Boiler pressure drop to 0 when off - fill valve closed

cdvo871 · February 2021

Hi Folks,

I have a relatively new (1-2yrs old) Weil-McLain oil-fired boiler that is doing a good job of keeping the house warm. I have a one-story rancher style home with fin/tube baseboard heating throughout the structure.

A couple weeks ago my county was performing work on the water main line and I was getting some pretty significant leakage from the vent pipe off the boiler fill valve - the water supply fill valve to the boiler was open at the time -as it has been since installation. I spoke to a plumber and he said that's normal when there's a severe drop in pressure on the supply side of the line, due to the water supply being cut off to the house. He instructed me to close the valve off to stop the leaking - so I did and the leaking did stop.

This was about two weeks ago now, and I have not yet opened the water fill valve back up - it's been closed the past couple weeks. I have not noticed any issues with the home getting warm, but today I did notice some gurgling in the pipes when the heat first kicks on/and the circulator starts pumping the water through the pipes. I also noticed the pressure gauge was all the way down to 0 when the system was off and cold (temp was about 78F) - it does get up to about 20psi when hot (180F) and running (this is what it typically runs at when hot).

I will say that I had been checking the pressure gauge over the past couple weeks and to my memory I don't recall it ever getting all the way down to 0 since closing the fill valve; but to be fair I was only checking every other day or so - and not always when it was ice cold.

My question is - should I open the fill valve back up like it was before? Or should I just leave well enough alone and keep it closed as it doesn't seem like there is any issue with the house getting warm? Do I have an issue keeping it one way or the other?

Looking for some advice from the heating pros.

Thanks,
Chris

Jamie Hall · February 2021

Your system should be around 12 psi when cold, but it shouldn't rise much -- perhaps to 15 -- when hot. I'd open the fill valve and bring it up to 12 psi when it is cod, and then watch and see what happens when it fires up.

EBEBRATT-Ed · February 2021

@cdvo871

What leaked when the city had the water off was the backflow preventer, It was doing it's job which is to prevent dirty boiler water from backing into the city water if the city water pressure is low. That's normal

In your city water make up pipe to the boiler you should have 3 things. A shut off valve, a backflow preventer and a prv (pressure reducing valve) which automatically feeds water into the boiler.

So open the valve back up and watch the boiler pressure on the gauge. It should be 12-15 psi when the boiler is cold and get up to maybe 20-22 psi when the boiler is hot.

If you here gurguling it means you may have lost some water or it could be an expansion tank issue.

Just open the valve, You may have to bleed some baseboards of air. Just keep an eye on things.

Call your service tech if need be

HVACNUT · February 2021

Your WM boiler likely came with a Hydrostat 3250 Plus, which incorporates a Low Water Cut Off. So it should sense it if the actual water level, not pressure got too low.
I wouldn't think you should lose any water pressure while having the valve off. Now comes the big question. Is your house on a concrete slab? The water went somewhere.
If the boiler is on the same level as the living space, it doesn't need much, if any pressure to run. But you don't want air in the system.
Open the fill valve and let the feeder do its job. When it stops feeding at about 12 psi, then close the valve again. Watch the gauge for a few days. If the pressure continues to drop, then the water is going somewhere.
Keep in mind you will lose some pressure due to air removal through the air eliminator.
Can you find a warm spot on the floor? Is there a crawlspace or any way to check from underneath if no basement?

cdvo871 · February 2021

Thanks for all the replies so far.

I should have clarified more - before I initially closed the feed valve there was a good amount of water that leaked from the pipe coming out of the backflow preventer while the city water was shut off. I assume most, if not all, of the water loss in the system occurred during that time as I have not opened the feed valve back since the water was turned back on.

I also meant to say that while I do live in a rancher style home there is a basement, which is where the boiler actually resides. However the main floor is the only floor that is heated by the boiler heating loop.

Based on the info received so far, I’m going to open the feed valve back up once the system cools down and I have a chance to monitor it for a while, while it runs. Probably tomorrow morning.

Will provide updates as I go along - thanks all.

HVACNUT · February 2021

Well there goes my theory.

mattmia2 · February 2021

It shouldn't go from 0psig to 20 psig if your expansion tank is working correctly. It also shouldn't lose pressure with the feed off. If it is loosing water you need to figure out why because constantly adding fresh water will corrode the system quickly.

EBEBRATT-Ed · February 2021

@HVACNUT

LOL, most of my theory's are bad. None of us has x-ray vision LOL

cdvo871 · February 2021

Opened the feed valve back up this morning, gauge went quickly up to about 10-12 and held there. Once the boiler fired up it got up to about 20-22psi (which is normally what it typically gets to once the water temp is around 180F).

Will keep an eye on it over the next few days.

Thanks for all the advice.

mattmia2 · February 2021

Close the valve again, at least after you bleed the air out and make sure it holds the pressure with the valve off. if it doesn't, find where it is leaking or your boiler will have a short life.

Boiler pressure drop to 0 when off - fill valve closed

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