I have a fully pumped system with a three way mid position actuator. For some unknown reason the pump stops when the hot water tank is satisfied which of course means that the radiators are not working. When first switched on the central heating system works fine with the actuator in the mid position. It seems that when the tank is satisfied the system shuts down despite the room thermostat calling for heat for the radiators. The actuator remains in the mid position.The only way I can get the pump (and thus the boiler) to work is to tweek the tank thermostat up a fraction which immediately fires the boiler and starts the pump. I have changed the tank thermostat with a new Danfoss one but the problem remains. Any advice would be very welcome please.
Boiler Switching Off
- Thread starter Farleyjim
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I seem to remember from when I had a Y-plan system that there's a microswitch in the mid position actuator that supplies the boiler and pump when demand is for heating only, so if the actuator is not moving to the heating only position then, yes the boiler and pump will go off when HW demand is satisfied. Does the acuator move at all? I'd suspect the actuator, although wiser heads will probably be along in a minute.
This question crops up quite frequently. Unfortunately there are several possible causes. The cylinder thermostat being one of them, which you seem to have investigated already.
As the motorised valve is moving OK to the 'mid' position when the room thermostat is calling for heat, that would indicate that the room thermostat and programmer for the heating are working as they should (ie the motorised valves white wire is being made live) and also that the neutral connection is OK.
When hot water is not required (ie because the hot water cylinder is up to temperature, or the hot water is switched off) the motorised valves grey wire becomes live (from the cylinder thermostat and /or programmer as applicable) At this point, if heating is still required, the live on the grey winds the valve fully across to close off the hot water port and at the same time it operates an internal microswitch to operate the boiler. As the valve doesn't complete its journey it doesn't operate the microswitch and that's why the boiler goes off.
If you have a multimeter and can use it safely, I would start by checking to see if the motorised valves grey wire is live under the following conditions.
Hot water 'on' at programmer and
Hot water cylinder up to temperature (satisfied)
If it is live, the cylinder thermostat is OK and there is a fault with the actuator. If it is not live, then the hot water required signal is not coming from the cylinder thermostat. This may be due to a wiring fault and not necessarily the thermostat itself.
As the motorised valve is moving OK to the 'mid' position when the room thermostat is calling for heat, that would indicate that the room thermostat and programmer for the heating are working as they should (ie the motorised valves white wire is being made live) and also that the neutral connection is OK.
When hot water is not required (ie because the hot water cylinder is up to temperature, or the hot water is switched off) the motorised valves grey wire becomes live (from the cylinder thermostat and /or programmer as applicable) At this point, if heating is still required, the live on the grey winds the valve fully across to close off the hot water port and at the same time it operates an internal microswitch to operate the boiler. As the valve doesn't complete its journey it doesn't operate the microswitch and that's why the boiler goes off.
If you have a multimeter and can use it safely, I would start by checking to see if the motorised valves grey wire is live under the following conditions.
Hot water 'on' at programmer and
Hot water cylinder up to temperature (satisfied)
If it is live, the cylinder thermostat is OK and there is a fault with the actuator. If it is not live, then the hot water required signal is not coming from the cylinder thermostat. This may be due to a wiring fault and not necessarily the thermostat itself.
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I had ruled out the programmer / timeswitch based on the OP's description. My reasoning is:
1) We know that the 'heating' is getting power from the programmer, otherwise the motorised valve would not move to the 'mid' position
2) We know that the 'hot water' is getting power from the programmer because the cylinder thermostat switches the boiler 'on' and 'off'
3) The fault occurs when the hot water is set to 'on' at the programmer so the 'hot water not required' signal wouldn't be present from the programmer then anyway. Yet the fault still occurs.
1) We know that the 'heating' is getting power from the programmer, otherwise the motorised valve would not move to the 'mid' position
2) We know that the 'hot water' is getting power from the programmer because the cylinder thermostat switches the boiler 'on' and 'off'
3) The fault occurs when the hot water is set to 'on' at the programmer so the 'hot water not required' signal wouldn't be present from the programmer then anyway. Yet the fault still occurs.
Reading your post a few times it’s seems like you have hit the nail on the head. Most likely is the actuator.
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