Boiler losing pressure when cooling down, then overpressurize during heat cycle

[Jddiy]

Hi there, I have a hydronic boiler system in the basement with 5 rads on the first floor. It was out of commission for several years and I recently serviced it and brought it back online. I'm a DIYer for most things and am new to hydronic heat.I have a couple of issues I've noticed that I'm hoping to get some guidance on.

First, the boiler fill valve seems to be working well. It will keep the pressure around 14 psi when the system isn't running. But when the heating cycle begins, of course the water expands and the pressure eventually gets high enough to trigger the relief valve. Not flooding or anything, but I can always see some moisture under it when I check. This has me confused as I expected the boiler to maintain an appropriate water level such that the relief valve wouldn't be triggered so often, or at all.

Second, when the system cools down, and the water contracts, I can hear air bubbles coming up through the radiators. Again I'm confused here because this is a sealed system and I thought the negative pressure would not be enough to suck air in through the vent. But that's the only place I can think of where air is being introduced.

The last thing I'm curious about is whether it's normal for the boiler to always run until the high temp shutoff point. I expected it to boil at a stable temp long enough to satisfy the thermostat, but instead it goes through cycles of getting as hot as possible until the shutoff kicks in. This seems inefficient, but I'm new to boilers and wondering if this is normal.

Thanks for reading my questions, looking forward to any ideas or advice you can give.

 

[GrumpyPlumber]

There are tons of projects that are safe to DIY, just not boilers.

The "bubble" sound as described is concerning but likely a need to flush zones, that aside, I'd have a licensed gas fitter, HVAC or plumber take a look at, you'll sleep better at night.

 

[Sponsor]

 

[John Gayewski]

There should be no air. There should be no boiling. The pressure change is normal, but your reilef should not be tripping and there should be no negative pressure. You should have an air seperater that'll eject air from your system.

Yes a boiler from a cold start will overheat the cabinet inside of the boiler and shut off. There is a limit switch inside of your boiler that measures the temp near the gas valve. When temp gets too high it'll shut the boiler off to protect the wiring and controls internally.

 

[Reach4]

Your thermal expansion tank may have a problem. Air precharge should be 14 pounds, I think, since that's when your fill valve starts bringing in more water.

Normally thermal expansion tank air precharge is measured and set with zero pressure, but zero pressure might admit oxygen.

If the Schrader valve is down, and the tank has failed, water can come out when you check the pressure. The air pressure will read the max of the water pressure and precharge pressure. So if the precharge is way low, you cannot tell while the water pressure is 14 psi.

 

[Jddiy]

Thanks all for the ideas and suggestions. Really appreciate the pointer toward the expansion tank. I didn't check that when I did the maintenance and turned the boiler back on, it makes sense that it would be involved in equalizing the pressure.

I should also mention there is only one zone.

Still wondering about it hitting the high temp limit during every cycle. I'll have to check further into the manual to see if there are any other components responsible for regulating the temperature.

 

[Fitter30]

All boilers have a operating and a high limit. Operating 160 - 180° limit 200°. Tools needed to check expansion tank. 50lb tire gauge & a tire hand pump. If water comes out when schrader is depressed tank is bad needs replacement. With a cool boiler and all pumps off ex tank needs the same pressure as system.

 

[Jddiy]

Thanks everyone, the expansion tank was definitely bad. I should have checked that when I did the maintenance before firing the boiler up. Replaced it and everything is working great, no more air bubbles or water from the relief valve.

Getting the old tank off the scoop was a real bear. Involved a lot more than a pipe wrench lol. That thing was stuck in there real good.

I wanted to replace the scoop while I was at it but couldn't find one locally.

I think the thing I was asking about with the high limit is by design. I expected this unit to have a high, medium, low heat output depending on water temp. Seems that is not the case and it just goes 100% when it kicks on, and waits until either the thermostat says no more heat or the high limit is reached. After the high limit is reached it waits til a lower temp before coming back on.

Really appreciate the help!

 

[Fitter30]

Scoop is all cast iron no moving parts except the air bleed at the top.