I recently moved into a house which has a really old natural gas fired hot water boiler (repco) The pressure on the gauge goes up to 40-50psi if I don’t manually let out some water from one of the zones.

The pressure relief valve seems to be blocked by the wall (maybe boiler moved closer to the wall over time?)

I recently replaced

  • the pressure expansion tank,
  • air vents
  • pressure reducing valve
  • pressure gauge on boiler

When I bleed the zones, the pressure drops to 12-15psi, but after 4-5 hours it goes up to 40-50psi.

If the pressure relief valve wasn’t blocked by the wall, I think there would be a constant flow of water coming out.

Does anyone know what would increase the pressure over time other than already replaced parts?

2 plumbers already looked at it but couldn’t figure it out.

  • 1
    Have you checked if the thermostat is working? Pressure increases with water temperature.
    – crip659
    Jan 7, 2023 at 20:03
  • 2
    boiler water makeup supply might be defective. Can you turn off the water make up valve and see if the problem continues? Jan 7, 2023 at 20:41
  • 1
    Danger Robinson, Danger...the pressure release valve should no be blocked.
    – Traveler
    Jan 7, 2023 at 21:11
  • The thermostat is working, water temperature doesn’t go above 170F, and the same problem happens even when the water supply valve is turned off.
    – John
    Jan 7, 2023 at 21:24

1 Answer 1


If this boiler also produces your domestic hot water, then there is a leak in the heat exchanger that is allowing water from your cold water supply to leak into the boiler.

Depending on how your domestic hot water is made, the leak is either internal to the boiler (aka tankless water heating) or in a separate tank (aka indirect water heating). This illustration shows tankless water heating, with the leak at the red dot.

enter image description here

The following illustration shows indirect water heating with a tank such as a Superstor that pumps boiler water through a loop in the external tank. The leak would be at the red dot.

enter image description here

(Images from U.S. DOE)

Test this theory by turning off the cold water supply to the tankless or indirect water heater or the house main water supply. The pressure rise will stop if this is the source of the pressure rise.

Of course if you have a completely separate water heater, then this theory is all wet.

And please arrange for a working pressure relief valve.

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