Alpha C27 Combi Boiler over pressurises once or twice a day then vents

[kez1]

System is gas central heating via a pressurised combi boiler system. The combi is a Alpha C27.

Often the user is operating only two radiators. The balancing radiator and one other. Operating 2 out of 7 rads in total.

Recently this combi has been de-pressurising once or twice a day soon after the heating is turned on.
After the heating is switched on from cold, with starting pressure of 0.75 - 1 bar, the system pressure rapidly builds up to above 2.5 bar, then vents.
0.75 - 1 bar is the manufacturer recommended cold starting pressure.
After water venting there is not enough pressure in the system for the system heating to continue working. So the heating circuit has to then be re-pressurised before operating again.
This cyle happens once or twice a day when the heating is being used.
User is re-filling to only manufacturers recommended operating cold filling pressure, no higher.

Boiler works in every other way. Running hot water is totally Ok. And radiators heat up to scorching very quickly before water venting.

Any ideas?

 

[Dandngassafe]

Sounds like expansion vessel has either gone or needs to be recharged

 

[ianmcd]

As above expansion vessel needs recharging

 

[kez1]

I've fitted a new 18 L expansion vessel to the Return (from toolstation), and
also fitted an expansion vessel kit (also from toolstation) with guage and exaust which has been piped outside.

The system still over pressures when the heating is on and has to be re-pressurised about twice a week. Ideas?

Also the red over pressure release valve on top of the new expansion vessel kit is leaky after gently releasing to pressure test the release, so there is now a bucket sitting undeneath catching any drips. It seems to dry up after a while. Can I fix this as taking the kit back is quite a bit of work?

I've yet to find out how to get to the boiler internal expansion vessel and re-charge this. When I get a moment I will do this also, if it can be pumped up and has not burst.

 

[DP]

Kez, is this your boiler?
You mention the ‘user’, who is the user, seems to be not yourself

While expansion vessels do fail, often your issue is due to neglecting regular prescribed service
Sometimes external Expansion vessels are fitted without good reason- have in the past pressurised boiler EV and ignored external vessel and boiler has functioned correctly

 

[kez1]

Hi DP,

User is 79 year old lady, not me.
No idea if it has been serviced annually.
I will ask her when I'm next there.

Are you saying I should find out how to re-pressurise internal boiler EV and that may fix issue?

 

[ServiceNumpty]

Is it actually over pressurising since you fitted the external expansion vessel, or are you loosing pressure slightly over the period of a week.

 

[ianmcd]

The Alpha C27 has a different heat exchanger to the more common combis, it is called a bi-thermal heat exchanger sometimes called a twin pass, it could be perforated, need more information from you though to determine this, turn the cold water inlet off and then put the heating on and see if it still happens

 

[DP]

Yes.

 

[McCombi]

You say rads are scorching hot, I noticed the thermostat in picture is half way, turn it lower and see if still scorching hot, I have a feeling it’s temperature related, ie overheating and not shutting off, but would expect overheat stat to kick in before this.

Not familiar on them.
Are you landlord or engineer?

 

[kez1]

Is it actually over pressurising since you fitted the external expansion vessel, or are you loosing pressure slightly over the period of a week.

It was over pressurising before I fitted the EXV.
I gather it looses pressure over half a week.

 

[kez1]

The Alpha C27 has a different heat exchanger to the more common combis, it is called a bi-thermal heat exchanger sometimes called a twin pass, it could be perforated, need more information from you though to determine this, turn the cold water inlet off and then put the heating on and see if it still happens

Not sure what you mean. The cold water inlet is the filling loop right? This is always off when heating is operating. Do you mean turn the supply/ stop cock off and run the heating / hot water, or something else?

 

[kez1]

You say rads are scorching hot, I noticed the thermostat in picture is half way, turn it lower and see if still scorching hot, I have a feeling it’s temperature related, ie overheating and not shutting off, but would expect overheat stat to kick in before this.

Not familiar on them.
Are you landlord or engineer?

I've had a feeling this may well be the problem, that it is heating and not shutting off.
Suspect also it could be a temperature sensor switch / the thermostat that is not turning the heating off at temperature.
I will turn the temperature on the stat down and see what happens.

I'm not a boiler engineer to trade, but I've done lots of engineering. Helping out.

 

[ServiceNumpty]

PRV?

 

[kez1]

PRV?

I've checked the internal boiler PRV. It seems solid, and the 15mm exaust pipe to the extrnal wall seems dry, though, it's high up. Next time I am there I will check for a drip.
The PRV on the new EXV I fitted from toolstation leaks a bit after it's tested. Talking it off and returning it will be a pain. I will check it's seating next time I'm there.