Expansion vessel or relief valve?

[Chris Bartram]

I've had to top up my Alpha HE CB33 combi boiler (which is, I think, about 6-7 years old) every day this week, and prior to turning up the CH temperature, it was every few days. Obviously this shouldn't be needed.

Today I finally had time in daylight to check the pressure relief exit (which is in an old coalshed), and sure enough, it's dripping very slightly when the boiler is running. Each time the boiler has failed to start I've topped the pressure up to the middle of the green area on the indicator- about 1 bar. When running, it's hitting around 2.5 bar when fully hot, which seems a bit high to me, but I have to admit I cannot remember what is normal, and the manual is no help, apart from stating that all pipework etc should be able to handle 3.0 bar.

Can anyone suggest if this is normal, or has perhaps the expansion vessel failed? If it makes any difference, it's a 7-radiator system on microbore in a 2 storey house.

If that's normal, seemingly the relief valve must be knackered. I'll be getting my friendly local boiler chap to do it, but it'd be nice to be able to be clued up beforehand. It's no emergency, as it's working, but obviously I don't want to introduce too much water, diluting the inhibitor.

[John Rumm]

If you need to extra expansion room as a stop gap, bleed some water from a radiator and leave a pocket of air at the top.

[Chris Bartram]

Cheers, but it seems OK, in that I top it up each morning and it continues to work until it cools again overnight.

[harryagain]

The expansion vessel might just need some air. See if there's a valve on it. Foot pump does the job.

If it works OK for while & then you have to put more air in, the diaphragm in the pressure vessel has ruptured & you will need a new pressure vessel.

[Chris Bartram]

I've just looked at the manual again to see if a valve is present, and it actually does help- I missed this last time:

"If the system pressure exceeds 2.5 bar when operating at maximum temperature, the heating expansion vessel should be checked and re-pressurised, if necessary."

It does have a valve, but that suggests that it may be the relief valve, perhaps, as it's not exceeding 2.5 bar.

[Roger Mills]

AIUI, the relief valve shouldn't open unless the pressure reaches about

3.5 bar - so if it's opening at 2.5, it seems like it's faulty.

However, have you actually watched the system heat up, and monitored the pressure as it does so? Is it possible that it *does* get to 3.5 and then falls back to 2.5 when the relief valve has let some water out?

As others have said, it would be worth checking the charge in the expansion vessel. With the system cold and the wet side depressurised, the air pressure should be around 0.7 to 0.8 bar. You can check it with a car tyre pressure gauge, and top it up with a car tyre pump if necessary.

That may be all that's needed. However, if water comes out of the air valve, the vessel is shot!

[John Rumm]

The more times you let the blow off valve operate though, the more chance you get of it getting some crud in it and then refusing to seal again in future.

[harryagain]

You might also find the "tyre valve" itself is leaking air. (Quite unusual though) Check in the usual way with a bit of spit. (Always check the easy stuff first!)

[Chris Bartram]

I'm actually thinking that ship has sailed and the valve itself has failed, as well as /possibly/ the expansion vessel. Back in the summer/autumn, the filling tap was leaking and overpressurised the system a couple of times, and I think the valve is now failing to seat. Certainly it's a steady drip from the valve rather than a surge.

It will have to be an external vessel, unless repressurising it works, as there's not enough clearance above to get it out.

Following my initial post I found a bit in the manual saying operating pressure could be up to 2.5 bar when fully hot, and it's running just past that when the burner runs, so I'll turn down the CH temp control a bit to try to keep it below that.

[Chris Bartram]

Cheers both. Just cranked the temp up to max and let the boiler run; it got just past 3 bar before I turned the temp control back down, so clearly the vessel is not working. The manual states the relief valve opens at 3 bar. Manual states vessel pressure at 1.0 bar.

Think I might light a fire and let the heating cool down and see if I can reach the valve (it's accessed from above and the boiler is fairly close to the ceiling- within spec by 5mm at 240, but the manual states that if there's less than 450mm and the vessel has failed, you need an external one). I've got in touch with my friendly neighbourhood boiler chap.

[Chris Bartram]

As an update: the vessel was f***ed. New external vessel fitted, system now holding pressure and running below 1.5 bar even at full temp. Relief valve seems to be OK.