combi boiler - central heating - expansion vessel

[Stephen]

Hello,

I have a very old boiler, I suppose I should buy a newer more efficient one, but that's a big job and costs money, so while it works, I'm leaving it as it is. Only it's not quite working...

The highest radiator in the house, a small one in the en suite, keeps filling with air. I haven't topped up the heating loop with fresh water and I know I added inhibitor a couple of years ago.

I'm not aware of any leaks; certainly there are no puddles or damp patches anywhere. Could it be that I have a pinhole leak in a radiator and air is gradually getting in this way?

If I try to bleed the radiator, I know that it will depressurise the system and I will have to open the filling loop briefly to restore the pressure. Is this normal, or does that suggest the expansion vessel needs replacing?

I remember reading an old post here about expansion vessels but I can't quite remember what the signs and symptoms were if one failed.

The EV is at the rear of the boiler and is very difficult to get to. The valve is right between the boiler and the wall, so there is little clearance to get anything in to measure the pressure.

I would have thought if the EV was broken, as the water heated it would expand and there would be nowhere for it to go, so it would escape through the PRV, which I do not see happening. But if that's the case, that has nothing to do with air getting in the system, so should I stop worrying about the expansion vessel?

Thanks, Stephen.

[Jethro_uk]

If the expansion vessel is ****ed what happens is the pressure builds up until it triggers the safety relief valve, and you lose water. When it cools, you've lost pressure and need to repressurise.

Rinse and repeat.

(One way of confirming is to locate the safety valve, and put a bag over it. If it fills ....)

If it is the EV, don't waste any time trying to deal with the internal one. Just fit an external one, and be done with it.

I did this last year.

If you are messing around with your system, be aware that there's a high chance that flushing/refilling could dislodge some crud that will cause problems elsewhere. After I replaced my EV, an auto-bleed valve on the pump failed and pissed everywhere.

[TimW]

I used to find that when I opened the bleed valve on the top radiator air would be sucked into it not out. The same mystery leaking was in the boiler, always evaporated so no drips, but eventually revealed by a build up of salts around the leaking joints.

TW

[John Rumm]

Unless you have massive gas bills, a new boiler is unlikely to repay its installation cost in savings any time soon. So it not normally worth replacing a working system unless there are other justifications for doing so.

Inhibitor does get "consumed" with use (its oxygen scavenger gets used mopping up free O2 in the water), so a top up may be due.

Yup its possible - even really small leaks that essentially don't let any water out, can let air in.

Bleeding significant amounts of air out would usually require some refilling.

The symptom of a failed pressure vessel, is that you will see a significant rise in system pressure as the system heats up. On most systems the "cold" pressure will be around 1 bar, and it may rise to approaching 2 as it warms up. With a failed PV, you will see a much larger rise (say over 3 bar). If it get high enough the pressure relief valve will let some water out through the emergency pressure relief pipe (normally goes straight out through the wall). Then after system cools you will often find there is no longer adequate pressure, and it won't run at all until you top up.

See above, and also

If its not exhibiting tell tale signs, then its likely ok anyway.

You can always fix a sandwich bag over the outlet with a rubber band - to collect any evidence.

Probably. ;-)

[Chris Bartram]

Air (or other gas) will naturally collect there.

If the pressure guage reads normal, stop worrying.

Let the air/gas out. Top up so the pressure guage reads normal when cold. Run the heating. If the EV has failed the pressure will go sky-high and blow the relief valve. Put a bag over the end of the outlet to be sure.

As Jethro said, if the EV has failed, fit an external one.

[Jethro_uk]

To be honest, unless someone can explain in words of one syllable, I struggle to find anything good in the design of putting the EV in the most inaccessible place. For the job it does (plus ease of design, fitting and spares) it would have been just as simple to supply the (slightly slimmer) boiler with an external EV as standard. Especially when there are situations where the internal EV isn't able to cope with the radiators fitted.

I wonder if "that's the way we've always done it" plays a part ?

[John Rumm]

Modern Vaillant and WB boilers manage to do it sensibly (as might others). e.g.

(EV on the right with the Shrader valve pointing straight at you)

[Harry Bloomfield]

It happens that Stephen formulated :

The signs of a failed EV, are that you can fill and pressurise the system, but the first time it heats up - it will over pressure and blow the surplus pressure out of the pressure release valve.

The EV is just a bottle, with a flexible diaphragm across the middle, sealing the water from pressurised air. As the water expands, the diaphragm will compress the air. They use a tyre valve, on the air side and need to be blown up to 1 bar. If you get water expelled from the tyre valve, you know the diaphragm has split.

Pressure release valves them selves can be faulty and leak. Put a plastic bag over its outlet, to check them for leaks.

[Harry Bloomfield]

The idea will be one of - if the EV is installed in the boiler, then the manufacturer can be certain one has been fitted. No risk of the installer omitting an EV.