Nest Thermostat
In 2014, Sandra and Ian installed a Nest Thermostat, previously they had a programable thermostat. The NEST offered more control via the internet and has some intelligence / learning built in. The thermostat is wall mounted in their hallway, so not in direct sunlight.
Cats, Catio and a problem
Sandra and Ian love animals, in particular their Maine Coon cats, who are all indoor boys, though they like to provide the cats with some outside space. In late spring, summer and early autumn weather permitting, they open a window in their sitting room that leads straight into the Catio.
The problem that arises is when the window is left open for the cats to come and go as they please from the Catio, the temperature in the sitting room, then hallway drops. When the temperature drops, it triggers the Nest thermostat to call for heat (start the boiler).
How to automatically stop the Nest thermostat calling for heat
Ian had already setup Home Assistant on RPi3B+, then made the decision to move to RPi4B, details of this can be found here.
In the UK, boilers are normally triggered to ‘fire’ by terminal “3 – Heating relay normally open/call for heat” in the Nest Heat Link. Dependant on how your boiler, pump and valves are wired i.e. S-plan or Y-plan, 230v is switched live as the call for heat.
Utilising terminal 3 in the Nest Heat Link, the switched 230v can be utilised to energise a relay such as the following:
Taiss/AC 220V Coil Electromagnetic Power Relay 5A 2DPT 8 Pins 2NO+2NC ←(paid link)
Using the relay above, Ian has wired it as follows:
- Terminal 13 – Terminal 3 of Nest Heat Link
- Terminal 14 – Neutral feed
- Terminal 8 – Switch wire
- Terminal 12 – Switch wire
The output from terminals 8 and 12 become connected when 230v is applied to terminal 3 of the Nest Heat Link, in effect turning a switch on. Using the ‘switch on’, we can wire into the following Zigbee device
SONOFF ZBMINI ZigBee Mini Smart Switch, 2 Way Light Switch ←(paid link)
Wiring in to the ZBMini as follows:
- Terminal S1 – Relay Terminal 8
- Terminal S2 – Relay Terminal 12
- Terminal N In x 2 – Neutral feed
- Terminal L In – 230v live feed to power the ZBMini
- Terminal L Out – Wired to where previously terminal 3 of the Nest Heat Link was connected
When the Nest thermostat calls for heat, the ZBMini senses a switch has been turned on and applies 230v to terminal L Out. The ZBMini can also be directly controlled by HomeAssistant when a ZigBee gateway is used. This configuration means that the Nest thermostat can be overridden by HomeAssistant.
How we setup HomeAssistant to take control of the home heating.