Central heating controlled by hw exclusively

[mbc]

Hi!
A recent problem turned up with my central heating.
It is a classical gas open vented system: gas boiler, loft cistern, indirect hot water cylinder, cylinder thermostat, living room thermostat, controller with separate knobs for hw and ch.

Symptoms:
1) if HW is off, nothing happens (irrespective of the thermostat setting and/or the CH switch)
2) if HW is on, both radiators and cylinder will be heated, but only until the cylinder thermostat is triggered
3) when 2 applies, the CH setting or living room thermostat have no influence on anything

I'm quite puzzled, because I cannot imagine of an explanation for this set of symptoms, given the fact that my system has two separate valves and a single pump (see pic).

Any suggestion on what could the problem be?

I also attach below a pic of the controller/programmer (SANGAMO 410, Form No.: 1).
Further details of the components:
- grundfos 59506180 (pump, left on the pic above)
- Honeywell v4043H1056 ( two-way control valve/diverter, bottom of the pic above)
- bgmvsp-2 (two-way control valve/diverter/actuator, top of the pic above)

I found some relevant discussions here and elsewhere, but they seem to deal with a three-way valve, which makes for a plausible explanation for the symptoms; however, my case looks a mystery to me.
Thanks in advance!
mbc

 

[Riley]

Had any recent work done??

 

[Chuck]

My guess is that the CH zone valve is stuck partially open.

 

[Riley]

Would make sense I suppose if it’s literally seized part open part shut then the micro switch can’t make

 

[ShaunCorbs]

I would check the clock out make sure it's not sending power to both when ones on

 

[mbc]

Hi, thanks everyone for the answers. @Riley : no, no recent work done.
What I don't understand is, if the CH valve is even partially stuck, how can symptom 1) happen.
Also, any idea which valve is which?
I'll report on any progress, thank you again to everyone who answered.

 

[SimonG]

Would need the testers out to follow the voltage. Maybe a clock and motorised valve issue but needs to be tested.

 

[Dane Plumbing]

From one of the valves the pipework will lead to the HW cylinder. The heating will only come on because the CH valve is stuck. The valve can not fully open and depress the actuator that would trigger the boiler to fire up. When the HW is turned on the boiler fires up because the valves works. Then the CH valve just let’s the water by because it is stuck partially open.