Boiler overpressuring

[ArlingtonHH]

I've read a few threads that helped me troubleshoot, but I am unable to solve my problem.

My system is having pressure issues. We just moved into a smallish 2-story house this summer. We had the heating system checked out this fall and used the heat a few times without incident, except that one radiator on the second floor wasn't heating up. Turns out its input valve was closed. Opened it and bled it, air came out, no water. Tried a few more times with less air coming out. I finally checked the pressure and it was below 5 psi cold. I opened the quickfill valve and the pressure very quickly came up to 15 psi. I bled the radiator and water came out.

Now the system sits at 15-16 psi cold. If we open the drain to drop it to 12, it comes back up. Seems like the autofill valve wants to be at 15-16 (it's a Watts 1156SF - tag says it's set to 12-15psi).

When we turn on the heat, the pressure rapidly rises. Tonight it went from 16 psi cold to 37 psi at 70 degrees before we turned it off. (PRV never opened but is a Watts M3/174A set to 30psi - after we turned the system off, I manually opened it by pulling on the tab and some black water came out. It is now slightly dripping).

The expansion tank is a 3' x 1' steel tank. when tapped, it resonates a bit and sound is uniform top to bottom. It has a drain stem that is inaccessible without destructing a wall, and drain is capped with no valve anyway so I don't see an easy way to drain it.

I suspect the expansion tank is waterlogged and this was possibly masked by the large second story radiator acting as a backup expansion tank (being connected to the system on the drain side and mostly filled with air). If that's the case, how can I drain the expansion tank? Would draining the entire system also drain the expansion tank or would it vacuum seal the water in? The drain is connected to the output side of the boiler (a Weil McLain CGM series 10 circa 1991) via a separate 3/4" or 1" pipe. I could possibly disconnect this pipe below the expansion tank valve and drain that way, but the fittings are currently all sealed up - I can't get close enough to the pipework to see if the fitting is threaded or welded. How hard would it be to replace this tank with a new bladder tank? I think I would need a pro to connect the pipes since there's not an obvious easy threaded connection point.

Also - how high can I safely let the pressure go before I shut the system down if the PRV isn't tripping? If the PRV isn't tripping, should I power down the whole system?

It's also possible that the problem is with the autofill valve over-pressurizing or even the pressure gauge. The system also has a Bell & Gosset "little red booster pump" (LR20BF) on the return side that seems to be operating normally.

At a loss & welcome any advice...

 

[ArlingtonHH]

Just ran the system again. PRV had mostly stopped dripping @16psi, but it picked up as pressure increased past 22psi and was quite a steady drip (almost a stream) by 33psi. Shut down again at that point.

Can I safely run this system by shutting off the mains pressure to the autofill valve, then bleeding pressure way down (say 4-5 psi cold)? (assuming the pressure at the top stays in the 30-35 range)?

 

[Davegasman]

Hi, your expansion vessel has either perished or gone flat, the radiator that youve bled would have been acting like the expansion vessel, hence the problem has only now shown up since.

You have a couple of options, drop the system pressure, release the shreader valve and then top up the pressure again, this will push out any water that has oozed past the rubber diaphram inside, this can happen over a long period due to water molecules creeping passed the rubber molucules. Drop the system pressure again and pump up the expansion vessel using a normal foot pump ect to about 1 bar. If it holds then job done, set it to run.

If the problem continues then you'll either have to pay a gas man to replace the vessel, or it is possible to retro fit a remote vessle onto the system, preferably not too far from the boiler, and on the return pipe.

Be sure to test that the new vessel is at 1 bar pressure, some I've had have been 3bar plus!!

 

[Davegasman]

Also while your at it, add some inhibitor.

The prv may now have some crud on the seating, so may still run after your repair, may need to be removed and cleaned up, or better still, replaced.