3 Way mid position valve always calling for CH

19 Feb 2005
Reaction score
United Kingdom
I have just recently installed a new pump, boiler controller, room stat & cylinder stat in my system and now having problems when there is a demand for HW. When boiler programmer demands HW, the mid position valve moves in to CH mode, when demand is cut the valve returns to normal position. The cylinder stat when adjusted up or down ignites/cuts boiler respectively .The CH works fine, and I have rewired the old programmer and is still CH on HW demand. The valve motor is working, is the rest of the valve on its way out? Or have I messed up the wiring somewhere. Any advice would be most helpful. Cheers

Honeywell ST6450 replaced with Drayton LP722 boiler programmer (wired as per instructions and set to P for a pumped system)
Honeywell L641A replaced with Drayton HTS3 cylinder stat
Honeywell T6360 replaced with Drayton RTS1 room stat
Mid position valve Myson MPE 322
Glowworm Economy Plus 30F boiler
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You guessed it; you messed up your wiring.

A three port valve will typically have four or five wires coming out of it. Of these, one will be neutral (blue) and, if there are five, one will be earth (green/yellow). It's the other three you have to sort out.

One of them drives the valve to mid position in response to a CH demand. The boiler/pump are still being powered by the HW demand at this point.

A second one drives the valve right over to the CH end IF AND ONLY IF it is already in mid position. This wire should be connected to the No-HW terminal on your controller. Have you connected it to HW by mistake?

The last one does not drive the valve anywhere at all. Once the thing reaches the CH end, a switch inside feeds power OUT of this wire and back to HW on the controller. This is needed to power the boiler/pump because the HW demand itself has gone.

All you have to do is figure out which of the three wires is which. If you have no info to work on, isolate them all and then connect live power to each in turn. Watch those other ends while you do this; volts can come OUT as well as IN.

Only one will make the valve move. That one goes to your room stat. A different wire will move the valve right over IF you push it to mid position with the manual lever. That one goes to No-HW. The third wire goes to HW. What could be simpler? (On second thoughts, don't answer that!)
Switched live from room stat should lead to 'white wire' on valve. Cylinder stat has two outputs (A) when heat called for output wire leads to boiler/pump as does the 'orange wire' from the valve.(B) when heat is satisfied the other output leads to 'grey wire' on valve. When hot water is off at programmer there is an output wire from the programmer that also leads to the 'grey wire'on the valve.
:rolleyes: :rolleyes: :rolleyes:
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Hi guys

Sorry for the late reply, just got round to sorting it out.
Thanks for all your input, wiring is all correct. I found that the head of the 3 way valve was duff, replaced it & is all back up & running.

Thanks again


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