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Wiring Diagrams

Wiring diagrams for the most common central heating systems than use motorised valves, most are based on  Honeywell products mainly because most other manufactures have followed there lead

these are simplified wiring diagrams so is only showing how the live wire sends the power through the controls to the motorised valve and the boiler &pump. at the bottom of each wiring diagram is a line diagram of how the boiler, pump, motorised valves and cylinder should be configured for the system shown, note all central heating systems should be fused at 3Amp with some warm air units fused at 5 Amp, on no occasion should a domestic CH system be fused at 13Amp as this could cause damage to the system in event of an overload


Honeywell S plan system using 2 or more moterised valves, Note that all the moterised valve are in wired in Parralel and all the end switch does is sawitch on the boiler when the valve is open


Honeywell Y plan system is only for 2 zones so uses just one 3 port 3 way valve and the wiring must have a HW off wire for this system to work corectly, note this can only work with 240Vac switching systems  


this is a low cost version of the Y plan system, but with the pump on the return and used in conjunction with a primatioc cylinder, a Y plan valve can be used instead  

Drayton Flow Share

Drayton Flow share does the smae job as the Y plan but in a more complcated way, when icome across this system that need a new moterised valve i fit the Honeywell Y plan and alter the wiring accordingly

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this is a very cheap way to put thermostatic controls and reduce heating bills on  a gravity Hot Water system, once both the Cyl & room stata are satisfied the boiler turns off and allows the boier to cool down, without the cyl stat the boiler stays at a high temperture & burning gas, very similare to the C plan but without the extra valve 



the G plan is similare to the S Plan but there is no spring rutun so they drive open when needed and also drive shut when satisfied so all stat have 1 wire in and 2 wires out  


Alternative SMC wiring diagram




Gravity Hot water with cylinder stat and zone valve

  this is a gravity HW system but with a 6 wire moterised valve fitted on the HW return, very similare to the Gravity HW with cyl stat (above) but with the extra moterised valve 

  Warm aire units always have an automatic fan, so when the heat exchage heats up the fan will start automaticly and these often have a switch that will run the fan for cooling in the summer

Most warm are units have 24V switching and many DIY repairs have resulted in 240Vac sent into the 24v circuit blowing all the compoments, so timers with voltless switching must be used