Baxi combi heating coming on randomly

[dalboy]

Hello all

I've searched the forum for similar but now a bit stuck. My baxi duo tec randomly started turning on the heating over the last few weeks.

I've cleaned the heat exchanger and replaced diverter valve and motor and still the same. When I done all this I turned boiler back on and left it a good hour without using any taps etc. Heating came on and checked the boiler and the heating light wasn't even on!?

Hot water is fine

I tried taking batteries out of thermostat controller to see if it was the cause but above still happened.

Any help really appreciated

Thanks!

 

[gas112]

the main ch sensor on these also acts as a frost stat and one of faults when they go is it thinks its cold and brings boiler on
part number 5114725 sits in a dry pocket in pipe on left of divertor valve

 

[dalboy]

the main ch sensor on these also acts as a frost stat and one of faults when they go is it thinks its cold and brings boiler on
part number 5114725 sits in a dry pocket in pipe on left of divertor valve

Ok thx will check that out.

If I isolate the heater flow (1st pipe under boiler) issue still persists. If I isolate return flow (last pipe under boiler) the problem seems to go away but obviously no heat to rads. Was just trying to isolate issue. Would that add up to the frost stat thing?

Thank u!

 

[ianmcd]

If I isolate the heater flow (1st pipe under boiler) issue still persists.

Sorry but that is impossible

 

[gas112]

sounds like when you are isolating it the knob on flow isnt actually turning the valve off

 

[dalboy]

the main ch sensor on these also acts as a frost stat and one of faults when they go is it thinks its cold and brings boiler on
part number 5114725 sits in a dry pocket in pipe on left of divertor valve

Is it this red one on the right with green wiring?

Thx!

 

[gas112]

The clue was on the PIPE to the LEFT of divertor
but it looks like that

 

[dalboy]

The clue was on the PIPE to the LEFT of divertor
but it looks like that

This thermistor is to the right though? Apologies, I'll blame lack of sleep due to a newborn

 

[dalboy]

If I open first valve and run hot tap, the pipe goes warm which I believe isn't meant to happen (I do have a loft extension which I've noticed first pipe has a run off then goes up)

And then ch light comes on as per pic

Edit. After several minutes light has gone off but can still hear boiler doing something for few minutes then boiler does go off

 

[Anon_E_Mouse]

You'll probably find you need a shock arrester and one way valve fitting to the cold water inlet pipework to the boiler. What's happening is when you run cold tap pressure in pipework drops. When tap is turned off the pressure returning activates the flow switch momentarily. The boiler will see this as a hot water demand and will go through its start up sequence. Then it will shut down and go through shut down sequence.