Vailant VCW Sine 18 T3W heating can only be switched off with the Control switch on the panel, even with the thermostat removed.

[Engineer11]

I am new to this forum. I am an electronic engineer, but I have a problem with my dad's VCW Sine 18 T3W. Over the years I have fixed the PCB and many other problems as well as a new pump, diaphragms, pins, and motor run capacitors in the pump. It has been a great boiler and is still working well on its 3rd wax capsule too! It is a decent boiler and great quality, so we think it can go for just a bit longer.

This repair is for my dad, he is getting a new boiler on the scheme for older people eventually if there are any allocations or funding left, or he is willing to pay to get another boiler fitted if not. However, for now I could get it through this Winter that would be good as an interim measure.

OK, now to the problem. Do you think it is a microswitch in the base behind the main switches? The heating and hot water work OK, but the heating when turned on with the left-hand switch comes on, and then when the rooms are warm even removing the thermostat the boiler keeps on running as if the thermostat is calling for heat.

The pin and the diaphragm are rising and falling as they should as it is shown in the picture on this thread, and the hot water will turn on and off OK with the heating switched on.

I notice that there are two microswitches. Is one for the heating and one for the domestic hot water? Could one of these be faulty?

I can vouch for the main PCB being OK, and the switch PCB, as I have replaced the relay and capacitors etc. so this is all good.

If someone could suggest which microswitch may be causing the heating to stick on, then I can see if I can reach in with my meter and check it.

Thanks.

 

[ericmark]

I would not change one switch in motorised valve head, as if muck in one, then likely in them all.

In fact I would change whole head.

 

[Engineer11]

Thanks, I take it that's the one on the left side with the plug going into it with a few wires coming out of it and you can remove the top can from it.

So it could be the microswitches on the top of the diverter valve that's opened by the wax capsule?

 

[Engineer11]

Hi everyone, it’s nice to see some follow up on here, so here it is! I thought I should go into detail in case it helps anyone. BTW if anyone has any questions about the VCW Sine 18 or for that matter a Swiftflow 80 then fire away!

I checked all the micro switches and all OK, Main PCB and switch PCB soldered, new relays and capacitors replaced!

No fault found, so convinced that the boiler is OK, and the fault is external! I wasn't bothered about the time spent doing the service, and it didn't cost me anything in money because I already had the parts in old stock.

Here we go then, removed two pairs of wiring from terminal 3 and 4 where the link was originally. This is for the wall thermostat which is a "set back temperature controller" so you can put in a temperature profile in to keep your home above a certain temperature at different times of day, so the heating is never truly completely off, but just below ambient temperature at the time, so then it is off.

One pair is for the setback temperature controller, and this was 100% working, closed when on, and open when off, so this left the other pair of wires!

I found out that they went to the frost thermostat and they were shorted out, so hence the heating would be on all the time. It was cold in the house but not 3 degrees, so it should have been open!

My parents had their house rewired, so it seems that the electrician moved the frost stat from behind the plinth and buried it under the floorboards, so I was unable to get to it having a laminate floor put down since, so it was a case of disconnecting the frost stat and using the setback thermostat instead to keep the temperature at 10C out of hours or 20C when the full heating temperature was required!

Job done, conclusion was faulty frost stat, and old Honeywell one fitted in 1988 had stuck shut, because no matter how warm the house was, the heating kept on running!

I should have checked the link between 3 and 4 first for shorts, but from my check and service I think 2 new micro switches on the diverter valve head sometime would be a good idea because they do seem a bit "old"!