Regular Auto Air Vent Failure, Any Ideas?

[stephens]

I have a Worcester Bosch 28i RSF combi, whole sytem fitted new December 2004 (not sure if it was flushed correctly on install.) The Auto Air vent leaked and was replaced after 9 months, then 7, after which it was cleansed for two weeks with Sentinel X400 and Fernox MB1 inhibitor added. The vent then leaked again after 12 months, 15 months and just now at 9 months! It never loses pressure unless bled, but when on in the colder months the bathroom radiator (which is always on to prevent short cycling) gets cooler at the top and is bled once per month (losing 0.1bar approx per 2 months.) The other radiators never need bleeding. It smells gassy (hydrogen) even though cleansed/inhibitor 3 years ago but looks clean when bleeding. Can anyone suggest what may be causing the vent failures and what to try next? Many thanks for any advice given.

[Mike Jackson]

AAV failure is almost always down to muck in the system. The presence of hydrogen when you bleed tends to confirm this. Your system needs a proper flush.

Mike

[stephens]

Thanks for that Mike. When you say proper flush, what would be appropriate? Should I powerflush (with something like Fernox F5 or even DS-40) to be thorough. Or would that be overkill in a system under 5 years old and would it be best to just to do the chemical flush again, with Sentinel X400, but leave it in a full 4 weeks this time? I would really like to sort it so there is no expectation of coming home to a pool of water on regular occassions! Thanks.

[Mike Jackson]

I find a mains flush tends to work best on a sealed system. Make sure you've got a drain off adjacent to the boiler on the flow pipe, assuming the filling loop is on the return. Put the F3 or F5 in and run for a few hours minimum. Check everything is hot top to bottom. Once you are getting an even heat to all radiators attach a hose to the drain off. Shut off the boiler open the drain off, attach the filling loop and turn on. Flush until the water runs clear. Now shut the isolating valve on the boiler flow pipe and continue flushing until the wtare runs clear again. Now shut all the rads bar one and flush until it runs clear. Open another rad and shut the first and flush until it's clear. repeat this for the whole system and then open up everything you've shut and give it a few more minutes flushing for good luck.

Now shut off the water, shut the drain off, add the inhibitor and refill.

Mike

[stephens]

Thanks for the info Mike and for taking the time to reply. Much appreciated.