Excessive pressure swings - any thoughts?

Old 01-08-07, 08:36 PM
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Excessive pressure swings - any thoughts?

Hello all - I'm new to the forum.....
I have a 9 year old Weil Mclain CG-6 natural gas boiler. In the past, the boiler pressure would stay in a narrow band (around 15psi dead cold, and 25psi when at 180 deg F.). In recent weeks, however, the boiler pressure guage reads 0 psi when dead cold and up to 40 psi when at 170-180 deg F.

This alarmed me, but since the 30psi relief valve has never popped off, I'm wondering exactly where the problem lies...is it a defective pressure guage, a bad expansion tank, the relief valve or pressure reducing valve? All of the above parts are original to the 9 year old unit.

The Watts ET-30 expansion tank seems like the most likely culprit, but I don't know of any way to troubleshoot it. Can I isolate it from the system (turn its ball valve off when the system is cold) and check the pressure on the air side with a tire pressure guage? Could its original air charge of 12psi have slowly reduced to zero, leaving no air cushion space on the air side of the bladder? I don't think it's a ruptured bladder, since the tank still sounds hollow, not waterlogged...
If the 30 psi pressure relief valve has never popped off, but yet the boiler pressure guage is reading 40psi, could I assume the pressure guage has gone south?
And finally, could the pressure reducing valve (Watts 1156F) be operating incorrectly?

Any insight you folks could share on this would be appreciated...Thanks!
Old 01-08-07, 08:57 PM
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The expansion tank has to be removed from the system for an effective check; you should be able to quickly check it by shutting off power to the boiler, closing the ball valve between the piping & the ET & unscrewing the tank from the system; use a tire gauge to see if you have at least 12 psi at the schrader valve; also fill the other side to the brim with water & check for any bubbles; any bubbles or water coming out of the schrader valve end indicates a dead tank.

The pressure reducing valve can be checked by turning off the inlet water valve that allows fresh water into the system; if the ET checks out ok, continued pressure problems would point to the reducing valve.

The relief valve should pop at 30 psi; put a pail under the discharge pipe & manually open the valve several times briefly; sometimes there's just a small piece of crud inside preventing it from operating normally.

You can buy a psi gauge at a plumbing supply house or Home Depot/Lowe's for ~$10 that you can temporarily attach to the boiler's drain valve, or any other handy fitting to see if you can get a more accurate reading of the true system pressure; if the relief valve still won't pop at 30 psi, it should be replaced to protect the system.

Last edited by jack horner; 01-08-07 at 09:09 PM.
Old 01-08-07, 09:18 PM
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Treat that tank as if it's full of water. Don't break any fingers or feet.

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