Erratic Pressure Greenstar

[steviecochrane]

Hi, i have a 2 year old Worcester Bosch 30si Greenstar Boiler and recently its been playing up, pressure has been very erratic.
First of all the pressure has been dropping to 0bar when boiler not in use, when running it will go to approx 2bar.I checked the EV and there was no water coming out just air, i checked the PRV line and it shows signs of water at the end of the pipe if it has been leaking so i proceeded to pressure up the EV to 10psi it showed pressure on the boiler guage of approx 1 bar, not sure if it should do this, ran the boiler and the pressure went to 3 bar.
Any suggestions would be appreciated on what the problem is......cheers

Stevie

 

[MANDATE]

Sounds like the usual problem of expansion vessel.
The vessel loses some of its 'air' charge over time, this reduces the 'air' capacity. This smaller amount of air when compressed sends the pressure way too high.
Normally water pressure would rise from 1bar to 2bar, but in this situation it more likely to go from 1bar to well above the 3bar and due to this the PRV opens (as it should)
There is more to correcting the expansion vessel, than just pumping it up to the correct pressure. The most important thing is the 'air' capacity.
The diaphragm has to be fully flexed so the vessel contains only air and no water. Its function depends on the air being compressed.
So there must be no pressure on the water side of the diaphragm when charging the vessel.
Its fairly obvious, the water pressure has opened the PRV at some time which leads to the second problem. Once opened the PRV does not seal again properly and you end up with a slight leak and pressure drops.
First you need to ensure the expansion vessel is functioning correctly and then replace the PRV

 

[steviecochrane]

Thanks for your response. i topped up the EV with no pressure on the boiler,i will get a new PRV, how would i know if the Expansion Vessel is faulty.

Cheers

Stevie

 

[MANDATE]

If the diaphragm was ruptured, there would be water on both sides of it and it would leak when you press the schrader valve.
The clue to its correct functioning lies with the amount of pressure increase. It should only be rising about 1bar so ideally if set at 1bar then it should go to 2 or maybe 2.2bar. (So it pays not to set it too high)
If it approaches 2.9bar/3bar (chance of PRV opening)and still rising then that tells you the air capacity is lower than it should be.
The PRV can be dismantled and cleaned, but just how effective it becomes is questionable.

 

[steviecochrane]

Thanks, i just looked at the FAQ on this site and followed the step by step guide for topping up the Pressure Vessel, now with the heating on it goes up from the 1 bar to 1.3bar, will check and see what the pressure does when switched off.

 

[MANDATE]

I would have expected the pressure to go higher than 1.3bar.
I assume some is being lost through the PRV, so with CH off it will most likely go lower than the original 1bar

 

[steviecochrane]

I agree PRV is probably passing, i have put a bag round it and see what it gathers. Looks a bit of a pig to change out.

thanks again

stevie