Cannot switch off heating when water on

[FMT]

I have a Baxi Solo HE boiler and a honeywell ST9400 programmer. I have an intermittent problem where if the water is on sometimes the heating comes on even if the thermostat is down. The light on the programmer for CH will be off but the radiators are hot. You can move the CH switch to off or override but it just keeps working. There is no way to stop the heating other than switching the water to off. With the water off the heating can be controlled perfectly with the buttons on the controller and thermostat etc.
I have had a plumber out who changed the switching valve and then an electrician changed the programmer so these parts must be ok

Does anyone have any other ideas please as I don't know what else to do?
Thanks
FMT

 

[Goldberg]

Do you have one three-port valve, or two two-port valves?

 

[FMT]

i have a three port valve

Thanks

 

[Goldberg]

If the heating is on when it shouldn't be, then either the valve is being moved to the wrong position, or it's sticking in the wrong position.

Is it the three port valve that the 'plumber' changed?

If so, did he change the complete valve, or just the wet part, or just the powerhead?

Also, what make and model is the valve?

 

[FMT]

As far as I know he changed the whole thing (he drained down system) and it is a Tower mid position Valve Val328MP.

 

[doitall]

If you have a multimeter, check if you have power on the grey or white wire in the hot water only mode.

 

[FMT]

Unfortunately I don't have one of these !

 

[Goldberg]

Unfortunately I don't have one of these !

http://www.maplin.co.uk/Search.aspx?criteria=multimeter&source=15

 

[kevplumb]

op said he didn't have one

didn't ask where to get one

 

[Goldberg]

 

[kevplumb]

very nice you should wear a hat in the sun so you don't fry what little grey matter you have left View media item 14331

 

[Goldberg]

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You don't need to thank me - I'm just pleased to be able to help.

Have a nice day y'all.