# Smart Thermostat

Can anybody tell if this thermostat is low or high voltage from the picture alone?
Low voltage, is says on it 24-230v.

Some good explanations but the OP hasn't really described what he expects the SMART thermostat to do or what technology he expects it to use to achieve his objectives. Simply, save money or reduce energy is a bit vague.

Do you have a condensing boiler, is it non-condensing or oil?

If a condensing boiler the EU reckons a simple thermostat offers a 1% improvement on a condensing boiler seasonal space heating efficiency. 2% if weather compensation control. 2% TPI (controls cycling, the closest I guess to a smart stat). 3% if a modulating room thermostat but that depends on your boiler model. 4% if modulating thermostat and weather compensation. 5% if three or more modulating thermostats working together with a three zone heating system.

Can anybody tell if this thermostat is low or high voltage from the picture alone?
There is no such thing as high or low voltage.
Term you are looking for is zero voltage switch or contact just by itself applies 240 volts to the contact.

Zero voltage switch is like your electrical switch. It switches whatever is on the other contact.

The back plate in the right picture is I assume zero voltage switching most likely- not 100% sure ( not knowing what that Backplate belongs to)

The back plate on the left puts 240 volts onto terminal 2. The grey wire takes the switched voltage to wherever it needs to go.
On the right. A would have the supply voltage ( one volt being an example to a million volts to be pedantic being switched and presented at B

I hope above is clear ( as mud?)

There is no such thing as high or low voltage.
Term you are looking for is zero voltage switch or contact just by itself applies 240 volts to the contact.

Zero voltage switch is like your electrical switch. It switches whatever is on the other contact.

The back plate in the right picture is I assume zero voltage switching most likely- not 100% sure ( not knowing what that Backplate belongs to)

The back plate on the left puts 240 volts onto terminal 2. The grey wire takes the switched voltage to wherever it needs to go.
On the right. A would have the supply voltage ( one volt being an example to a million volts to be pedantic being switched and presented at B

I hope above is clear ( as mud?)
Thanks - sorry for not being clear, I’m talking about the right photo, the backplate of the DT90E thermostat. I ask as I read the nest can only work with low voltage wiring?

I ask as I read the nest can only work with low voltage wiring?
Nest Thermostat dial requires 12V dc which is either fed from the heat link via new wiring or existing, or fed from a plug socket outlet. Everything, bar the L and N, in the heat link is then volt free switching, so as @DP explained, whatever is being supplied to/from it.

Just to note, the simplest solution may be to just change the thermostat.

Your current thermostat does not have a power supply (of its own); without the addition of extra wiring, a battery operated thermostat such as the Nest Thermostat E, could be a direct drop-in replacement.

There is no such thing as high or low voltage.
I really wouldn't want to say that on the electrician's forum!
For the record, according to BS 7671:

Low voltage is defined as a voltage: "exceeding 50 V ac or 120 V ripple-free dc. but not exceeding 1000 V ac or 1500 V dc between conductors, or 600 V ac or 900 V dc between conductors and earth"

Extra low voltage is any voltage below that, and High voltage above!
But for most of us, when a DIY'er talks about low voltage, we know what you mean

...and the definition of 'Zero volt switching' is another issue for semantic discussions. A switch has to have a different potential on each side (when open), or else it wouldn't make a difference if the switch was open or closed!
The definition can be taken to mean a switch that has no electrical connection to the output contacts (such as a relay, as apposed to a transistor), or it could mean that the controlled device is energised when there is zero volts across the switch (i.e. when the switch is closed).

A would have the supply voltage ( one volt being an example to a million volts to be pedantic being switched and presented at B
I'm afraid you've lost me there!

Last edited:
Just to note, the simplest solution may be to just change the thermostat.

Your current thermostat does not have a power supply (of its own); without the addition of extra wiring, a battery operated thermostat such as the Nest Thermostat E, could be a direct drop-in replacement.
Still could have a regular Nest, but the St9400c would need to be replaced with the heatlink and use the existing wiring of DT90e for the power of 12v to Nest thermostat.

Just to note, the simplest solution may be to just change the thermostat.

Your current thermostat does not have a power supply (of its own); without the addition of extra wiring, a battery operated thermostat such as the Nest Thermostat E, could be a direct drop-in replacement.

I really wouldn't want to say that on the electrician's forum!
For the record, according to BS 7671:

Low voltage is defined as a voltage: "exceeding 50 V ac or 120 V ripple-free dc. but not exceeding 1000 V ac or 1500 V dc between conductors, or 600 V ac or 900 V dc between conductors and earth"

Extra low voltage is any voltage below that, and High voltage above!
But for most of us, when a DIY'er talks about low voltage, we know what you mean

...and the definition of 'Zero volt switching' is another issue for semantic discussions. A switch has to have a different potential on each side (when open), or else it wouldn't make a difference if the switch was open or closed!
The definition can be taken to mean a switch that has no electrical connection to the output contacts (such as a relay, as apposed to a transistor), or it could mean that the controlled device is energised when there is zero volts across the switch (i.e. when the switch is closed).

