vokera excell 96E no CH control

[Wilr]

Hi,

With the temperature control knob set at a position 1/3 round the dial, the CH temperature control on my boiler races up to 80degc and the boiler makes no attempt to control at what should be about 60degc for this setting. Adjusting the temperature control knob down to zero position has no effect, the boiler just carries on at full power. I then have to switch to manual override to stop the CH and allow my house to cool down!
HW is ok.

I guessed temperature measurement fault and replaced the thermistor with a new one today, but the fault is still there.

not sure if it's relevant , the PCB was replaced in the summer but the boiler has worked ok from then until the start of this week. Boiler is 7 years old.

where should I look next to solve the problem?

 

[Slugbabydotcom]

As I remember, there was a problem with the control knobs on these. Are you sure the control knob is moving the potentiometer on the pcb?

 

[scatmanjohn]

Isn't that the linea or do these do that as well?

 

[Wilr]

Thanks for replies.
I've checked CH control knob and it is moving the pot spindle ok, also quick check of in-circuit pot value shows a varying resistance similar to HW pot so I assume also ok.

I've noticed that when on CH and demand for HW comes the "problem with fan/flue" LED comes on briefly and there is a short clicking sound at the same time ( I guess a relay) before the boiler swaps to HW ok. But when the HW demand goes away and the boiler tries to go back to CH there is a series of about 5 or 6 short flashes of the same LED and each time the short clicking sound, then finally the boiler swaps to CH. One click on changeover with this boiler is what normally happens. Hope this sheds more light ?

 

[Wilr]

Decided to read MI and followed fault-finding flowchart for CH controls. found v.low voltage at modulator coil terminals (replicated at control terminals) and no fault with wiring and connections, so flowchart suggested replacement of modulator coil.

Replacement of modulator coil as described in MI looks simple except final step of procedure which is check burner pressures. My question is this, do the burner pressures really have to be checked after modulator coil replacement or could they be left till next years service ?
cheers
Rob