Worchester combi oil boiler 'goes to sleep'

[oilhead]

Tony,
The Heatslave oil boilers do not have NTC sensors. All the stats are mechanical. They do not control the pump, only the burner. If the pump is cutting off, then the problem swith the PCB or the diverter valve head.
Which diverter valve is fitted?

 

[OilLecky]

Let me throw this in.
Looking at my marked up copy of the wiring diagram for the Heatslave (page 22 of the manual) there are 4 possible supplies to the circulating pump.

1. If both the Boiler Termostat and the Central Heating Control Thermostat (both in the boiler)go to "high temp" then a supply is fed through them to energise Relay 2 and thus power the pump. (Unlikely source of supply!)

2. If a hot water tap is opened the flow switch operates and a supply is fed to the pump via Relay 4. (Doesn't apply, this is hot water demand.)

3. If the Domestic Hot Water Thermostat (in the boiler) is calling for heat, then relay 3 energises supplying the boiler. (Doesn't apply once hot water is up to temperature.)

4. When the Diverter Valve (in the boiler) is at full central heating it makes the diverter valve end switch and supplies the pump.

Now, if the (No.3) Domestic Hot Water Thermostat calls for heat the boiler and circulating pump will run BUT when it is satisfied the (no.4) Diverter Valve end switch MUST take over the circulating pump supply. If it doesn't then the boiler will over-heat and either the Boiler Thermostat or the Central Heating Thermostat will switch the burner off.

So I would check the Diverter Valve end switch output.
With the Hot Water up to temperature and switching off the electrical power before poking fingers anywhere, check for 230 volts at pin 2 of connector X8 (grey wire on the PCB.) Or if you do not have a multi-meter, put the power off and put a shorting link across pins 5 (orange wire) and pin 2 (grey wire) of connector X8. then, mind your fingers, put the power back on.
If you get 230volts at pin 2 or the shorting link causes the circulation pump to run, then the Diverter Valve switch is not operating.