Vaillant boiler cuts out repeatedly

24 Mar 2008
Reaction score
United Kingdom
Hello. Its a Vaillant ecomax 635, about 3 yrs old. Its cuts out most days with error codes that translate to overheat/air blockage. I have heard what happens prior to cut-out; a boiling (bubbling/kettling) noise emanates from the boiler. Various corgi engineers have claimed to diagnose the problem, and have attempted to bleed air through the boiler pump central screw, and subsequently by draining down and refilling the system, with no effect. The problem is most likely to happen when 2 of the 3 central heating zones are turned off. When I restart the system (by pressing the ignition button) there is always a gurgling noise as if gas bubbles are being flushed through the boiler, even if the boiler has been allowed to go cold (which makes me think they are not steam bubbles). There is rarely any gas to bleed off from the radiators though. I have tried a descaler, and the system has rust inhibitor added. Have emailed Vaillant UK with details but get no reply. Any suggestions welcome
Sponsored Links
Have any of these 'engineers' actually used the purge program on the boiler P0?

Has the AAV cap been confirmed it is loose?

Has the EV pressure been checked, see FAQs?

Have they checked the auto bypass is functioning correctly, or indeed set correctly for your system?

Is it always the same zone working on its own when the fault occurs?

Has the burner co2 % been checked and set correctly?

Has the boiler been range rated for your system?

These are a few of the simple things that should be checked, before indulging in thinking it is a major fault
i think its air near the heat exchanger but i have drained it down twice and bled the pump
Sponsored Links
I can never understand how anyone can imagine that draining anything will solve an air problem.

Dave has listed all the things to be checked with a problem like this.

The original post was almost certainly a problem with the auto bypass, either not correctly set or missing!

Tony Glazier
The 'problem' in all these cases is that the appliances are not commissioned correctly, and if further 'services' are carried out they are strangely by people who stab at problems and mis-diagnose, ignoring the basics.
Commission a boiler?

Any Installer knows that you just bang them on the wall, turn them on and dont adjust any settings!

Then you just copy the specifications onto the Benchmark Certificate if you have a fussy customer.

For a really fussy customer you sometimes have to buy a bottle of inhibitor and peel off the label and stick it on the side of the boiler. You can sell off the bottles ( without labels ) on Ebay !

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local