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Loosing Pressure and Loosing Water Out The Pressure Relief Valve


Folks, I've got a 1927 house with a mix of old radiators and some new in-floor heat zones. It's gotten into a death spiral lately where pretty much every day it will burp a quart of water out the Pressure Relief Valve and then the system will be low in pressure and the in-floor zones stop working. And, sometimes it just looses pressure even when nothing has come out of the Pressure Relief Valve. These seem like opposite problems.
I have brought the system pressure down to close to zero and checked the pressure on the pictured Expansion tank is 12 psi. I also went and bled the seven radiators in the system and there was zero air in any of them. Please help me understand what is going on.
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Albert Einstein
the_donut, Yes, after water is burped out of the pressure relief valve the trindicator shows low (~2psi) and after I add water to the system it goes up to 10 to 20 psi. So, it seems to work.
I have included pics of the system and a drawing of the system for your information. Help! I came home from work today and a quart of water had burped out of the Pressure Relief Valve again and the system was down to 2 psi and barely working, and we are in Minnesota, so there is plenty of winter left. I'm thinking of setting up some spy cameras to see what temp the trindicator says when the Pressure Relief Valve blows. Perhaps the thermocouple that controls the gas is failing and the water in the firebox is getting so hot it is boiling for a short while making the pressure skyrocket?
isolate it?
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
Mr donut, I see your point about filling to too high of a pressure. I'll double check I'm not doing that. It's a two story - so highest lift from bottom to top of 2nd floor radiator is about 18 feet.
BUT, I JUST CAUGHT IT IN THE ACT! The loop with seven 1927 style radiators - and some of them pretty big - was the only one running and the trindicator was up to 30 psi; whereas before that loop started heating up it was 10 psi. The trindicator was only up to 110F. This loop has several times more water than any of the other loops, so a lot more expansion going on. The air side of the expansion tank was 12 psi when the trindicator was at 10 psi and still at 12 psi when the trindicator was at 30psi (it seems like that should be changing, at least some). The pressure control valve burped a quart of water out and now that that loop with all the radiators is cooling down the trindicator is down to 2 psi. It seems the expansion tank is not doing its job even though the air side has air in it at a reasonable pressure (12 psi). Or, could there be a big blob of air in that loop that expands a ton when that loop heats up (I don't think so since I bled all those radiators just yesterday).
Should I change out the expansion tank. Or, what?
You can shake it and if it feels sluggish, it's water logged.
You can do this, when the boiler is cold, take a compressor and pump up the bladder side of the tank and watch the tridicator and when the tridicator reaches 25 psi, stop putting air into the bladder side. Open a hose bibb and release water until the tridicator indicates the fill pressure of the setting on the pressure regulating valve. Then repeat the process until you can't raise the pressure on the tridicator, which means that there isn't any more water in the tank to be pushed into the system.
If the sys pressure is 12 psi, set the pressure in the tank to 12 psi.
This will fill the whole tank with air and you will have 2.5 gal of system water acceptance.
I would also check over loop for blocked or pinched line or a turned shut off valve on return side.
You say. "...it will burp a quart of water out the Pressure Relief Valve and then the system will be low in pressure...". How can the system be low on pressure if the fill pressure regulating valve is open. It should reach (stabilize) at a set pressure of about 12-15 psi and not go lower than that without more fill water moving into the sys.
You need to watch the tridicator (T & P gauge) and see what the pressure is when the pressure relief valves start discharging water. It should be about 30 psi. Anything less than that indicates a faulty PRV.
The #30 tank has an acceptance volume of about 2.5 gals. If this is a new problem, then I would expect the tank size to be acceptable as it worked earlier. However, look at the tridicator and see what the cold sys pressure is and what the hot sys pressure is. The difference should not be more than 3-4 psi. If there is a big difference, it might mean you are at the very edge of the tank's acceptance capacity and a larger tank would be warranted. A Bell & Gossett HFT-60, 7.6 Gallon is the next size up.
This doesn't make any sense to me. You would have to shut off the boiler fill valve and then drained the water in the sys down to around 0 psi. The elevated pressure in the X-tank would push all the water in the tank into the sys. Then you measured the air pressure on the bladder side of the tank thru the schrader valve and the pressure measured 12 psi? I don't get it. There was no water coming out of the schrader valve, correct? This is important, no water, right?
One other thing, you do have the correct flow through the boiler and the boiler water isn't boiling forcing the water pressure up rapidly to 30 psi with the release of water out of the PRV and then a rapid collapse of pressure close to 0 psi before the boiler fill valve can refill the sys to 12 psi?
Shake the tank and if it's sluggish, it is water logged.
The X-tank is sized on two factors: the temperature rise of the boiler and the water capacity of the sys.
https://www.supplyhouse.com/Bell-Gossett-1BN328-HFT-60-7-6-Gallon-Hydronic-Heating-Expansion-Tank
Compression tank alone isn’t sitting well with me, unless the in floor added a lot of volume to the system, in which case the next largest size would make sense if this was a new issue as only seen when all zones, or most of the zones call for heat at the same time.
It is also possible that some cast iron rads were holding a head of air but still heating adequately. If you bled them or they somehow lost that air head then your little tank would have to pick up the entire expansion load. If this were the case then you may have to step up to the next size. Or drain down the system and regain the air head.
Drain system and remove compression tank. Check connection for restriction or blockage. It may have a pinhole leak, but if you ran shredder valve for a while and only air came out, I am leaning towards something else causing problem. You may want to watch entire cycle, or video it on 1927 radiator side.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch