Weil McLain 68 Pressure Problem


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Old 03-17-09, 06:39 AM
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Weil McLain 68 Pressure Problem

I've got a 15 year-old Weil McLain 68 oil furnace servicing 3 closed loops and hot potable water that continually builds pressure over a period of several hours until it hits 30 psi.

I first discovered the problem when I got home the other day to find the Weil McLain popped off at 30+ psi. I replaced the Amtrol expansion tank, figuring it was possibly the problem since the tank was waterlogged. Within several hours, pressure was again approaching 30 psi. (I've since monitored the pressure and bleed off water as needed to avoid additional spikes.)

I then closed the valves for incoming and outgoing potable water, thinking it might be the coil. Pressure seemed to hold steady over night, maybe edging from 18-19 psi to 21-22 psi, but I wrote that off to the pressure difference going from cold (140 degrees) to hot (180 degrees). To get a second opinion, I called in my local furnace guy who is pretty good, and we ran the test again a second night. Same results. So we figured it was likely the coil and replaced it yesterday (and we tested the new coil prior to installing just to be safe!).

I woke up this morning to find the psi at about 26-27. I double-checked to be sure the cutoff valve preceding the automatic intake valve for the furnace was closed, and it was, as it has been the whole time.

Does anyone have any additional suggestions of what to troubleshoot? I'm at a loss as to what else could be wrong. There's no evidence of any leaks around or under the unit. I've wondered if the cutoff valve and/or intake valve is leaking, but it seems highly unlikely to me that BOTH are leaking as would have to be the case with the cutoff valve closed.

Any help would be most appreciated!
 
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Old 03-17-09, 09:00 AM
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Usually there is a pressure regulator that acts as an autopmatic fill valve, allowing water to enter the heating system from the water supply system whenever the pressure falls below a certain level, often 12 PSI. That regulator may be failing, allowing water to enter at a higher pressure.

I'd check for that.
 
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Old 03-17-09, 10:51 AM
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Originally Posted by SeattlePioneer
Usually there is a pressure regulator that acts as an autopmatic fill valve, allowing water to enter the heating system from the water supply system whenever the pressure falls below a certain level, often 12 PSI. That regulator may be failing, allowing water to enter at a higher pressure.

I'd check for that.
That occurred to me, but I've closed the 1/2-inch cutoff valve between the water supply and the regulator (I referred to it as an intake valve in my OP). I suppose BOTH the cutoff valve and regulator could be failing, but that seemed improbable (at least initially!) to me. I guess I could shut off the entire house supply for a couple of hours. That might give me some indication of the possibility of both the cutoff valve and regulator failing.
 
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Old 03-21-09, 07:37 PM
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Boiler Pressure

What size are the boiler & tank?
Did you check/adjust the pressure on the tank prior to installation?
After replacing the coil, did you check the "hot" pressure soon after the boiler came up to temperature?
 
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Old 03-24-09, 09:21 AM
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Originally Posted by Grady
What size are the boiler & tank?
Did you check/adjust the pressure on the tank prior to installation?
After replacing the coil, did you check the "hot" pressure soon after the boiler came up to temperature?
The boiler's model number is P-368-WT. The expansion tank is an Amtrol Extrol 60 (closed diaphragm tank - 7.6 gal - pre-pressurized at 12 PSIG). It's the same model/size expansion as was previously installed.

After replacing the coil, we re-filled the boiler to approximately 12 PSI with cold water. Restart of the furnace (set at 180 degrees F) brought the pressure up to about 18 PSI. It appeared to hold steady, but by the next morning (about 14-16 hours later), pressure was just short of 30 PSI. This occurred with the cutoff valve preceding the regulator in the closed position.

If I close the cutoff valve that controls any water from entering the furnace, it seems to slow greatly the the pressure build, though it still seems to edge up 2-3 PSI overnight.

This problem is particularly frustrating because I've already replaced the usual suspects. And unless I'm missing something, about the only things left to replace are the cutoff valves, regulator, and/or boiler itself (though again, I see no water dripping whatsoever underneath or around the boiler to suggest a cracked boiler).
 
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Old 03-24-09, 03:50 PM
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Pressure

Since you still get an increase in pressure overnight, I can't help but belive you have a valve leaking thru AND either a bad coil or pressure reducing valve. It is quite rare but I have seen new coils leak.
 
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Old 03-24-09, 06:56 PM
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Thanks for your replies, Grady! I too have wondered if the new coil might be bad, as well. I appreciate having another person such as yourself to bounce ideas around before I spend another day tearing apart my furnace...again.

I think I'll start with the valves and regulator first, however.

I'll let you know how it goes!
 
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Old 03-25-09, 04:46 PM
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Valves

All I use is full port ball valves & don't buy the cheap ones. Apollo, Watts, several others are US made & well worth the difference in price.
 
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Old 03-25-09, 05:53 PM
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If you can disconnect the inlet and outlet of the heating coile you should be able to either air test or water test that for intergrity with a little immagination and some fittings.
 
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Old 04-08-09, 01:39 AM
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UPDATE: Just thought I'd post for closure. I finally had time last week to cut lines and test valves and the regulator for leaks. Turns out ALL were failing! Installation of two new cutoff valves and a new regulator has finally solved the problem! Beer 4U2

I'm still thoroughly perplexed how two cutoff valves, a regulator, and a coil could have failed - it seems an almost unbelievable coincidence. Anyway, thanks for the much welcomed input from all!
 
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Old 04-08-09, 03:28 PM
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Leaking components

With the valves & the regulator (pressure reducing valve) all leaking thru, the coil may not have been leaking. In general we trust that valves shut off unless there is some reason not to trust them. When they don't fully close & the reducing valve is leaking thru it can throw the best a curve.
Glad you got it fixed & maybe we can all learn a lesson from this one. Beer 4U2
 
 

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