240V / 24V Thermostat confusion.

[LegionOfBrad]

Hi there,

We have recently had a nest put in to replace the our old room stat and timer setup.

The old setup was a Honeywell dial stat. With a dial timer on the boiler. The only cable coming out of the boiler apart from power was T&E connected to the 24V switching section (not the 240V section which the nest is connected to) This cable is 100% not live having been disconnected.

Rather stupidly i assumed the cable would lead directly through the walls and terminate at the Honeywell dial stat thus i could use it as a hard connection for the heatlinks T1 and T2 connections without having to power it via usb. But i've found that at the stat end it is connected via T&E but the cable is live (tested) and the stat is a T6360B which is a 240V.

The cable run can't be seen as it snakes through the house. I'm assuming at some point the 24v signal from the boiler is being converted to 240V - is this something that is possible? And if so shouldn't the converter be somewhere inspectable?

I'm happy to run the nest via USB but would rather have got it hard wired in.

Thanks

 

[stem]

The 24V is produced by the electronic circuits inside the boiler, and will be an integral part of the boiler's PCB.

It may be that the thermostat is switching a mains operated motorised valve that has an internal potential free contact that switches the 24V, (not ideal to have 24v and 230v wires running in the same cable though) Or there may be a relay in the circuit somewhere. Only an examination of your system will reveal what the situation really is.

The Nest Thermostat requires a 12V supply to operate. It can get this by connecting the T1 and T2 terminals at the Nest Thermostat to the corresponding terminals at the Nest Heatlink (As with the boiler, the Heatlink has an internal power supply to generate the 12V) or by using a separate 'plug in' power supply.

If the old room thermostat is still in operation (I wonder this because of the fact that you say that the wires are still live), Because it should have been disconnected froand the wiring modified when the Nest was installed.

 

[LegionOfBrad]

The 24V is produced by the electronic circuits inside the boiler, and will be an integral part of the boiler's PCB.

It may be that the thermostat is switching a mains operated motorised valve that has an internal potential free contact that switches the 24V, (not ideal to have 24v and 230v wires running in the same cable though) Or there may be a relay in the circuit somewhere. Only an examination of your system will reveal what the situation really is.

The Nest Thermostat requires a 12V supply to operate. It can get this by connecting the T1 and T2 terminals at the Nest Thermostat to the corresponding terminals at the Nest Heatlink (As with the boiler, the Heatlink has an internal power supply to generate the 12V) or by using a separate 'plug in' power supply.

If the old room thermostat is still in operation (I wonder this because of the fact that you say that the wires are still live), Because it should have been disconnected froand the wiring modified when the Nest was installed.

Turns out the wires were not live. My live tester pen was giving false readings. I checked the wires with a multimeter and they were dead. Then connected up both ends and turns out the cable was indeed the same one.

So the basis of the this thread was incorrect. I can use the t&e to power the nest as soon as the filler around where the old thermostat was has dried.

 

[stem]

If the existing cable is completely disconnected from everything. You can use it to provide the 12V for the Nest Thermostat. By connecting T1 at the Heatlink to T1 at the Thermostat, and T2 at the Heatlink to T2 at the Thermostat. It must not be connected to anything else.

I'm assuming now that the rest of the Nest installation has been done correctly and that the boilers 24V switching terminals previously connected to the Honeywell have been wired to the Nest Heatlink terminals 2 and 3. [As you are switching 24V, the Heatlink 'Live' terminal should not be connected to the 'Common' terminals. 230V installations only require this]