GB142 Low Pressure

dgkula

Hi,

Yesterday we lost heat and the error indicated the fuse in the UBA3 had blown. I removed the UBA3, checked the current fuse continuity, determined it was blown, checked spare fuse continuity, it was good and installed the spare fuse and it started right back up. I will say that whoever designed the spare fuse holder is a genius.

This morning I am getting low pressure errors. Pressure is usually between 19 - 22 and it was down to 7psi. The analog pressure valve on the manifold agreed roughly with the pressure reading on the GB142 (maybe it was 10psi and the GB said 7psi). Bizarre - I dont have any leaks that would result in that type of overnight drop.

I tinkered with the pressure regulator on the loop supply, increased the pressure and restarted the unit and the pressure climbed and it was fine. Later in the day I had the same problem, back down to 10psi and the analog gauge on the manifold agrees.

I just changed the pressure regulator to get the pressure back up to around 30psi (GB agreed at 27psi) and closed the supply ball valve. other than the boiler, zone valves and hot water tank, the only thing on this loop is the expansion tank.

Is it possible that my hot water tank has a perforation in the loop and is leaking into the domestic hot water lines? Anything else I should consider? I dont see how pressure in the loop can just drop - I have checked all the radiators and dont see a leak ...

pedmec

if your hot water heater is an indirect fired water heater and the coil has sprung a leak then the boiler will increase in pressure and the boiler relief valve will discharge, most likely weeping. the domestic hot water will be at a higher pressure then the pressure in the boiler so the water would go from the water heater to the boiler.
did you take the cover off the boiler? under the cover is an air vent that could possibly be leaking. check the obvious first

dgkula

My son is taking a shower; he is 30 mins in (he has OCD and often takes very long showers).
At the start of the shower the analog pressure gauge on the manifold showed 29psi @ 172F
Now, 30 mins in the analog pressure gauge shows 27psi @ 122F.

So in 30 mins, a loss of 2psi and a temp decrease of 50F. I know that PV=nRT for a gas but dont know the formula for water but pressure must be proportional to temp or there wouldnt be an expansion tank I think.

Anyone have any thoughts?

dgkula

My son just finished the shower after another 10 mins: 26.5psi @ 110F. So s tiny further pressure drop that seems to correlate to the loop water temp drop. The indirect water heater doesnt seem to be the cause.

I will check the air vent but if it was leaking enough for the pressure to drop over 10psi I think I would see water on the floor and I do not. I think the air vent you are referring to is the vent that is used when filling the boiler, it lets the air out and always seems to leak a very tiny bit, drying before dripping onto the floor.

Jamie Hall

The pressure/temperature relationship for water is very different than the one for gas, since water starts with a large finite volume and an ideal gas (which is the law which you quoted) has a volume of zero and absolute zero (absolute temperature -- Rankine or Kelvin -- must be used in the formula). The thermal expansion coefficient of water is 0.00021 per degree Celsius at 20 Celsius. It's not linear, either -- water expands faster at higher temperatures. On the other hand, the container will also expand -- but not as much. A very rough rule of thumb is that water will expand -- if allowed to do so -- about 2.5% for the temperature rise between 40 and 140 Fahrenheit. Now if it isn't allowed to expand freely -- such as in a closed heating system -- the pressure will rise about 4 psi for every degree Fahrenheit in water temperature.

Or to put it much more simply -- if the pressure only changes 2 psi for a temperature change of 45 degrees, you have to have a working expansion tank on that closed system.

This is one very real reason why a pressure measurement on a closed system with an expansion tank absolutely MUST be done at a known, constant temperature. Pressure measurements as a means of detecting a leak are almost useless if there is an expansion tank present.

So...

