?   Sorry, but I don't fully understand your comment about the leaking indirect as I'm not sure what the indirect is.  Thanks for any further enlightenment!

icesailor

Leaking PRV Valve:

If the Boiler PRV is 5 years old and leaking, it probably needs to be replaced. Codes require the PRV to be installed in an upright position with the pressure coming in from the bottom. It then becomes a source of air entrapment. Where the seat pushes against the gasket, the air may accumulate. When the boiler is really hot, and water has worked its way under the seat, there is no pressure. Therefore it isn't protected by the boiler pressure. It will boil off and leave a residue like you find in a pan where you boiled all the water out of. After a long period of cycling, the build up is enough to let it leak more. When they start doing it, I always just replace the valve.

If you have an expansion tank mounted in the ceiling, you may have not drained it properly to let more air in when you drained it. If it is an Extrol type bladder tank, it may be too small especially if the boiler is a cold start. Or the bladder may be leaking and not have enough air pressure to do its job. You can check it with a tire gauge but you must drain water out of the boiler to get the gauge to read zero. Then check the air pressure at the  Schrader Valve. If you don't have at least 12#, the tank may have a problem. You need a minimum of 12#. 15# might be better. You can pump it back to 12# or 15# but if you need to do it again, replace the tank. If you hear bubbling in the tank when you add air, replace the tank.

RobbieDo

Did you say.....

It was at 200 degrees and 35psi?? What doe the PRV on it say? Usually they release at 30psi......