I'm afraid you've lost me there!
High and low voltage was in reference to what OP said about thermostat.

Rest I agree with you. But to quote BS to a lay person is like talking rocket science to a pie maker

you got 2 channel programmer which control hot water and heating.
no 4 on programmer send current to thermostat and than thermostat send it to motorised valve.
thermostat most likely getting 240v when heating is turned on from programmer, if you want to make sure it , just replace programmer on backplate, than check current on thermostat both wire with tester,
than turn on heating on programmer and check again on both wires of thermostat , tester will lit on anyone of the wire that means 230v.
so technically when heating demand turned on from programmer, power goes to thermostat,than motorised valve and last at the boiler and boiler fire up.
if your system is on two seprate motorised valves, you can fit any two channel smart thermostat but you have to connect both wires together in thermostat to send the power to motorised valve.

Low voltage, is says on it 24-230v.
In general as said already low voltage is 230 volt and 0 - 50 volt is extra low voltage, so pointless misleading statement, and in USA what we (international standards organisation) call extra low voltage they call low voltage, so better to read the actual voltage and not go by LV and ELV.
5% if three or more modulating thermostats working together with a three zone heating system.
I would say unusual to have less than three heating zones, but the zones can be simply temperature zones, or temperature and time zones, my main house as 12 heating zones, both temperature and time, but as to saving since never used central heating without the programmable TRV heads it is hard to say.
There is no such thing as high or low voltage.
Term you are looking for is zero voltage switch or contact just by itself applies 240 volts to the contact.
Well in the British home correct there is no high voltage, our supplies are well under 1000 volt, so we have low voltage and extra low voltage, but as far as thermostats go we have volt free contacts in some versions, which means low and extra low voltage can be taken to the same device, the device does not care if the voltage at the contacts is 12 or 230 as not connected to the supply voltage for the device.
I ask as I read the nest can only work with low voltage wiring?
Nest can work with low or extra low voltage, both Nest e and Nest Gen 3 both have volt free contacts, so does not matter, however both are also able to work with OpenTherm I seem to remember that's 0 - 40 volt and those contacts are not volt free. Also Nest Gen 3 supplies 12 volt to the controller, the controller contains the thermostat, the PIR, and is the human machine interface (HMI) however the lack of interconnection with Nest to date in UK (USA is different) with extra temperature sensors means it is of very limited use.

I had every intention of fitting Nest to my mothers house, with this in mind I fitted Energenie MiHome TRV heads with the intention of adding Nest later. However the electronic TRV heads gave me all the extra control I needed, so never added Nest.

So when we sold my late mothers house the old TRV heads were put back on and the Energenie heads brought here. I had a problem no assess to void under floors, and just two wires from main house to where the boiler was, which were also suspect as colours changed, so since Nest Gen 3 controller connects with just two wires for power and all control, it seemed ideal, the connecting to the TRV heads was a bonus.

However when I tried to connect the TRV heads I had problems, major one was they seemed to work wrong way around, instead of the heads telling the wall thermostat when to turn on the boiler, it was the wall thermostat telling the TRV what temperature to set to.

Using the Energenie app the TRV's and Wall thermostat changed together, but turn the dial on the controller, or set a schedule and the TRV's failed to follow, phone call to energenie and I was told when Google took over Nest support was withdrawn, so there is no interconnection between my TRV's and my wall thermostat.

So my cheap eQ-3 TRV heads actually do a better job to the expensive Energenie.

As to the "Smart" features, anti legionnaires, geofencing, life style learning, etc, they have been one by one switched off, for one reason or another they have caused problems.

Geofencing relies on speed to reheat room to work, theory may seem good, as it detects your phone is getting closer it turns one the heating, but the TRV head needs to have anti hysteresis software which will not stop the room reheating fast, but also will not over shoot, the Drayton Wiser claims to do this, but Energenie did not, moving from eco (16ºC) to comfort (20ºC) it did the first 2ºC fast, but took an age to get next 2ºC so I cheated and at 7 am it was set to 22ºC and at 8 am 20ºC and then it was at 20ºC by 8 am, but can't do this with geofencing.

The other thing was Nest does not allow you to set distance, all the control you have is Eco temperature and Comfort temperature so a bit hit and miss if you will come home to a warm house.

I have also had other problems, it stopped working with wife's phone, and she was unable to get it to work not helped by every so often it want one to confirm by an email it is actually you, not clever when you don't have the email coming to your phone. And when the local EE mast went down in high winds, it would only switch on heating when we walked past the thermostat, so enough for us not to realise it was the thermostat turning heating off all the time which was reason we were cold. It was second day before I realised what was going on, and removed the geofencing.

So in the main simple time seems to work best, and when we had regular trips out at the same time, we could set a sequence of events, heating came on at 5 pm together with kitchen, 5:15 pm dining room heating came on, and 5:25 pm living room heating, as the evening progressed to dining room heating goes off, and then kitchen and finally living room and at the same time the bedrooms come on, so kitchen heats up faster when other rooms are off, etc. However this needs radiators which are big enough, boiler output is 19 kW so can't use all that in kitchen, but it is also heating DHW and hall/landing and shower and bathroom, which is never turned off.

