Baxi Combi starting up on pre-heat when COLD tap is switched

[fresco2009]

Hello,

I have a four year old Baxi 133 Combi boiler.

When I turn on the cold tap, for example, to fill a kettle, wash, etc, when I switch it off quickly (as one normally does) the boiler starts up as though it is doing a pre-heat - after about 20-30 secs it shuts down.

BUT, when I trun the water off slowly it doen't happen.

The CH system pressure is OK and the are no error lights coming on. The CH and hot water work as normal.

I suspect it's a very sensitive sensor somewhere that activates.

Any ideas?

Thanks

 

[mysteryman]

There's probably an upward facing dead leg on the hot water circuit. When you close a cold tap sharply, a pressure surge can occur into this air pocket, and this can be enough to trip the DHW flowswitch briefly.

 

[fresco2009]

Thanks mysteryman - is there an easy fix for this or is it a big job?

 

[swbjackson]

Double check valve and shock arrestor(mini expansion vessel) on the cold pipework to the boiler.

 

[mysteryman]

I would prefer to find the cause of the problem and fix that, rather than add extra kit to fix the symptoms. Did you have a hot water cylinder, and is the old 22mm pipe from that still there in the airing cupboard, with a cap end on it? If so, have the deadleg removed.

 

[fresco2009]

Thanks to both again, I'll have a look at the pipework. There's quite a lot of old stuff hanging about so I'll track down their route and then look at your solutions,

again, many thanks - all very helpful

Rob