Why does my boilers pressure relief valve keep leaking?

bender1227

Hey everyone-

My pressure relieve valve was leaking on my boiler so I decided to change it since it was old. My boiler is a Weil Mclain CGT. After I installed the new pressure relief valve and blead the lines; when I turned the 3 zones I have on in my house the pressure would raise above 30 psi and the pressure relief valve would leak. I'm thinking my pressure relief valve wasn't the issue and possibly the expansion tank in the system. Any help with diagnosing the issue would be great. Below are two pictures of the boiler and one of the side chart.

Also, after the zones were turned on, heated up, the pressure relief valve would leak, then the pressure would stabilize under 30 psi and work fine.


https://imgur.com/a/ctsAMKq

https://imgur.com/a/NUdgD7B

https://imgur.com/a/NFc6H2W

Thanks!

STEVEusaPA

Can you add the pictures to the post instead of a link? This way the pictures will always be with the post if the link breaks.
All the usual reasons or a combination of them.
-Air in system
-Bad fill
-Bad expansion tank
-Doesn’t look like you have a tankless coil, but that could be leaking.
Get a pro, or keep changing parts.

STEAM DOCTOR

Agree with above. More often than that it's the expansion tank. You can take off that little black cover on the bottom and push in that little thingy jiggy. If water comes out, then you know it's a bad expansion tank. 

bender1227

I added the pictures good call. I pressed on the valve on the bottom of the expansion tank and nothing came out. No air or water. Maybe it lost it's charge or maybe it went bad.

Jamie Hall

bender1227 said:

I added the pictures good call. I pressed on the valve on the bottom of the expansion tank and nothing came out. No air or water. Maybe it lost it's charge or maybe it went bad.

See if you can adjust the precharge, and if it will hold that precharge. Remember that the precharge must be done with the tank empty and not connected to the system...

Big Ed_4

If it leaks only if the boiler fires , it is the tank . If it leaks constant I would check the feed valve or the indirect .......

EdTheHeaterMan

I have a quick and easy way to check the expansion tank charge.

1. Shut off the feed water to the boiler
2. Release any pressure to the boiler system by opening a drain valve.
3. Pump air into the expansion tank and listen for bubbles going into the system pipes.
4. If no bubbles are heard then test the air pressure in the tank and adjust the air pressure to about 12 psi.
5. Make sure the boiler pressure is still below 5 psi this is important
6. After 5 minutes see if the expansion tank is still at 12 psi
7. With a successful air pressure test of the tank, you may fill the boiler with water to 12 psi cold
8. Fire the boiler and see if the boiler pressure stays below 18 psi. Success!

Since there is no pressure in the expansion tank, and there is no water coming out of the Schreader valve your diaphragm is probably still intact. When you pump air into the tank the water in the tank will go pack into the boiler. That is why you need to check #5 above. You don't want that water to cause the boiler pressure to increase during this test procedure. If there is more pressure in the boiler after filling the tank with air, you will need to drain more water from the boiler drain valve to get the pressure below 5 psi.

The boiler model number you posted has a tankless water heater inside the cast iron sections. there is a possibility that your control system is still making the heater fire for hot water even though you disconnected the hot water piping. That control system does not know if the pipes to the water heater are connected or not. There may be a possibility of saving 5 to 10% of your gas usage by rewiring the boiler for a "Cold Start" operation just like other Weil McLain boilers that do not have tankless DHW Coils.

bender1227

I re-bled the 3 zones to see if that would help and it did not. We don't have an indirect. Our hot water heater heats it's own water. It only leaks when the boiler fires so believe it's the expansion tank. I'm going to look into replacing that. The feed valve is only a few years old so I doubt that is the problem.

EdTheHeaterMan

The boiler model number you posted has a tankless water heater inside the cast iron sections. There is a possibility that your control system is still making the heater fire for hot water even though you disconnected the hot water piping. That control system does not know if the pipes to the water heater are connected or not. There may be a possibility of saving 5 to 10% of your gas usage by rewiring the boiler for a "Cold Start" operation just like other Weil McLain boilers that do not have tankless DHW Coils.


This control is located inside the boiler front cover and has an adjustment that can be lowered to 140°. That is still very hot for a boiler to stand by in case you want hot water that is not even connected. I would recommend removing one of the wires from that control so the boiler does not maintain a temperature for hot water. When you disconnect that wire, cover it with an insulated wire nut or cover the bare end with black electrical tape so stray voltage will not contact any metal parts of the boiler.

Also... If it is not too late, the tankless coil gasket will shrink after 500 hours of use. This shrinkage is usually enough to cause a leak around the bolts and gasket. If the bolts are not frozen form corrosion, you can avoid this leaking gasket problem by snugging up on the bolts to the tune of about 25 ft pounds. in some cases that can be as much as 1/4 turn. If it is too late then there is nothing to do except to monitor the leak and replace the gasket in the future when the leak gets bad enough to see water on the floor. .

HomerJSmith

I had a situation where every time the boiler turned on the PRV would pop open and discharge a small amount of water. The problem was that the circulation pump was too large creating a pressure surge above the 30# limit.

bender1227

I believe I have solved the problem. I ended up replacing the expansion tank and pressure relief valve. The expansion tank was the issue. Once I installed it the problem went away. I also bled the lines before running the boiler to test it.

The expansion tank was a Amtrol EX-30 but the guy at the plumbing store said was an equal to the one I already had on the boiler. It came precharged at around 13.5 PSI. I checked the feeder valve and the tag said the pressure that it releases is 15 psi. I'm assuming I don't need to add any more pressure to the expansion tank.

Thank you everyone for the help.

HVACNUT

bender1227 said:

I believe I have solved the problem. I ended up replacing the expansion tank and pressure relief valve. The expansion tank was the issue. Once I installed it the problem went away. I also bled the lines before running the boiler to test it. The expansion tank was a Amtrol EX-30 but the guy at the plumbing store said was an equal to the one I already had on the boiler. It came precharged at around 13.5 PSI. I checked the feeder valve and the tag said the pressure that it releases is 15 psi. I'm assuming I don't need to add any more pressure to the expansion tank. Thank you everyone for the help.

What brand and model "feeder valve" ?
They're usually set for 12 psi.

Check the boiler pressure when it's cold. If it's 15 psi, then remove the extrol and add 1.5 psi to it.

hot_rod

Trust a pressure gauge before you trust the dial or indication on the fill valve.
Match the pre charge to the actual fill pressure, confirmed by a gauge.
Having the pre charge a pound or two below fill just allows a small amount of fluid to enter the tank. Unless the tank is undersized, that should not be a problem. Many gauges have that much error, + or- 10% is common for gauge accuracy.

bender1227

I had an issue when I turned my boiler on recently due to the weather getting colder; the pressure would raise to 30 psi then lower back down.  The new expansion tank I had put on only had 13 psi in it and the fill pressure is 15 psi from the water infill valve. I released water from the system, added 2 psi to the tank, filled the system back up, and drained it for air.  Everything seems to be working fine now.