F.20 after refilling central heating system

Today I partially emptied the central heating system (1st floor and loft) so that I could disconnect an old towel rail and install the pipe work ready for the new towel rail. All went well and I have two capped pipes sticking out of the wall where the new towel rail will be fitted.

Having re-filled the central heating system, the boiler is playing up and refuses to heat the water or the radiators. I noticed it was giving an S.54 message, but after turning off and back on it is now displaying F.20.

There's a lot of splushing sounds going on in the boiler, so I guess there's some air in the system, but the pressure is at 1.4 bar.

The boiler is a Vaillant ecoTEC plus 637.

Any thoughts on what the issue might be and how I can get it back up and running?

So, I found an article about running the P.0 test purge program on vaillant boilers after the system has been re-filled. Although the article related to F.76 fault codes, the advice for running the test purge program was to purge air from the boiler whenever the system had been emptied and re-filled.


Unfortunately it didn't make any difference. Tried turning off and back on again but I can now hear "kettling", so I've turned it off and will have to wait until Monday to get my heating engineer out to look at it unless someone can suggest something I can do to clear the air from the boiler.

There is an air vent inside the boiler just above the pump that needs to be opened (looks like the same cap you find on tyre valves) but as you have to remove the front cover of your boiler I could not recommend that you try this.

Thanks picasso. I am aware of the automatic air valve and I did check it (although I shouldn't) and to my surprise found it was screwed fully down! Not sure how or why as only my heating engineer tampers with the boiler internals. I unscrewed it 1 turn and some air and a bit of water did splutter out, but it didn't resolve the issue and I continued to get F.20.

I tried running the P.0 test program a few more times and also turned down the heating/hot water temperatures to try to force the pump to run without heating the water. I also increased the pressure from 1.4 to 1.6 bar. Eventually, the boiler stayed on for longer and longer and now appears to be running okay and is heating the hot water cylinder as this is priority.

Thanks for taking the time to post.

Milleniumaire said:

I am aware of the automatic air valve and I did check it (although I shouldn't) and to my surprise found it was screwed fully down!

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Lots of engineers screw them down after initial filling of the system and open them again when bleeding, I know they are supposed to be automatic air vents but they leak for fun and cause serious damage on some boilers, never seen a decent make, they are all cheap rubbish

ianmcd said:

Lots of engineers screw them down after initial filling of the system and open them again when bleeding, I know they are supposed to be automatic air vents but they leak for fun and cause serious damage on some boilers, never seen a decent make, they are all cheap rubbish

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My ecotec 637 plus boiler is now 10 years old (how time flies) and the auto air vent was changed in February 2017 at the same time as the pressure relief valve as it was thought to be causing the issue of water trickling out of the overflow. Later it was determined it was the built in expansion vessel that needed to be recharged and, in fact, an external vessel was also installed, so not really sure if the AAV was faulty or not. There was a noticeable trail of scale leading from it, so it appears that water had been very slowly leaking from it but I also noticed last night that when I unscrewed it and air came out, so did a little bit of water.

The 637 has been absolutely brilliant, with most of my earlier pressure issues being caused by the internal expansion vessel. Since the external vessel was added I haven't had a single F.75, which just goes to show how important it is to size the system even with a large boiler like this. I keep forgetting how old the 637 is and inside the case everything looks brand new!!!

Last night is the first issue I've had for a while, but I've now learnt that I need to perform the P.0 test purge program prior to starting it up after the system has been drained and refilled. I'll add that to my notes