boiler firing up when toilet flushed

[mrplumberman]

just fitted a new loo.when you flush the toilet the boiler fires up.the guy i going bush.any help?

 

[GQuigley67]

have you piped it up with the hot water pipe ??

 

[mrplumberman]

could it be the divertor valve on the boiler

 

[shorticus]

what's the make of boiler?

 

[GQuigley67]

don't see how it can be the diverter valve, as the diverter valve should be in the hot position when not in use.

 

[Billy Bob Bob]

water turned off to replace the bog, dead leg somewhere now full of air there's your issue IMHO

 

[mrplumberman]

the boiler is a vokera flowmatic.checked for air locks and run off water from all taps and bled the rads what else can it be.?

 

[GQuigley67]

stick a check valve on mixer taps

 

[Billy Bob Bob]

has the system been converted from a cylinder,cwsc,bbu/wm set up to a combi?

 

[shorticus]

i believe that vokeras have differential pressure switches in them, it might be mis-reading the pressure drop, can you be a bit more specific on the model and I'll check the manual?

 

[mrplumberman]

its a 40/80 flowmatic.the cwsc was feeding the shower until we put in a new electric one.the cwsc has been put in the skip now.

 

[mrplumberman]

the boiler looks old could it be the flow switch?

 

[shorticus]

haven't got a manual for a 40/80, got a manual fro a 20/80 though and within the operation sequence description it states

'Opening a draw off tap will operate the dhw differential pressure device which in turn operates the the 3-port valve and the dhw flow switch'

I'd have my money on this being the cause, if the wc is now off the mains it would give a pressure drop that the boiler would pick up, I'd suggest making sure that the ballvalve has a high pressure jet in it and maybe throttle back the service valve a bit otherwise the boiler might need a bit of attention

 

[mrplumberman]

should i fit a prv to the cistern then?

 

[shorticus]

should i fit a prv to the cistern then?

that might be a little overkill, sure it'd probably work but it'd cost £25+ plus your time, check the wc jet first, how quick does that toilet fill? If it's super quick then it'll be the jet for sure if not then I'm fairly confident that it's a fault in the boiler...or it's an apalling supply, maybe try a dynamic pressure test, pressure gauge on the kitchen cold tap then get someone to flush the toilet or run another tap...just thinking out loud here...

 

[oneim]

hi had this [problem on a baxi 28, it needs a mini expansion vessel cutting to the main water supply near the boiler