Erratic pressure on Vokera Option Combi Boiler

[wen53]

Yes, I have the dreaded Vokera Option combi boiler!

The problem is that the pressure keeps dropping when the boiler is either off or when the heating stops cycling. Noticed that the red and yellow lights kept coming on so kept topping up the system. Eventually lowered the control panel to access the pressure gauge, topped up when cold and then switched the heating on, the pressure went right up past 3 bar so let a bit out to relieve the pressure. Once the heating had got up to temperature and stopped cycling the pressure could be seen dropping so checked outside to see if the PRV was leaking and nothing coming out. I have also checked all the pipes and radiators for leaks and nothing.

I watched the pressure gauge for a long time! I noted that when the boiler kicks in to heat, the pressure rises by about 0.5 bar but when it stops cycling it drops about 1 bar so eventually it gets so low the yellow light comes on.

I have searched the forum and I keep seeing 'expansion vessel' as the culprit. Is it likely the case in this instance or could there be any other explanation?

Many thanks

 

[glasgowgas]

if the water is 100% NOT coming out of the PRV, then it is coming out of your pipework somewhere

 

[wen53]

As I watched the pressure drop I checked the prv outlet and it was bone dry. There are no leaks, checked under downstairs floor and there are no wet patches on any ceilings.
When the heating is switched on the pressure rises right up to 3 bar so relieve the pressure down to about 1.5 - 2 bar and it rises again up to 3 bar. After doing this for 2 - 3 times the pressure stabilises but when the heating is switched off the pressure drops like a stone to 0.

I checked the pressure in the expansion vessel and it reads just less than 1 bar. If I need to repressurise it do I hold the prv open at the same time or do I do something else?

 

[WDIK]

heat ex?

 

[wen53]

would that account for the funny hissing noise I can hear coming from the boiler now and then?