Combi Boiler Pressure Fluctuations

[Harry Singh]

Every year we get our boiler serviced and every year the engineer says the pressure is low (zero) I'll just top it up. On the odd occasion I've had a peak at the dial and topped it up myself. So it seems I might have had a problem for some years.

This year was different the engineer topped the system up but then talked about causes. We checked the PRV outlet and it was dry so not a suspect at that time; he said it might be a leak under the floor.

I ran the CH and next morning the pressure was down at zero. So, I topped it up and observed the dial and the PRV outlet. Quite soon the pressure was in the red zone and the outlet dribbling nicely.

I followed (as much as I could) the procedure to re-pressurise the expansion vessel and let the water in to push it up to 1 bar.

I ran the system for 45 minutes and it jumped 2 bar and threatening to creep into the red zone so I switched it off and it settled back at 1 bar so no water lost this time.

I think, however, if it runs for an hour say it might get into the red zone and trip the PRV and lose pressure.

What does a 2 bar jump point to? Could it be I didn't re-pressurise the expansion vessel properly? It was showing 0 bar when i pumped air to 0.7 (this showed on the pump and the boiler dial). I then opened the radiator drain point and zeroised the boiler dial. Next: 0.7 bar of air and a top-up of water to 1 bar.

The Worcester Bosch web site suggest the expansion vessel pressure needs fixing but I'm not sure what else I could have done. On the plus side the boiler is running much quieter than it used to – at least for that 45 minutes!

 

[terryplumb]

You havent charged the EV properly by the sound of it . when did engineer attend ,why didnt he do it ?

 

[Harry Singh]

The service engineers visit once a year - they've only ever topped up. When they service it maybe they have never run it long enough to see the pressure spikes. The latest one was a "pre-service" inspection so they should be back and I'll mention it to them then.

Maybe the water hadn't fully drained from the EV when I did the last charge but I'm not sure how I'd tell other than the pressure dial and a spurt becoming a dribble at the drain point.

Thanks for your help.

 

[terryplumb]

When charging the ev ,the system needs to be depressurised and open to vent ( like leaving a drain cock open ) the means of pumping air into the ev should incorporate a pressure gauge so you can ascertain the AIR pressure.as opposed to the system pressure on the boiler / system side. Probably best left to your engineer.

 

[Harry Singh]

Ah - the pump had a gauge but I closed off the drain cock between charges so it would have been sealed for the last charge. And yes, one for the engineer I think. It's a familiar story with my tinkering I'm very much a nearly man. On the plus side it sounds like it might be an easy fix when done properly.

Thanks again.

 

[MeldrewsMate]

I assume you have a pressure gauge to measure the EV air pressure.
The EV may have a large amount of water in it, so you need to allow water to drain from the system as you recharge the air side of the EV.
Charge the air side to 3 bar, and drain the system until the boiler pressure shows zero. Recheck the EV pressure. Repeat until the boiler pressure is zero and the EV pressure stays at 3 bar. At this point the EV has no liquid in it, just air.
Reduce the EV air pressure to 1 bar, only then do you top up the water system pressure to just above 1 bar.

The system pressure will increase as the heating temperature increases, but should not increase beyond the 'red line'. If it does your EV may have been specified too small.

I ran the system for 45 minutes and it jumped 2 bar and threatening to creep into the red zone so I switched it off and it settled back at 1 bar so no water lost this time.!

This statement tells me that there appears to be no problem with the EV, though it may still be too small for your system.

 

[Harry Singh]

I assume you have a pressure gauge to measure the EV air pressure.
drain the system until the boiler pressure shows zero.

The boiler side indicator was zero when I started the procedure does the 3 bar air charge force any residual water (not under pressure as the dial was zero) out of the EV?

 

[MeldrewsMate]

Yes, so if it has nowhere to go it'll raise the system pressure. The procedure I gave you is the way it is so that you evacuate any water from the EV before setting it's pre-charge pressure.