Loosing Pressure and Loosing Water Out The Pressure Relief Valve

castover

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.

Zman

Usually a failed expansion tank causes an overpressure. The relief valve opens and does not close until the pressure has dropped. The system refills and the cycle continues.
Usually...

the_donut

Are you see low pressure at trindicator? It could be trindicator is plugged up reading wrong pressure. Also pics of pump and feed water in relation to compression tank please.

castover

Zman, the expansion tank seems to be functioning to me. I can add water to the whole system and the pressure doesn't spike and when the system heats up the pressure stays pretty level. Also, I am measuring 12 psi on the expansion tank when the system pressure is near zero.

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?

neilc

where is your make up water?
isolate it?

the_donut

If thermocouple is bad valve will shutoff. If pressure is above 20 cold fill it could increase during heating. How much lift to highest radiator?

Jamie Hall

There should be an aquastat or some such device which should shut the boiler down if the temperature gets too high. I'm kind of liking your idea that the thing may be boiling -- but it's not the thermocouple on the boiler you want to check, it's the aquastat and any other high limit shutdown you have -- which you haven't mentioned.

castover

neilc, I added a notation for the make up water (between Pressure Relief Valve and expansion tank). When you say isolate it do you mean turn off the valve that supplies water to that valve? Good idea, could be leaking!

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?

HomerJSmith

I'm with Zman, it's your expansion tank. It may be at 12 psi, but connected to the sys while measuring it doesn't tell the whole story. It is the acceptance value of the tank that's important. I think that the tank is water logged because of the loss of air on the bladder side. It can still register 12 psi, but have no room for the heated expanding water so it exits out the only way it can, the PRV.

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.

HomerJSmith

You are fighting two pressures. The pressure regulating valve street pressure down to 12 psi and the X-tank air charge. They must balance. As long as there is some air in the bladder side, that air will be compress by the pressure regulating valve until the pressures equalize no matter how much water is in the tank.

the_donut

Air in the system would compress, not expand. Pressure set to 12 is fine with that height. Isolate makeup water like said before and verify air charge like instructed above. You could also check by depressing Schrader valve if it looks water logged, water should come out rather quickly.

I would also check over loop for blocked or pinched line or a turned shut off valve on return side.

kcopp

If your system has as much water content as you suggest a #30 extrol tank may not be large enough.... may want to try a #60.

HomerJSmith

Is this a new phenomena that's recently happening or something that was happening from the start.

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.

HomerJSmith

You say, "I have brought the system pressure down to close to zero and checked the pressure on the pictured Expansion tank is 12 psi."
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.

pecmsg

kcopp said:

If your system has as much water content as you suggest a #30 extrol tank may not be large enough.... may want to try a #60.

That's a lot of piping . Agreed larger X-Tank

HomerJSmith

A larger X-tank doesn't hurt. Just more expensive. A Bell & Gossett HFT-60, 7.6 Gallon is the next size up.

https://www.supplyhouse.com/Bell-Gossett-1BN328-HFT-60-7-6-Gallon-Hydronic-Heating-Expansion-Tank

castover

Homer, thanks for adopting me. Your right it doesn't make any sense. The X-tank is not waterlogged - no water coming out of the Schraeder valve - but, it doesn't seem to be doing any pressure regulation. When the system is operating at 12 psi on the Trindicator I can let a quart out of the pressure release valve and when the system cools off it will be 2 psi on the Trindicator. The pressure release valve is reliably letting out water at 30 psi - no problem there. The system worked for many years with this size expansion tank, so I doubt that is the problem. I'm investing $40 in a new expansion tank and will install it this weekend and start from scratch. I'll let you know how it goes. Thanks!

the_donut

How new are the radiant in floor zones? This started happening after they were installed right? When you caught the issue, had other zone valves been opened? If you run only the 1927 radiator zone, and other zones closed, do you see a pressure rise? What is the mixing valve temp for the in floor zone set to?

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.

castover

Donut, The system has worked well with this expansion tank for 10 years with the "New" in-floor zones in play. I'm pretty sure I would see the pressure spike if I closed off all the zones except the big zone. I'm trying that now. Mixing valve is at 130F, I think. To me it acts like there is something blocking flow to the expansion tank since it never changes pressure on the air side not matter what is going on with the water side (2 psi to 30 psi).

JUGHNE

Have you removed the tank from the system, held it in your hands and shaken it? Any sloshing of water? Then with it removed you check and set the air pressure.

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.

castover

Donut, I shut down all the other zones and closed valves at those pumps and just cranked up the zone with all the old fashioned radiators on it. The Trindicator said 95F / 7.5 psi when I started and an hour later is was 130F / 30 psi and it had barfed out a quart of water out of the pressure release valve. I then let two quarts out of the Pressure Release Valve and the Trindicator was 130F / 5 psi. And, during all this maneuvering the air side of the expansion tank stayed steady at 12 psi. It seems like that x-tank ain't talking to the system, but your the expert.

the_donut

If the tank is functioning right it should be able to handle close to 1700 gallons at 2.5 gallon displacement at that pressure and temperature change (26 psia and 95*F to 140*F).

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.

Jamie Hall

The expansion tank isn't connected to the system. Plugged pipe or fitting, closed valve, something of that sort.

castover

OK, I threw away the old expansion tank and put a new one in the same size and now the system is controlling pressure. There was no closed valve or obvious blockage leading to the expansion tank. I'm thinking the diaphragm was stuck to the top of the expansion tank and not allowing any water in - though, that sounds a bit unlikely. It's always something new! Another happy customer. Thanks guys!