This weekend I came into the house and it smelled like rusty steam. The air was humid. Also the temperature in the house dropped.
I went to the boiler and saw that the aquastat had been tripped. The aquastate was set to trip at about 220F. As its a hot water boiler and no a steam boiler the warer should really not exceed 212F.
After resetting the aquastat the boiler fired up. About 3 minutes into the boiler running I heard a hiss from the Pressure Relief Valve(PRV) drop pipe. Water began to drip on the floor. It was only then I noticed that there was water on the floor the whole time. So I went to home depot and purchased the last PRV they had in stock for about $40. After a lot of struggling and the use of a butane torch and some banging I was able to replace the PRV. Turned on the the boiler feeling proud that the problem was solved.
Later that night around 2AM I started to feel a draft. I went to the radiator and it was cold. So back to the basement I went. I again reset the aquastat but stood there watching the boiler for some time. Again the PRV was letting out water. I wondered whether I had damaged the temperature sensitive spring inside with the butane torch. So I decided to turn down the internal temperature limit of the boiler in hopes that the pressure would not increase to the point where the PRV would release.
So I stayed there watching the boiler. Still the PRV released water.
So I looked at the built-in pressure and temperature gauge. I was able to see that the pressure in the boiler was increasing constantly as the temperature increased. Also the PRV was doing its job correctly as it released the pressure at 30psi exactly.
God inspired me to suspect the expansion tank. So I googled what are the sypmtoms of a bad expansion tank. There I found that the expansion tank is supposed to absorb approximately up to 50psi of pressure(may be different for different boilers). So I went to Lowes the next day and purchased their biggest(same size as mine) 4.4 gallon expansion tank.
Using my butane tank and a vice grip(not the best tool) as my two plumber’s wrenches were too big to grip the hex area of the tank I was able to eventually replace the existing unit.
I skipped descrition of some steps in the story: I had to drain the boiler a few times and then bleed the radiators. I also reset and reprogrammed my thermostat. I was able to get the heat to function for about 18 hours with the bad expansion tank connected but a lot of water was wasted during that time possibly up to 10 gallons. The expansion tank dropped as soon as I screwed it off as it was filled with water and heavy. I used the butane torch to dismantle the copper PRV drain pipe and to soften up the iron threads to assist removal.
So in conclusion if your PRV is leaking but the pressure gauge on the boiler says the PRV is working correctly then the PRV is fine. Its something else probably the expansion tank.
This applies to “hot water boilers” but does not apply to “hot water heaters” as they have combination Tempereture and Pressure Relief valves. You can tell the difference between the valves as the T&P valve(for heaters) has a rod sticking out of it while the PRV(for boilers) does not.