Is it a problem with Diverter Valve?

[TimA]

My 5 year old Bosch system has recently started the CH sytem when the heating side of the controller is in OFF periods. The HW side still working to set times, and although not getting full heat on the towel rails, some heating is occurring outside the CH times set.
I turned off CH side for 24 hours to do a diagnostic check, and there is still a detectable temp change in the towel rails at random times.
Could this be a diverter problem and are there any tests I can perform before calling someone out? Any help will be appreciated!
Cheers

 

[Riley]

Are you on a combi or a regular heating set up with two port valves/three port valves

 

[Stu-B]

Can you set HW times on a combi?

 

[Scott_D]

Can you set HW times on a combi?

Yeah, you can set the preheat to be on/off

 

[Riley]

I know where you’re going but I’m just making sure

 

[TimA]

Are you on a combi or a regular heating set up with two port valves/three port valves

Sorry, should have given full spec. System is a regular set up with, with a twin coil tank (gas powered AGA also providing share of hot water in winter months)

 

[Darren Jackson]

Sounds like a sticking CH motorised valve or 3 port valve depending on set up.