Pressure rising causing PRV to slowly leak

Bsp8nt

I have a newer (4 year old) hydronic gas fed system. I have town water. I set the PSI at 15. When the system turns on, the PSI slowly rises to 30 making the PRV open. There is a pressure reducing valve but I’m not sure how old that is. I shut off the town water feed and the next day my PSI is below 5. When I open the town water it goes up to 15 when the system is idle. When it fires up the PSI slowly climbs to 30 and slowly drips from the PRV. My guess is the pressure reducing valve is no good. My expansion tank is also 4 years old. Most of my house is carbon radiators with some baseboard being added in a new bathroom and kitchen before I brought the house. What am I missing?

thanks!🙏

LRCCBJ

My guess is the pressure reducing valve is no good.

You most likely guessed correctly.

When the boiler is at 15 psi, find the shutoff valve that is probably right before the pressure reducing valve. Shut it off.

Run the boiler normally for a day or so.

If it still remains near 15 psi, you confirmed the issue.

Just leave the shutoff valve off (some people leave it off all the time) until you replace the PRV.

Jamie Hall

You're missing the expansion tank. Being that young, it's most likely ether lost its precharge completely — or was never charged properly in the first place. It could, also, have failed.

First thing to do — on the end of the tank away from the water inlet there is a Schrader valve, just like a car.tire. Take the valve cap off and give it a little poke. Air should come out. If not, you haven't really finished. On the other hand, if a little squirt of water comes out, the tank id done. Time for a new one.

Now if air came out, find a reliable tire pressure gauge and measure the air pressure. It should be right around 15 psi, to match your feed valve. If it is either much more or much less, the tank is incorrectly charged.

You will need to empty the tank (hopefully you have a valve to shut it off from the system. Even more hopefully, there is a drain for the tank…) completely. If you can do this without removing the tank from the system physically, so much the better. With the tank completely empty of water, set the air pressure with the Schrader valve to that 15 psi. Watch the tank for several hours. The pressure should hold. If it does, reconnect the tank to the system and test the system. If the pressure doesn't hold — or if you can't pressuise it at all — the tank is done.

Bsp8nt

I checked the expansion tank at the air fill valve on the bottom and got no air just water coming out. I brought a new one, checked the charge and it was 40 PSI. I hooked it up, bled the system and fired it up. The psi in the system was 15. As it ran for a bit, it rose to 30 and the pressure relief valve popped reducing the system psi to 20. The light over my head came on and I reduced the charge in the expansion tank to 15 PSI. It’s been running now for about five minutes and only gone up to 20 PSI at the boiler. Furnace whatever. I’m pretty sure we’re all good. Thank you for the great and timely advice.

EdTheHeaterMan

You got the wrong tank. that tank with a 40 PSI factory pre-charge is for a well or for a water heater. The one for a closed heating system comes pre-charged with 12 or 15 PSI.

No worries though. you can adjust the factory charge by disconnecting the tank form the system and reducing the air pressure to 12 PSI, then connect the tank back to the system.

EdTheHeaterMan

Here is a comment I made last year that explains how the wrong pressure in the expansion tank looks when you check it when it is connected to the system.

https://forum.heatinghelp.com/discussion/comment/1747354#Comment_1747354

In your case the expansion tank with 40 PSI air pressure in it will not accept any expanding water from the system before the pressure gets over 40 PSI. the problem is that the relief valve will discharge any water pressure over 30 PSI. So that expansion tank is not an expansion tank at all, it may has well be a closed valve.