Combi boiler question

[rocket scientist]

I've noticed for quite a while now that my combi boiler drops pressure now and again and I have to top the water up so that the pressure is around 0.5-1bar (as stated in the manual) . However after running the central heating for a while I then notice that the pressure has risen to just within the yellow portion of the pressure gauge (over 2 bar).

I can't find any leaks around the boiler area or the rest of the heating system (there are only 5 radiators as it's a very small cottage) so I'm wondering if the pressure side of the system could be leaking through the heat exchanger into the flow side of things?

My main question is though, why does the pressure rise so much? I can understand it rising a bit due to expansion in the sealed system but not by that much. Or is that normal?

 

[Tedlooney]

It's normal for the pressure to rise when the heating is on as water expands when heated. My Worcester Bosch combi boiler rises by about 0.5 to 1 bar when the heating is on which is green on my dial.

 

[needsabiggerfuse]

This guy runs on a bit (he's got a lot to get in actually), but I've found his advice to be sound. DT_Xtremez_30:

https://www.youtube.com/watch?v=4meZgMAUecw

 

[Teh Wal]

That's probably the best video I've seen that explains things in laymans terms DT_Xtremez_14:

Just for info... my expansion vessel needed replacing at around about the 9-10 year mark from new, much the same symptoms that you're describing RS. Mine's a pretty standard combi-boiler, Worcester Bosch Greenstar R35 HE Plus, that was installed just over 12 years ago. There were a few of my neighbours who also had the vessel replaced between the 8 - 10 year mark too, we all live on an estate where the houses were thrown up at around about the same time.


As a bit of aside... I've recently had the main gas valve in the boiler replaced too: on moving to a different gas provider (SSE) they sent their own man out to do an initial boiler survey. He discovered there was gas coming out of the external vent - my missus was convinced she'd smelt gas from there for quite some time but like the good man that I am I'd ignored her! DT_Xtremez_42: The check he used was something he called the "blip test"; when we told him that the boiler had only been serviced by Scottish (British) Gas 6 weeks beforehand he was astonished that they hadn't discovered the gas leak as the "blip test" was something that British Gas had invented/introduced! So, he closed us down! Actually physically cut the gas supply pipe to the boiler and switched off the supply valve; this left us without heating for a couple of days until he replaced the valve.
Anyhow I got him to leave the old gas valve with me and eventually I stripped it down. I discovered that the reason it was leaking was because the faces and seats on 2 internal rubber valves were coated with impacted dust - obviously the gas isn't particularly "clean" - once I cleaned the dust off they sealed perfectly (a manual blow through check before and after I stripped it down proved it).
It should have cost 240 quid to replace it but by a strange quirk of fortune SSE covered the repair bill otherwise BG would have received a big style snotogram!
Gotta say that the SSE customer service people and the boilerman were brilliant, some of the best service and customer support that I've received in the past 20 years if I'm honest.

 

[Shiny10]

Ex Gas Fitter

Ex Gas Fitter

In only takes a small leak for the pressure to drop in a Sealed System. These leaks are normally on a Radiator Valve and when the heating is in use the water evaporates before it can be seen. I used to wrap RAF Blue Paper towel around the valves and return a week later and any leaks would show up as white staining on the Blue Paper.

Another area to check is the Safety Valve outlet which will be outside. Crud gets under the seating of the valve which can be cleared or, normally, the valve replaced (15 minute job).

The last place is the Air vessel which contains a rubber diaphragm that deteriates with age. To check this let the system go cold and then observe the pressure gauge as the system is turned on. If the pressure rises rapidly the Air Vessel is f****d and needs replacing. The system will go to 'Lock Out' if the pressure goes too high and the Safety Valve will open.

If it's not any of the above

 

[rocket scientist]

Thanks for the advice - I'll have a look at that video when I'm not on DII.