With a modulating boiler it would work better, but if the boiler can turn down to 6 kW you want at least 6 kW radiators to stop boiler cycling and losing the latent heat. Not such an issue with me, my oil boiler does not modulate.

DP
Can anybody tell if this thermostat is low or high voltage from the picture alone?
Reply was in response to above
Well in the British home correct there is no high voltage, our supplies are well under 1000 volt, so we have low voltage and extra low voltage, but as far as thermostats go we have volt free contacts in some versions, which means low and extra low voltage can be taken to the same device, the device does not care if the voltage at the contacts is 12 or 230 as not connected to the supply voltage for the device.
Understand all that.

In general as said already low voltage is 230 volt and 0 - 50 volt is extra low voltage, so pointless misleading statement, and in USA what we (international standards organisation) call extra low voltage they call low voltage, so better to read the actual voltage and not go by LV and ELV.

I would say unusual to have less than three heating zones, but the zones can be simply temperature zones, or temperature and time zones, my main house as 12 heating zones, both temperature and time, but as to saving since never used central heating without the programmable TRV heads it is hard to say.

Well in the British home correct there is no high voltage, our supplies are well under 1000 volt, so we have low voltage and extra low voltage, but as far as thermostats go we have volt free contacts in some versions, which means low and extra low voltage can be taken to the same device, the device does not care if the voltage at the contacts is 12 or 230 as not connected to the supply voltage for the device.

Nest can work with low or extra low voltage, both Nest e and Nest Gen 3 both have volt free contacts, so does not matter, however both are also able to work with OpenTherm I seem to remember that's 0 - 40 volt and those contacts are not volt free. Also Nest Gen 3 supplies 12 volt to the controller, the controller contains the thermostat, the PIR, and is the human machine interface (HMI) however the lack of interconnection with Nest to date in UK (USA is different) with extra temperature sensors means it is of very limited use.

I had every intention of fitting Nest to my mothers house, with this in mind I fitted Energenie MiHome TRV heads with the intention of adding Nest later. However the electronic TRV heads gave me all the extra control I needed, so never added Nest.

So when we sold my late mothers house the old TRV heads were put back on and the Energenie heads brought here. I had a problem no assess to void under floors, and just two wires from main house to where the boiler was, which were also suspect as colours changed, so since Nest Gen 3 controller connects with just two wires for power and all control, it seemed ideal, the connecting to the TRV heads was a bonus.

However when I tried to connect the TRV heads I had problems, major one was they seemed to work wrong way around, instead of the heads telling the wall thermostat when to turn on the boiler, it was the wall thermostat telling the TRV what temperature to set to.

Using the Energenie app the TRV's and Wall thermostat changed together, but turn the dial on the controller, or set a schedule and the TRV's failed to follow, phone call to energenie and I was told when Google took over Nest support was withdrawn, so there is no interconnection between my TRV's and my wall thermostat.

So my cheap eQ-3 TRV heads actually do a better job to the expensive Energenie.

As to the "Smart" features, anti legionnaires, geofencing, life style learning, etc, they have been one by one switched off, for one reason or another they have caused problems.

Geofencing relies on speed to reheat room to work, theory may seem good, as it detects your phone is getting closer it turns one the heating, but the TRV head needs to have anti hysteresis software which will not stop the room reheating fast, but also will not over shoot, the Drayton Wiser claims to do this, but Energenie did not, moving from eco (16ºC) to comfort (20ºC) it did the first 2ºC fast, but took an age to get next 2ºC so I cheated and at 7 am it was set to 22ºC and at 8 am 20ºC and then it was at 20ºC by 8 am, but can't do this with geofencing.

The other thing was Nest does not allow you to set distance, all the control you have is Eco temperature and Comfort temperature so a bit hit and miss if you will come home to a warm house.

I have also had other problems, it stopped working with wife's phone, and she was unable to get it to work not helped by every so often it want one to confirm by an email it is actually you, not clever when you don't have the email coming to your phone. And when the local EE mast went down in high winds, it would only switch on heating when we walked past the thermostat, so enough for us not to realise it was the thermostat turning heating off all the time which was reason we were cold. It was second day before I realised what was going on, and removed the geofencing.

So in the main simple time seems to work best, and when we had regular trips out at the same time, we could set a sequence of events, heating came on at 5 pm together with kitchen, 5:15 pm dining room heating came on, and 5:25 pm living room heating, as the evening progressed to dining room heating goes off, and then kitchen and finally living room and at the same time the bedrooms come on, so kitchen heats up faster when other rooms are off, etc. However this needs radiators which are big enough, boiler output is 19 kW so can't use all that in kitchen, but it is also heating DHW and hall/landing and shower and bathroom, which is never turned off.

With a modulating boiler it would work better, but if the boiler can turn down to 6 kW you want at least 6 kW radiators to stop boiler cycling and losing the latent heat. Not such an issue with me, my oil boiler does not modulate.
I use Evohome. Works without a hitch. Planning an extension. Will have UF which will be hooked up to Evohome

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