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Discuss Boiler pressure dropping to zero when heating turns off. in the UK Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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Good afternoon,

I am experiencing a pressure issue with my heating system and was hoping to receive additional insight as to what the problem could be.

We have a biomass boiler and stratification tank set up and recently had an issue with boiler pressure dropping off when the system was cold. I checked the PRV and there was no evidence of it releasing water and then checked the boiler's expansion vessel which had failed. Ordered and fitted new vessel (checked pre-charge before doing so) and alas the problem still persists! (The old expansion vessel was definitely faulty).

When the heating system cuts out at temperature and the circulation pump stops, the pressure drops from around 1.6 bar to zero. The new expansion vessel is still charged, the PRV is not bypassing but the pressure still drops down to nothing. It is a 2 pipe system feeding 12 radiators.

The only possible solutions I can ascertain are:

1) the pipe leading to the expansion vessel is blocked
2) there is a massive leak on the central heating side of things - although there is no sign of this so far.
3) I have the Grundfos Alpha2L circ. pump set on the incorrect settings (currently on proportional pressure 2 PP2)

Basically, I was wondering if there are any glaringly obvious issues that I have seemingly overlooked before I start lifting floorboards!

Any assistance or insight would be hugely appreciated.
 
Faulty Pressure Gauge?

Do you have to re-fill it every time?
Does the boiler not start due to low water pressure?

Depending on the situation, you may not have a fault
 
Faulty Pressure Gauge?

Do you have to re-fill it every time?
Does the boiler not start due to low water pressure?

Depending on the situation, you may not have a fault
Thank you for taking the time to reply.

It could in fact be a faulty pressure gauge as the boiler was operating this morning despite the low pressure reading on the gauge. We were previously filling it each morning and bleeding air from system.
 
Sounds like the expansion vessel needs servicing and the prv needs replacing
 
Sounds like the expansion vessel needs servicing and the prv needs replacing
Thanks for the input!

Could I enquire why you believe the expansion vessel would need servicing when it is brand new and also why the PRV would also need replacing when it is not bypassing. These are genuine questions by the way, I am not trying to be flippant.

I am a mechanical engineer with limited experience of heating systems :)
 
Thanks for the input!

Could I enquire why you believe the expansion vessel would need servicing when it is brand new and also why the PRV would also need replacing when it is not bypassing. These are genuine questions by the way, I am not trying to be flippant.

I am a mechanical engineer with limited experience of heating systems :)

Pressure shouldn’t rise 1.6 bar with an expansion vessel fitted 1 bar at most im guessing you had water out of the expansion vessel pipe eg return pipe ?

Unlikely for them to block as normally 15mm

Try filling the system to 1 bar and watch the pressure on the guage
 
Test the pressure in the expansion vessel - with the water drained from the boiler.
I always check pressures on expansion vessels when replacing.
In fact, I always check pressures on expansion vessels on new boilers before filling the system with water.
 
Pressure shouldn’t rise 1.6 bar with an expansion vessel fitted 1 bar at most im guessing you had water out of the expansion vessel pipe eg return pipe ?

Unlikely for them to block as normally 15mm

Try filling the system to 1 bar and watch the pressure on the guage
Sorry, the pressure didn't rise 1.6 bar though. It fell from an operating pressure of 1.6 bar to zero static pressure when the boiler and circulation pump turned off. I will refill tonight and observe.
 
Test the pressure in the expansion vessel - with the water drained from the boiler.
I always check pressures on expansion vessels when replacing.
In fact, I always check pressures on expansion vessels on new boilers before filling the system with water.
With the system drained, the static pressure of the expansion vessel is 1.5 bar (it's pre charge value). The system operating pressure is supposed to be between 1.2 and 1.6 bar ideally according to the manufacturer's literature. Yesterday, the boiler continued operating despite the low pressure on the gauge however, the central heating flow and return remained cold until i topped up the pressure to 1 bar.
 
That means you have a leak as the system doesn’t drop to zero from a pre fill of 1 bar

When you fill at 1 bar then turn the heating on what does the pressure rise to ?
 

Reply to Boiler pressure dropping to zero when heating turns off. in the UK Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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