Wiring a low voltage (zigbee?) switch in series with a conventional thermostat

Hardware
1

Hi all, I am just getting started down this home automation rabbithole; I want to run this idea past people with experience before starting to buy things.

The problem:
For reasons I can’t fathom and can’t seem to get any answers about from management, the common areas in the building where I live are kept at about 27°C. Even at times like last night, when I had the heat off, all the windows open, outdoor temperatures dropped to 5°C, and the temperature in my bedroom got down to 10°C, the temperature near my thermostat stayed around 23°C because of all the heat coming in from the hallway. I am at the stage in my life where I have a very strong preference for cooler temperatures, and that could cause big trouble this coming winter when there’s a cold snap, if the outside-facing walls, where the heating pipes are ,get really cold but the thermostat stays toasty.

I called my local plumber/HVAC place, hoping to get some kind of mechanical/hardwired solution, but they didn’t know of one and tried to sell me an ecobee. If the solution is home automation, I don’t want it to be over wifi, and I don’t want anything that needs to talk to the internet.

My current (non-smart) setup:
About as basic as it can possibly be. There is a mechanical thermostat with the metal spiral inside, connected to two wires (24V) that stick out of the wall, and (I assume) connect to the zone valve for the radiators.

An interesting twist: because of the very cold climate here, the zone valve is fail-open rather than fail-closed.

What I want to do
I read this other discussion where someone wanted to wire a smart switch to his AC thermostat so that it could be controlled manually or by HA, but it didn’t quite work: Relay with old-fashion thermostat as a backup plan

I think, because my zone valve is fail-open, I might not have the same problem.

I want the heat to turn on/the valve to open, when it’s too cold at the outside walls, or when it’s too cold at the thermostat, and otherwise I want the valve closed.

The valve stays closed when it is getting current from the thermostat.

If I wire the thermostat in series with the switch, then if the sensors at the wall are warm enough, the switch should turn on/close the circuit, and if the thermostat is warm enough, it also closes the circuit, and the valve stays closed.

If either gets too cold, it will break the circuit, causing the valve to open and the heat to turn on.

If it isn’t doing what it’s supposed to, manually futzing with the switch and the dial on the thermostat should be able to force the state you want until you can solve the actual problem.

So:

  1. Is there anything about this plan that sounds impossible, dangerous, or otherwise bad?
  2. Is there a particular switch that you would recommend for this application? The wiring diagrams for the switches I have looked at, don’t look like they are meant to be wired in series (but it would help if the captions were in a language I know, and if I had pursued electrician skills past 7th grade shop class)

Thanks for reading my novel!

2
  • Using a battery powered Zigbee switch(/relay) is obviously safer that using a 240V powered zigbee object.
  • Avoid switching the relay too often as every switch degrades the relay (the number of switching cycles is specified in the relay’s datasheet);
  • There is no issue to put your switch in series with another, and certainly not in this case: you are on 24V, the zigbee switches I know all use relays and the current is limited.
  • I would choose a zigbee switch that has the following command and the choice between NO/NC (normally open/normally closed):
    image
    image701×69 4.85 KB

    That command allows you to turn the switch on for a given duration and it will turn off all by itself.
    So that should avoid that the switch remains in a state in case Home Assistant fails.
    As the switch should be battery powered, you do not need to worry about the initial state on power on.
    None of the zigbee objects I have support the 0x42 “On with timed off” command :slightly_frowning_face: .
3

Not related so much on the question on how to turn it off, but more on a possible why this may not be a good idea:

A lot of heating systems need there to be a way to let the water flow at all times, in order not to destroy the pump. This can be achieved using an overpressure valve. But most of the time, there’s no such valve installed and the system relies on at least one radiator to be always open.

So there’s a chance that if you close it, the pump will fail at some point in time, leaving you out in the cold. Something to keep in mind.

4

I’d make sure of these statements, first.

Is it possible that your thermostat wires go back to some central control box, as do the wires from the radiator valve?

5

True, but here he’s action on the thermostat on/off signal so it turns off everything or nothing.
With individual thermostats you need to be careful. I installed an adaptive pump and I’ll leave one radiotor open. Unfortunately, I can’t share experience - after insdtalling it I discovered a major issue on my system and I am waiting for parts to fix it.