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Purchased smart thermostat from China still in transit, my old thermostat has two wires only from ducted gas heating unit was powered by 2-1.5 volt batteries.

The new thermostat requires a hard wire power source ,no batteries. The connection at back of thermostat states 95-240 ac volts L and N , I gather line and neutral . See image. My question is why do you need so many volts to power the display ? Is it simply the fact that most households already have 110 or 240 volts (I live in Australia) supply so that's how they design them. I just want to make sure that I don't damaged the unit by hooking up 240 volt power.

thermostat back panel

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  • I had vague idea this might be the case, thanks for clarifying in detail, much appreciated.
    – Tonedown
    May 9 at 8:21
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You kind of answered your own question: "hard wire power source, no batteries". If the thermostat requires its own power source and doesn't use batteries, then normal line and neutral household power is needed, that would be 120/204 volts. The line and neutral more than likely feed a transformer inside the unit that steps down the voltage to what the display requires.

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  • Not sure if you concluded that my thermostat is a 'misfit' from what I implied from reading my post or you concluded because I mentioned the word ' china'.
    – Tonedown
    May 9 at 8:25
  • I never concluded it was a misfit. I do know that much of the stuff from China lacks certification.
    – JACK
    May 9 at 11:26
  • Sorry jack, your post answered my question for someone else posted it was a misfit, sent my reply to you by mistake, new to this forum, again sorry.
    – Tonedown
    May 11 at 11:33
  • @Tonedown Not to worry, we're all friends here trying to help people and learning along the way.
    – JACK
    May 11 at 11:36
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The way “universal power” devices or devices with a 90-240v input range work is with a power supply called a switching power supply.

These supplies take the available AC power and using electronics convert whatever is there to a lower voltage in most all cases , rectify it into dc (3-15v is the nominal range).

These particular switching supplies are called “buck supplies” as the voltage is reduced so any AC input in the range will be used to power the electronics.

You can usually tap the hot lead to power the electronics but a neutral may be needed.

In the past there were specific voltage devices because there was a transformer that is a fixed device to create the low voltage for the electronics, solid state electronics now make the power supply’s much smaller and lighter and the voltage is created from whatever is available thus the term universal. The mfg only has to make 1 model and and it can work anywhere. And you won’t damage it because it is designed to work in that range.

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  • I had vague idea this might be the case, thanks for clarifying in detail, much appreciated.
    – Tonedown
    May 9 at 8:18
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Why is your thermostat a misfit? Because you bought random stuff from China, instead of safety-certified domestic stock designed for AU homes from reliable vendors. Send it back and get something appropriate.

Well, I'm not exactly sure how your old thermostat worked. I mean, obviously, your thermostat is set up as a "shunt, or don't" -- just like every thermostat on earth prior to smart 'stats. The question is, which voltage does your system operate at?

For instance, US systems use a low voltage 24VAC pilot signal to operate thermostats on a ducted gas system. UK systems hotshot 240V straight to the thermostat. Australia tends to pick and choose whose conventions they follow, so I'm not sure which system you need. You can ascertain this by measuring voltage across the existing thermostat terminals when the thermostat is NOT calling for heat. (when it's on, the difference will be 0-1 volt).

If your controls are low-voltage, then this thermostat is a misfit for your application.

Otherwise, you have the classic problem of any smart-switch or smart-stat installation: no "neutral" or "C-wire". (different scopes but the same problem). You will need to replace your thermostat wiring with 3-core wire + earth.

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  • Do the UK thermostats that operate at line voltage (240 VAC) connect with the same small diameter wires we use for US residential 24 VAC thermostat control wires? I guess in the UK there are low current fuses protecting the wire? May 8 at 17:31
  • @JimStewart No, UK 'stats use normal power wires, they don't have a small thermostat cable. May 8 at 18:40

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