The auxiliary circuit allows several 2-port motorised valves to be used to control separate zones without interacting electrically. Otherwise, if several zone valves were used without auxiliary circuits, and all were connected to the same pump so that the Live wire that opened each valve also fed the pump Live, there would be problems. As we’ve said above, Honeywell make a large range of motorised valves. The Normal state of a valve is the de-energised state. This is the state when no electrical power is being used by the valve motor. A failed motor will prevent the motorised valve from opening but it is not the only cause of a valve failing to open correctly. After years of use, the O rings sealing the valve spindle (or actuator shaft) can fail. When this happens, central heating water will seep past and will corrode the metals. On modern Honeywell valves, if the powerhead fails it can be removed from the valve body without draining down. However,be warned!With very old Honeywell motorised valves the powerhead could not be removed without draining the water first. Honeywell V4044C1288 22mm 3-port diverter valve. This valve is not a mid-position valve. It opens either Port A or Port B (usually heating and hot water). Apart from the few moments when it is switching between ports, Port A and Port B are not open together. Note that it is a 3-wire valve, not a 5-wire valve

The Honeywell mid-position motorised valve has three ports in a T configuration. Water enters via port AB. Water leaves by either port A or port B or by both at the same time. Port A is normally connected to the radiator circuit and Port B to the cylinder coil (to heat hot water). The circulating pump is usually connected to feed water from the boiler into port AB. Left to its own devices with pump on and CH and HW on, nothing seems to be happening in terms of pipes near the valve getting hotI tried out of interest to see what would happen during this state, if I set the controller to switch on heating. Nothing happened. Honeywell V4073A1039 22mm 3-port mid-position diverter valve. These are 5-wire valves. When used with other Honeywell controls the setup is know as Y Plan When the valve is working should the lever move over to the open position when it is doing HW or CH? When using two 2-port valves, the control system may be known as a Sundial S Plan system or a Honeywell S Plan system. The manual lever of the valve offers no resistance with the power off and it does not return to the auto position when released, as it should. I have played with one of these valves in a shop and the resistance to movement is much greater and the valve does return to the auto position.

Does water HAVE to flow into the CH or the HW system if the pump is working? If so, then one or other of the outlet pipes should be getting hot, but there are times when neither are getting hot and the boiler is not coming on, as though there is no water flow. Motorised valves are often fitted in tight spaces with limited visibility so note the orientation of the powerhead.This can be difficult to feel for at first. It is most obvious when the valve has been in the Heating Only position and reverts to the mid-position. My first thought was that the boiler wasn't firing up because the micro switch(es) to do so were faulty.