I have a 4 zone heating timer (HW, Underfloor Heating, Towel Rail and the 4th zone is unused).

I moved into a new build a few years ago. After 3 years my heating timer unit (John Guest TM4) started making weird noises and then the touch display shutdown (no backlight to it or evidence of power to it). I replaced the John Guest TM4 unit with a like for like replacement myself, wired it up and it worked fine until last week. It started making the same weird noises again and once again the touch display shut down (again no backlight)

John Guest don’t make the TM4 anymore but Heatmiser now make the TM4-TS which looks the same and is meant to be a like for like replacement.

I ordered the Heatmiser TM4 last week and wired it up the same way but it didn’t work - no power to the display. I used a potentiometer and checked there was current to the unit through the live and neutral and there was. I assumed the Heatmiser unit was broken from the outset so I ordered another Heatmiser TM4-TS.

This time I just wired up the live and neutral (leaving the wires for the 4 switch zones unconnected). I then checked the unit turned on, which it did, lighting up.

I then wired up the 4 switch zone wires (4 the 4 heating zones) but this time no backlight or evidence of power to it when I turned it on. I then disconnected the 4 switch zone wires so just the live and neutral were connected but now no backlight.

I’m assuming connecting the unit to the 4 switch zones is somehow breaking the unit?

I have no idea why this could suddenly start happening. Any ideas anyone please?


  • Hello, and welcome to Home Improvement. Where are you located (I suspect NZ)? And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Oct 12 '19 at 18:22

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