To find a leak, if you are suspicious of one, turn off the boiler. Let the system cool (if possible, continue circulation to cool the whole system down. Using the inlet valve, bring the pressure to a desired value in range. Close the inlet valve. Close the valve to any expansion tank. Observe -- keeping the system temperature constant. If there is a leak, the pressure will drop significantly -- observably -- over the next hour or so. If it does, now you can chase the leak by isolating sections of the system, and repeating.

dgkula

Thank you Jamie for your detailed reply. I am running the boiler now with the cover off to reheat the indirect hot water tank. I am not seeing any leaking from the boiler itself. The temp is up to 194F but the pressure has dropped to 25 psi. So that is a further drop while the water temp has gone up a lot. 

I have turned the ball valve on the supply before the expansion tank so the items in the loop are: expansion tank, boiler, indirect hot water tank, zone valves and radiator. I do have the ability to take the expansion tank out of the loop with a ball valve so maybe that will be my next try. 

Could the expansion tank be causing the behavior I’m seeing?

dgkula

While I was typing that last message the pressure dropped to 21psi. The boiler is still running heating the indirect hot water tank. This is bizarre

dgkula

sorry it switched to zone heating and some zone valves opened not sure if that could account for the pressure drop. I don’t think so as the valves are only on the return side. Could I have air in a zone?

Jamie Hall

Yes you could have air in a zone. The only real ways to check what is happening are the approach I mentioned -- or if you have a water meter on the boiler makeup water line, observe if water is being added over time. A hot water system should not use ANY water over time, once it is filled and charged.

Widely varying pressures with no water added are almost always the result of a failed expansion tank. Steadily dropping pressures are usually a leak somewhere, if the makeup water is shut off (if the makeup water is on, you may see oddly varying pressures, as the pressure reducing valve interacts with the system pressure and temperature in somewhat unpredictable ways).

dgkula

All calls for heat have been satisfied and the pressure is back up to 27psi @ 175F. Looks like normal operation. I’ll run it for a few days with the ball valve closed that disconnects the loop from the cold feed, backflow preventer, and pressure reducer. If I don’t have any issues I’ll open the valve and see if reintroducing those items causes problems. Appreciate the input.

dgkula

@Jamie Hall I didnt see your last comment when I posted the above. I do not think it is a leak. I just ran the boiler with the cover off and no leaks not even a tiny drip at the boiler. I walked all the radiators - inside the house: a leak to drop 10 psi would be a lot of hot water I think and would be noticeable and everything is dry. After disconnecting the boiler makeup line the pressure swings I saw just now while in DHW and zone heating mode seem "normal" and the boiler returned to about 27psi which is normal. It never went below 20 psi - at the start of this process it was at 7 psi which was triggering the code.

My system has run for the last 10 years with the makeup line on which includes both a backflow preventer and pressure reducing valve in series. Turning that line off and removing those components from the system seems to have returned the system to "normal" operation. I do still have the expansion tank in the loop and so if I still see strange behavior I can take that out of the loop too with a ball valve.

When you say that "Widely varying pressures with no water added are almost always the result of a failed expansion tank," what would you define as "widely varying?" If my loop is going from 27psi to 20 psi and back again with no makeup line in does that fit inside a "normal" range or should I have my expansion tank checked?

Thanks you again for your feedback!

Jamie Hall

Leave the expansion tank in the loop! Without it you will see disastrous pressure swings! Take it out only if you are NOT going to run the system and are looking for a pressure drop!

dgkula

Appreciate that clarification!

dgkula

Had the ignitor fail, just replaced it. When I started it up the condensation trap started filling rapidly. Looks like the HX has failed ... that would explain the pressure drop for sure.

dgkula

Here is what a failed HX looks like

https://1drv.ms/v/s!Arn2MjygvR9ciuAHY63fQJbfIuWpWw?e=YZWccq

pedmec

not saying you don't have a hx leak but gb series boiler have excellent heat transfer (aluminum heat exchanger) rates so they do create a lot of condensation under normal operation.

what do you have for a circulator?

dgkula

With the unit turned off and the lower condensation plate removed I have a steady trickle of water from the lower right hand side of the HX. I have the standard Grundfos circulator in the manifold.