Nest - Adding master time switch override

[DamonBristol]

Hi there,

I'm trying to wire multiple Nest Thermostats (Nest Heat Links) as zone controllers in a multi-zone system with a Heatmiser UH8 wiring centre for the zone actuation.

The UH8 provides:
-> Live/Neutral
-> Timeswitch 1/2
-> Earth
-> Switch Live

The Nest provides:
-> Live/Neutral
-> Call for Heat
-> Common
-> Satisfied

How do I link the two such that the Nest will ONLY enable the UH8 Zone actuator if the Master TM4 Timer Switch is also enabled?

Cheers,
Damon.

 

[stem]

You don't. The Nest provides both time and temperature control in each unit and replaces the existing TM4 timer and thermostats.

 

[ianmcd]

Really cant understand why you would want to

 

[DamonBristol]

The reason we want to do this is that we each of the zones goes to a set of offices where different tenants are being given control of the Nest Thermostat for their zone.

Then centrally the building landlord who pays the electric bills (on a time of use tariff) controls the TM4 timer.

So they want to retain a level of master override that will allow them to switch off heating for across peak times (5-7pm weekdays).
Also they want to prevent use of the heating system over the summer from the master TM4 timer.

 

[stem]

In that case:

CH1 of the timer is connected to Nest 1 terminal 2 (common) and the grey wire that was in terminal CH1 goes to Nest 1 terminal 3 (call for heat)

CH2 of the timer is connected to Nest 2 terminal 2 (common) and the grey wire that was in terminal CH2 goes to Nest 2 terminal 3 (call for heat)

The two Nests should receive their 230V supply from the same mains supply as the rest of the system, so that they are permanently powered and the whole heating system is isolated together during maintenance.

 

[DamonBristol]

Ok cool. So if I understand then the TM4 needs to enable/disable the common wire and that will be sufficient to override the Nest's call for heat. Is that right?

 

[stem]

Yes, the Nest has a simple on/off switch connected to terminals 2 and 3. So, when the TM4 switches 'on' a live is applied to the Nest switch (T2), then when the Nest switches the heating 'on' (T3 also becomes live) the heating will come on.

This means that both the TM4 and the appropriate Nest have to be 'calling for heat' at the same time for the heating to actually come on.

 

[ianmcd]

And when the tenants realise that the tight fisted landlord is illegally interupting their heat supply, they simply get someone in to over ride it