Which cable for smart thermostat?

[nadimaj]

These new age "smart" thermostats:

https://www.amazon.co.uk/dp/B07VB6Y...abc_6MMX2FXSHQPZM3NE8X2N?_encoding=UTF8&psc=1

https://www.amazon.co.uk/dp/B07HQCW...abc_DH0PWS40GYDHDZQTHWTC?_encoding=UTF8&psc=1

...require 240v just like the hold fashioned (Honeywell) ones. I find this a bit strange. An alarm control panel will comfortably operate over 12v that is sourced via the main control box or a battery, but these for someone reason want 240v which they then convert locally into 12v. The current draw is identical so there's no reason for it. Is this simply because the manufacturers assume you will have the original (240v) wiring? I mean, wouldn't a better (future proof) to have versions that operate over 12v in the same way as your alarm control panel? New builds would benefit.

In any case, could some one please share a link to something I can buy online suitable for the above?

Thanks

 

[stem]

These new age "smart" thermostats.....for someone reason want 240v which they then convert locally into 12v.

Is that really the case? I've not seen a wired smart thermostat that has a 12v power supply built in.

An alarm control panel will comfortably operate over 12v

Alarm panels generate 12v from an internal power supply and all of their components are operated by 12v which draw tiny currents. Heating systems on the other hand have mains powered components such as motorised valves, circulation pumps etc. which are frequently switched directly from the thermostats, so would require 230V anyway, would you really want to complicate it further by running both 12v and 230V cables to that same thermostat?

Also, if the voltage were dropped, from 230V to 12v, to provide the same operating power (watts) the current draw would increase by about 20 times, even if you could find pumps, boilers and motorised valves that worked on 12v.

It's horses for courses really. Thermostats are a 'one size fits all' that have to work with all types of heating systems, OK some combi boilers that don't have external components such as motorised valves and pumps use 24V [produced by a PSU inside the boiler] that can still be switched by a mains rated thermostat that has voltage free switching. [The inverse wouldn't be true, a thermostat designed for 12v maximum wouldn't be suitable for mains.] However this 24v is for its control circuits only, the boilers internal components such as the pump etc. are still 230V.

With regard to the cable, it should be sufficient to handle the current that the fuse protecting it is rated at, which in the case of most heating systems is 3A, so 0.75mm2 would be sufficient, although I tend to use 1mm2 for mechanical strength of the connections. Also, if it comes into contact with an area that is hot, I use heat resistant cable.

The number of cores 3/4 is as required for the installation. So, for local connections such as between a receiver and a boiler, or to a pump that requires a flex connection, something like this:

heat resistant flex
standard flex

For longer runs such as out to remote wired thermostats I use standard grey PVC sheathed twin and earth at 1.5mm2 or three core and earth.

 

[CBW]

Is that really the case? I've not seen a wired smart thermostat that has a 12v power supply built in.

Nest 3rd generation learning thermostat? Heatlink has a 230v supply and 12v output at heatlink? Could OP be thinking along these lines?

Also as I know you’re aware @stem, it is often the boiler that supplies the voltage to the thermostat, eg 24v

 

[stem]

I wondered that initially, but the OP said "These new age "smart" thermostats:" and then posted links to a couple of 'non nest' thermostats that "require 240v just like the hold fashioned (Honeywell) ones"

And as you rightly point our the Nest thermostat itself doesn't require 230V, so the OP must be referring to something else.

Also as I know you’re aware @stem, it is often the boiler that supplies the voltage to the thermostat, eg 24v

Indeed, I said that already

OK some combi boilers that don't have external components such as motorised valves and pumps use 24V [produced by a PSU inside the boiler]