HW issue with Vaillant combi

[DIYspanner]

I drained the CH off and while it was drained, I moved the expansion tank further into the cupboard so the door can fully close.

To move the expansion tank, I loosened the lower compression on the expansion tank's pipe in the photo and let the water drain out of the boiler. I got the expansion tank move in then reconnected the pipe. I basically just rotated the tank in towards the back.

With that done, I started to fill/bleed the CH and when done and pressurised to a little over 1 bar, I switched on the boiler but the HW output is poor. The gas starts to fire but stops.

Can anyone advise please?

 

[Johntheo5]

There could be air trapped in the boiler HEX, most have a automatic air vent (AAV) but this is generally shut after venting to stop it leaking water, you probably can't do this if it involves removing the boiler casing. Can/have you tried it on CH, it might shift the air from the boiler to where you can vent it from the rads.

 

[DIYspanner]

There could be air trapped in the boiler HEX, most have a automatic air vent (AAV) but this is generally shut after venting to stop it leaking water, you probably can't do this if it involves removing the boiler casing. Can/have you tried it on CH, it might shift the air from the boiler to where you can vent it from the rads.

Indeed, that solved it. Thank you!

I turned on the CH and ran it for an hour then tried the HW and all was well.

After 30 minutes running the CH, air was accumulated in the small upper floor radiator on the landing that also had to be bleed twice when I drained and refilled the system last year.

The water drained from the boiler to expansion tank pipe was quite murky compared to what drained from the radiators so may fit a drain valve to the pipe as an easy way to drain the CH water from the boiler which I did not do previously.