Dual Oil-Fired Boilers different pressure readings, one is too high

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momentumep

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I have two Weil-McLain WGO-5 oil-fired boilers supplying 8 zones. One of them was releasing water through the relief valve every night until I, for a completely separate reason, redid all the pipe insulation. I was pleasantly surprised that after improving the insulation, that boiler was only occasionally releasing water, and in much smaller amounts.

I noticed that the pressure on that boiler is often at 40psi or slightly higher, while the other boiler is usually below 20psi. The temp on both of them is usually just under 170. Reading posts about high boiler pressure suggested it might be bad expansion tanks, but the boilers share the same expansion tanks.

Any ideas on why the two boilers have such different pressures and what could be causing high pressure in one of them?
 

Fitter30

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One expansion tank or two? One fill valve or two? Most boiler relief valves are set at 30 lbs at about 35 should be dumping wide open. See if you can post some pics of the boilers and related piping. Pipe covering shouldn't make a difference in boiler pressure.
 

WorthFlorida

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There will be Pressure Reducing Valve just before furnace before the cold water supply inlet. One may have went bad.

Check and drain the expansion tank. Old steel tanks get water logged, bladder type the bladder bag burst or it needs compressed air.
 

momentumep

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Thanks. There are two expansion tanks and one fill valve.

The basement is also my workshop. I added filters to the air intakes to keep sawdust out and went crazy with the pipe insulation because the manifold was like a big radiator heating up the basement. It's probably 15 degrees cooler now and the overflow has all but stopped. The bucket is almost dry vs. completely filling up every 24-48 hrs. I don't understand why, but nothing else has changed.

Here are some pics:

IMG_1245 copy.jpg
IMG_1247 copy.jpg

Left boiler:
IMG_1248 copy.jpg

Right boiler:
IMG_1249 copy.jpg

Expansion tanks look ok:
IMG_1243 2 copy.jpg
 

Fitter30

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One gauge is bad could be both. Could turn off the ball valves to zones off and drain boilers down. Both boilers should be the same pressure how there piped.
Tridicator gauge shows 45' red arrow which is 19.5lbs with boiler under 100° pumps off. The plastic nipple on bottom of ex tanks unscrew the nipple with a 0-50 lb tire gauge check pressure with a boiler under100° pumps off. Get water out bad tank has to be replaced. How many feet of height is there from bottom of boilers to the top of the tallest heat emmiter?
 

Jeff H Young

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Im looking at it and admittedly these systems arent my specialty as far as trouble shooting. but pressure I think should be the same I dont see how they cant be equall.
How about swap the guages? relief valves give problems too verify thier operation. I dont see how the expansion tanks make a differance (as to why only one boiler is acting up) both tanks appear to come from both units.
 

momentumep

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How many feet of height is there from bottom of boilers to the top of the tallest heat emmiter?
Do you mean the vertical distance to the indoor air handlers in the attic or, since everything is currently in a/c mode, do you mean the distance to the highest hot water outlet in the house?

The basement height is 9' and the boiler is about 16" off the floor. The first floor height is 10' and the second floor height is 9'. The air handlers in the attic are just above the floor, so perhaps the hot water has to rise 2' above the second floor ceiling. So the height to the attic air handlers is about (108" - 16") + 120" + 108" + 24" = 344". To a shower head on the second floor would be around 290".
 

Jeff H Young

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Anybody know why the pressure is differant in the 2 units? it could be mis adjusted or bad relief valve I still think Id swap the guages unless its supposed to have differant pressure.
 

Fitter30

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Do you mean the vertical distance to the indoor air handlers in the attic or, since everything is currently in a/c mode, do you mean the distance to the highest hot water outlet in the house?

The basement height is 9' and the boiler is about 16" off the floor. The first floor height is 10' and the second floor height is 9'. The air handlers in the attic are just above the floor, so perhaps the hot water has to rise 2' above the second floor ceiling. So the height to the attic air handlers is about (108" - 16") + 120" + 108" + 24" = 344". To a shower head on the second floor would be around 290".
Water is 2.31 feet per lb 24.2 ' 10.5 lbs add 4 lbs
15 lbs all your system needs and expansion tank air pressure. Still need to check gauges or replace them then fill the system, reset fill valve 15 lbs and bleed all the air out then run zone pumps to let air scoop do its job 24 hours. Fire boilers.
 
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Jeff H Young

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not the issue that dosent fix anything . Why dont the 2 boilers have equall pressure? and how do we know those numbers are correct meaning the actuall pressure ? not the theoretical prefered pressure which is important too
 

momentumep

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not the issue that dosent fix anything . Why dont the 2 boilers have equall pressure? and how do we know those numbers are correct meaning the actuall pressure ? not the theoretical prefered pressure which is important too
Right. I did have a chronic problem of the boiler on the right constantly overflowing (which I was procrastinating making a service call about), but then I inadvertently fixed the problem, which made me to take a closer look. How could adding insulation have possibly fixed the problem? That's when I noticed the pressure differential, did some googling about boiler pressure and posted my question: Why are the pressures different?

The right boiler has the high pressure and only that one was overflowing. It's possible that the pressure gauge is wrong and the relief valve is bad, but it's more likely that something is causing that boiler to have too much pressure. Given the way it's piped it doesn't seem possible that the boilers could have different pressures. Something must be causing a pressure drop when the water flows from the right boiler to the union with the left boiler.

Could it be a problem with the circulator pump on the right boiler?
 

Fitter30

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After looking at the boiler piping the crossover between the supply and return to the left of the boiler right behind right ex tank is wrong should be piped like this page9. Note1 -12" or closer. Like them as close as piping allows.
The water supply is teed into line running up and down. Does it tie into both lines?
 

momentumep

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If you rotate my system 90°, and move the supply and return lines to connect directly into each end of the crossover (eliminating a 90° turn from each side) on the layout on p.9, the piping is effectively the same. Please correct me if I'm wrong.
IMG_1245 copy 3.jpg

As to the distance. I happen to have an old photo, taken pre-insulation, when I was taking measurements for a contemplated enclosure to address the excess heat in my workshop. WIth some adjustment for the camera angle, the measure of the noted distance between tees looks almost exactly 12".
measurement.jpg

The fill valve is teed into the supply line, just after the crossover. The third side of the tee is connected to the expansion tanks. The layout on p.9 doesn't include a fill valve or expansion tanks, but mine are located where the system temperature sensor is.
 

momentumep

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Turns out one of the pressure gauges was bad. I checked the pressure on the drain valves with a separate gauge and they both matched the higher reading on the right boiler.
 

Fitter30

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Never trust a gage or a temp reading unless you checked them out. The other thing is going down the rabbit hole we all done it when looking into problem.
 
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