How can my boiler pressure be increasing when the heat is off

[MonkeyK]

I went down to my basement this evening to find a huge puddle around the boiler.
PSI on the boiler said 40, usually it is 10 or less - I drained it back down.
But the heat has been turned off for about a week now and baseboard water heat is all that we use the boiler for. The rest of our hot water comes from a standard tanked water heater.

What can cause this?
I was in the basement 2 days ago, I didn't think to check pressure then, but there was certainly no water on the floor. I asked my wife if she knew anything about it and she said she hadn't been to the basement today.

Normally when the heat is on, we need to add a little water every few weeks cause the pressure drops.

Has not been a good water heat year

• On a particularly cold day three months ago we had not topped off the boiler so it didn't kick in and we had a frozen spot in one of the pipes. Fortunately we were able to thaw the pipe and there was no apparent damage
  
• We also had the pressure relief valve replaced a couple of months ago when it got stuck opened after the refil valve was opened too fast.

 

[MonkeyK]

to make matters worse, I now see that the pressure relief valve that was installed was for 30psi. So I don't know how the boiler pressure got to reading 40psi.

 

[Sponsor]

 

[Jadnashua]

Does your boiler have an internal coil that was abandoned? If the water is still on, there may be a slow leak in it that is raising the pressure.

If there's a lot of air in your hydronic lines, and it got fairly warm, the air will expand lots more than the water, and that could raise the pressure...it would depend on how much the temperature rose.

The pressure gauge may not be all that accurate. You can double-check that by checking the air pressure in your expansion tank. While the system is full of water and under normal pressure, the air pressure in the tank will rise to the same pressure.

If you have an auto-fill valve, or even a manual one, that is leaking, the pressure can slowly rise.

There may be other possibilities, but those are the ones that come to mind right off the bat.

 

[MonkeyK]

Makes sense. I found a second valve after the refill valve and shut that overnight. Pressure did not increase by morning, so I opened it back up and over the next hour pressure increased again. I think that I can point to a leaky refill valve.

Thank you!

 

[Dana]

Makes sense. I found a second valve after the refill valve and shut that overnight. Pressure did not increase by morning, so I opened it back up and over the next hour pressure increased again. I think that I can point to a leaky refill valve.

Thank you!

That's really common, and there is no reason to use the autofill for anything other than a convenience.

But the fact that you have to "...add a little water every few weeks..." is an indication of a leak somewhere. It might just be an air-purging vent spitting a bit during heating cycles, but it's likely to be something else. These can be hard to chase down sometimes, especially if it's very small leak from the water side to the fire side of a cast iron boiler that only leaks at some higher temperature, with the leak water going up the flue never on the floor. Keep an eye on things, see if you can't figure out where the water is going.