*HELP*Boiler runs excessively/40psi/no bleeders

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Only In The North

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Duluth, MN
I have a Weil Mclain Gold CGa 4 SPDN boiler installed in '03. This is my second winter at the home, after crazy gas bill last winter I adjusted the HIGH temp 180 and was told to have roughly 38lbs PSI. Beyond that I haven't touch anything but extensive research hasn't helped my situation.
Basically it runs way too much with the thermostat always under 68. Usually set around 64 maybe 68 on negative days. The system cannot get hotter then 160 before hitting 40 psi then kicking back on 10 minutes or so at 140 and 22psi. There are no bleeders anywhere in the house and since I can remember you can always hear water running when it kicks on and off upstairs. The piping runs from the basement to baseboards one the main level and baseboards in the second level. Roughly 15ft above boiler. Really could use some help and should be able to answer any questions about the sustem. Currently have it cold to possible start over and drain/fill adjust auto fill.. .Etc

The 90 elbows in closet is on the second floor
The two 45s are on second floor
The last image is the end of the second floor heating returning


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I think your question may be "why does pressure rises so much?"

With the system pressure at 22 or less, knock on the gray thermal expansion tank, that is to the left of the boiler. That should sound empty of water. That tank may need to be replaced. It may need to be up-sized. I am not a pro.


Retired Defense Industry Engineer xxx
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New England
Unless you have a lot of stories in your building, you shouldn't need that much pressure. A typical, 2-story building can use something like 12-15 psi or so. Pressure drops at approximately 0.43#/foot...you need enough pressure in the system to prevent a vacuum at the top. If your home is at high altitude, you might need a few more pounds to prevent boiling, depends on how high up you are. But, when the pressure rises a lot during a heating cycle, it's usually the expansion tank has either lost air or has failed. If the tank doesn't have a good cap on it, the Schrader valve doesn't always hold pressure forever...the cap is a good secondary seal against air pressure loss.

To test, first, see if it seems full of water. TO actually test the air pressure in it, though, you must release the water pressure in the system, leave a drain valve open, then check the air pressure with a tire pressure gauge. IT should be set to approximately your normal system pressure. If when you open that valve, water comes out, the tank is shot. IF it won't hold pressure, the tank is shot. IF it does hold pressure, refill things, and you should now be okay (can be a pain purging all of the air out of the heating loops, but that's another story!).

Water expands when it is heated...the pipes aren't really elastic, so that causes the pressure to rise. If it doesn't have somewhere to go, it can cause the safety release valve to open to relieve it. Normally, since air compresses fairly easily, it just pushes back into the expansion tank, and the pressure barely rises in the system. FWIW, unless you tank's precharge is too high, the air pressure will always equal the system water pressure, so can be a good check on your water pressure gauge if you just take the air pressure reading at the tank.


In the trades
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What they said- first check the expansion tank to verify that it is good, and properly pre-charged to a reasonable system pressure.

If the auto-fill valve is seeping causing the system pressure to rise slowly over hours/days/weeks, bleed the system to the right pressure with the boiler drain valve by the pump & auto fill, then turn off the isolating valve between the auto-fill and the potable supply. The isolating valve might seep too (especially if it's an old gate valve rather than a ball valve), but it's worth a shot. The isolating valve isn't obvious in the picture, but follow the pipe back- auto-fills are almost never installed as the only valve between the potable supply and the boiler.

In your case there is a ball valve between the auto-fill and the boiler. If that's the only place the boiler can be fully isolated from the potable use it, but only if it's been verified that the auto fill is slowly seeping. When weather and time allows, reverse the order of the auto-fill and ball valve when you replace the defective auto-fill.

Hey, wait a minute.

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