Gas boiler pressure problem

[fizzy]

I am assuming the PRV?

Boiler loses pressure. Open the taps and get it into the green. Then a while later the needle is almost into the red. Starts dripping outside. Then later on pressure down to zero.
Is it just the valve or something else?
Tenant is pretty daft which doesn't help and can't get a picture what is going wrong.
The other day I went there. Electricity off for 2 days - in this freezing weather. All trips were fine. She said there was money on the meter. Wanted me to call her electric company and sort it out
Turned out she was in debt and hadn't put enough in to cover it

 

[Morrisman]

Totally uneducated on heating boilers, but I'd guess there's air in the system, that expands as it gets hot, puts the pressure up?

 

[Parm]

Nuclear Power as an option?

 

[Bornfree]

Probably the expansion vessel has failed

 

[OJCar]

Probably the expansion vessel has failed

This.

 

[voipio]

As above. It might be possible to pump the expansion vessel's air pocket back up. Just done that to ours, but it's not a combi system. Having said that, not sure why the water pressure should drop to zero. Do you have any pictures?

 

[Bornfree]

As above. It might be possible to pump the expansion vessel's air pocket back up. Just done that to ours, but it's not a combi system. Having said that, not sure why the water pressure should drop to zero. Do you have any pictures?

Without the air cushion the system overpressures when it heats up. the prv releases the excess pressure and when it cools down it drops to zero.

 

[fizzy]

Probably the expansion vessel has failed

I was afraid of that. As it is a gas boiler and in a rented house I will have to get someone in to do it - everyone seems fully booked though.

 

[fizzy]

As above. It might be possible to pump the expansion vessel's air pocket back up. Just done that to ours, but it's not a combi system. Having said that, not sure why the water pressure should drop to zero. Do you have any pictures?

Yes I did that before on our oil boiler. Won't risk it on a gas boiler for my tenant though.

 

[hunter27]

If it is the vessel. As a stopgap if you drain a large rad so it is full of air it will allow somewhere for the expansion to occur until the vessel is replaced but sometimes when a PRV has been unseated it won't re seat correctly due to dirt on its seat.

 

[addjunkie]

Probably the expansion vessel has failed

Agree had similar in mine last year, change prv and expansion vessel

 

[Blobber]

Just put an external pressure vessel on the pipe work near the boiler, anyone can do that and you dont need to touch the boiler

 

[Tim.S]

Losing pressure means its losing water, it coulds be the expansion vessel needs re-pressuring, the way to check is if the pressure rockets up when the heating or hot water has been on for a couple of minutes, but normally if theres no signs of leaking it will be the prv but even if its the expansion vessel it will probably still need the prv changing because when they've let go, which it would if the boiler over pressurises they rarely re-seat properly.

 

[RJY]

If the pressure relief valve has a problem and you can tell for sure by seeing it drain on its own, not just normal condensate from the condensate drain, there's the problem. If it's the expansion vessel, you are likely to see drips of see water coming from the fill valve on the side of the vessel if you depress the inner (just like a car tyre valve, note you may need to repressurise it if you let much air/water out). If the pressure has been increased sufficiently to cause the PRV to operate, it's likely that there is "stuff" stuck under the seat, which you may or may not be able to clear by causing it to operate briefly. It could also be caused by a leak in the pipework somewhere.

 

[addjunkie]

If the pressure relief valve has a problem and you can tell for sure by seeing it drain on its own, not just normal condensate from the condensate drain, there's the problem. If it's the expansion vessel, you are likely to see drips of see water coming from the fill valve on the side of the vessel if you depress the inner (just like a car tyre valve, note you may need to repressurise it if you let much air/water out). If the pressure has been increased sufficiently to cause the PRV to operate, it's likely that there is "stuff" stuck under the seat, which you may or may not be able to clear by causing it to operate briefly. It could also be caused by a leak in the pipework somewhere.

prvs lift quite often due to hydraulicing as there is no damping if the bladder is no longer pressurised. Once the PRV has lifted a few times it will cut the seat and never seal again

 

[Pigeon_Droppings2]

"If it is the vessel. As a stopgap if you drain a large rad so it is full of air it will allow somewhere for the expansion"

Mine broke last winter....no way I was paying for a plumber in the middle of winter in London!

I did exactly as suggested above...

Only problem is it worked so well I forgot to fix the vessel in the summer...

 

[Justme]

I was afraid of that. As it is a gas boiler and in a rented house I will have to get someone in to do it - everyone seems fully booked though.

Fit an external one if you cant replace the internal unit.