What could be causing bubbles in heating after newly installed combi?

[Strebor]

This is a weird one, for sure... I had an old Vaillant combi replaced by a new Worcester Greenstar 4000 by Boxt.

It was as simple a job as you can imagine - direct replacement... my house is a simple 2 up 2 down semi - nothing remotely complicated.

However for the last two months since it was done, I've been getting small amounts of bubbles ending up in one of the upstairs radiators. It's worst after the heating has been off over night but also get some bubbles during the course of the heating being on too. Pretty certain it no longer could be residential air from the replacement job.

The Boxt engineers have been back three times, and they've bled the radiators and added some sealant but they are at a loss. as to what it is - it's also not frequent enough to be easy to see it happening when someone is only here for a few minutes.

I'm pretty certain there are no leaks and the pressure is not dropping - it is about 1.1 bar cool and goes up to about 1.4 bar hot.

I'm aware that gas can be hydrogen, the result of oxidisation, but I doubt that - it doesn't burn when I bleed the rad and it didn't happen before the replacement, although this new boiler definitely runs hotter than the old one.

I've tried opening and closing the automatic air vent at the top of the expansion vessel - no difference.

All I can think is that there is something wrong with the new Combi itself that is somehow sucking a little bit of air into the pressurised system

Are there any experts who can help me - at least tell me what I should tell Worcester so that I can get them to fix it - I doubt they'll come out unless I can be a bit more sure it is the boiler.

Very much appreciated!

 

[ShaunCorbs]

It could be just air building up as it’s quiet common a month after to still get air as it’s brand new water in the system

 

[Strebor]

It could be just air building up as it’s quiet common a month after to still get air as it’s brand new water in the system

OK. But this was now over 2 months ago (end of November it was done) - it's been consistently bubbling all that time. Could that still be residual air? Seems unlikely to me?

 

[ShaunCorbs]

Nope what do you mean by bubbling can you hear it or you just need to bleed the rad ?

Also was any flush / cleaning of the system done ?

 

[Strebor]

Nope what do you mean by bubbling can you hear it or you just need to bleed the rad ?

Also was any flush / cleaning of the system done ?

Yes I ca hear it. Yes I’ve bled it a few times now and also I’ve tried to light the air coming out of the rad to see if it’s hydrogen from oxidation. The system was not flushed when the combo was replaced but I didn’t have this problem before.

 

[SJB060685]

Any plastic pipe work in the system? Does this air build up while the boiler is off for some time?

 

[Strebor]

Any plastic pipe work in the system? Does this air build up while the boiler is off for some time?

No plastic pipe work. Yes it’s worse after it’s been off overnight and comes on first thing.

 

[ShaunCorbs]

I’m leaning to corrosion as you say it’s worse when it’s been standing overnight eg boiler off

 

[Strebor]

I’m leaning to corrosion as you say it’s worse when it’s been standing overnight eg boiler off

Yeah except I didn’t get this problem for the 8 years I’ve lived with the old boiler - it’s just happened since the new one (all other pipe work and rads are the same)… plus, like I say, I can’t light the gas which I thought would be possible hydrogen off corrosion. It’s had a lot of inhibitor too.

 

[ShaunCorbs]

Try and leave it a day or two then try capturing some and see if it will safely flare off

 

[Strebor]

Try and leave it a day or two then try capturing some and see if it will safely flare off

Like I said I’ve tried this a few times now. I can’t light the gas in the rad. It is not flammable, which points to it not being corrosion?

 

[ShaunCorbs]

It’s points to it not being hydrogen still could be corrosion

 

[Chuck]

Yeah except I didn’t get this problem for the 8 years I’ve lived with the old boiler - it’s just happened since the new one

The default pump setting on Worcester boilers is normally 'maximum', which should be reduced to the minimum that gives satisfactory circulation by the installer. It's quite common for the installers to skip this step, which may be why you are now hearing noise (e.g. turbulence) when you didn't before. It would be worth checking the pump setting.

The pipework may have an 'inverted-U' section in it somewhere. E.g. a horizontal section that is bowed upwards or some other form of 'trapped volume'. This could give you a reservoir of air that is slowly being dissolved in the CH water and then transported around the system. If so, it could take months to disperse.

"All I can think is that there is something wrong with the new Combi itself that is somehow sucking a little bit of air into the pressurised system"

Seems unlikely to me. To "suck air in" the water at the site of the leak would need to be less than 0 bar (gauge pressure). Assuming you are measuring your '1.1 bar' (gauge pressure) at the boiler any such leak would need to be several meters above the boiler. In any case, a leak that is large enough to let air in when the pump is running would likely result in loss of water (and hence pressure) when it was stopped, which you don't seem to be seeing.

My guess is that if you check the pump setting and then leave it a few months it will resolve itself.

 

[Scott_D]

Might be a dead leg if pipework somewhere holding air.
Could try fitting a deareator

 

[king of pipes]

Any pictures of the install ?? Configuration of the pipework can cause issues like these the most common being the flow and return crossed ? . Kop