Baxi H80 pressure drop

[WornOutEngine]

Can anyone tell me what the cause of the following is please.

Pressure drops to Zero. No heating or hot water
Re pressurise to 1.5 bar.
Heating and water ok.
Pressure goes up to 2.5 - 3 bar.
Water overflows from outside overflow pipe.
Turn off C/H pressure drops to zero after a while as the boiler cools down.

Is it likely to be the expansion vessel?

 

[bazdaman]

Yep

 

[steve32]

Yep

+1

Common fault

 

[MANDATE]

Yes. Over time, air has been lost from the air side of the vessel. The lost aire has been replaced with water. This has left too little volume of air.
When compression takes place due to the expansion of water when heated the pressure goes beyond the 3bar needed to open thePRV.
Action required to re charge the vessel to 0.8bar, making sure the excess water taking up the air space is pushed out of the system.
Then set the water pressure to 1bar.
When heating is on, the pressure should rise to about 2bar.
If it reaches say 2.8 or 2.9bar then it ain't been done properly.
When vessel has been corrected you may still have a problem with the prv because they don't always close properly. Sediment on valve seat stops it sealing and leaves you with small leak.
Replacing prv is best option or cleaning it if you know how to dismantle it

 

[WornOutEngine]

Thanks Mandate,

Will take a look at that and pressurise the vessel but first how do I remove any excess water please?

 

[mogget]

Thanks Mandate,

Will take a look at that and pressurise the vessel but first how do I remove any excess water please?

Make sure the heating circuit is left open to atmosphere, whilst repressurising to 0.8bar at the schrader valve.

The higher pressure in the exv will displace the water out of it.

Instructions somewhere in the faq/wiki.