Can anyone help me with air in my central heating system?

[Mike Roberts]

I apologise for the length on this post in advance.
I have a noise that sounds like boiling water at the boiler a Veissman 100 WB1B on an open vented system. Once the boiler has modulated down after about 10 minutes or so it is a quiet as normal.
There is no Magna clean (or equivalent) installed.
The boiler is approx 7 years old.
The noise started March last year, there had been no changes to the system since the boiler was installed and has worked perfectly until this problem started.
I had the system powerflushed last June by the gas engineer who installed the new boiler and has serviced my gas appliances for many years. He didn't think that the powerflush was needed but I asked for it anyway to eliminate any corrosion or sludge that may be present. The noise persists to this day and now the boiler has started noticeably cycling which it never did previously, there is also bubbling that comes from one upstairs radiator when the pump kicks in. I have been bleeding a lot of air from this radiator once or twice a week.
I have a British Gas break down contract and asked them to take a look at it. The engineer said it was crud, he implied that the powerflush wasn't done correctly and needed doing again. They quoted £750 for that.
I had the pump replaced as a possible source of air as it was 9 years old but problem still persists. My gas engineer has had couple of attempts to cure this but I think is stumped.
I realise air is not good in a heating system, corrosion etc. I'm at a loss as what to do next.
Is there no one that can help me with this?

 

[ianmcd]

have you had the hoses changed in the boiler ?

 

[Mike Roberts]

Hi Ian,
Thanks for responding.
Could you elaborate about the hoses?
The boiler is a Veissman Vitodens 100-W WB1B heat only boiler on an open vented system.
Are you familiar with this type?

 

[wwwebber]

Powerflushing is an expensive waste of time in my opinion. I have always always used a cleaner, drained down and rinsed out twice then fill with inhibitor. Is the sound you are hearing kettling ?

 

[Mike Roberts]

It does sound like kettling yes.
I believe the power flush was done correctly so reckon the noise is caused by air getting into the system.
The problem is I don't know where.
With gas engineers having their busiest time of the year. I think I'll struggle to get someone to come for an indefinite time to find an air leak somewhere on my heating system. So it's down to me to try and find it.
As I said earlier I've had the pump changed incase that was the culprit but no improvement.
On a couple of occasions I've held a jug of water under the overflow above the F&E tank to see if any got sucked in when the pump started but the water level didn't change.
If anyone has successfully found and cured a leak I'd be happy to hear how they did it.
Any help would be appreciated.

 

[kirkyboy3]

Do you have any pressure drops on your boiler at all? - apart from when you've been venting air

 

[Mike Roberts]

Hi Kirkyboy3 the system is an open vented system so there is nothing on the boiler that detects pressure.
The system is not venting air it's sucking it in.