Bosch Combi Boiler keeps overflowing

[jammy]

This is the third time our Bosch Combi boiler has overflowed at the pressure relief valve.

The same thing happened on two previous occasions (in the last 3 years.) The expansion tank and relief valve were replaced both times. The inlet valve is set at 17 psi. The relief valve is rated at 30 psi.

I was able to stop the flow by opening and closing the relief valve and turning the handle a half a turn clockwise. The boiler is now reading about 10 psi and the flow has remained off.

Any suggestions for how to go about diagnosing this?

P.S. The house does not have a whole-house pressure-reducing valve, only one at the inlet to the boiler. A plumber suggested that this might be the cause. Could a main pressure spike override the boiler pressure reducer and make the relief valve open and stay open?

 

[Fitter30]

Take a pic of boiler and related piping including the pump and ex tank. The ex tank air pressure has to match water pressure with heater and pumps have to off and 100° or cooler water. Tools needed 0-50 lb tire gauge, hand tire pump or air compressor. If using a air compressor short bursts and check pressure. Valve opposite piping end. If water comes out steady stream ex tank needs to be replaced bladder is bad.

 

[Sponsor]

 

[jammy]

Take a pic of boiler and related piping including the pump and ex tank. The ex tank air pressure has to match water pressure with heater and pumps have to off and 100° or cooler water. Tools needed 0-50 lb tire gauge, hand tire pump or air compressor. If using a air compressor short bursts and check pressure. Valve opposite piping end. If water comes out steady stream ex tank needs to be replaced bladder is bad.

Thanks for the tip. Will do tomorrow.

I also spoke to the town Water Department tech who is very helpful. He mentioned that the pressure in our part of town can go as high as 90 psi!! Definitely need a pressure-reducing valve. Next project.

 

[Reach4]

From what I have read, there is usually a valve in series with the auto-fill valve. If you close that valve, it will stop the effects of a leaking auto-fill.