Smart replacement for 2-wire Brivis Thermostat

I’m just starting out in the world of DIY home automation. I have a tasmotized power plug that I’ve added to my Openhab server along with a bunch of other devices, and now I’m looking to get a bit more custom. I’m renting the house, so I can’t make any fundamental changes, but popping the BRIVIS thermostat off the wall and replacing it with a custom thermostat control that Openhab can control would make things WAY better for our heating bill.

I don’t want to do too much messing about with code on the device, since I’ve got enough work to do with the server, so I wanted to make something running Tasmota for easy integration.

My thoughts are to have an ESP2866 like this, connecting to a 24vAC relay switch that can make a circuit on the two wire control circuit for the thirty-year-old gas heater in the roof.

I was thinking that if I include a step-down voltage regulator/converter on the circuit, then I could get input power whenever the heater is running, and use that power to recharge a battery to keep it going at other times.

Does this sound workable? Does anyone have any suggestions for what to use for relay/transformer/battery components?

This will be my first attempt to make an IoT device, so please forgive me if this seems all obvious. Electronic circuitry is not a field I’ve gotten into before.

Does anyone know what the power draw is for the ESP2866 so I can work out how long it will be able to run on battery? It might need to go for a significant time period to stay alive over summer!

I’m not a lawyer.

That said, I would suggest some serious consideration of the possibility that making alterations to the control unit for a gas appliance would be against a few regulations and impact the validity of any insurance policy that the owner has on the property.

There is a reason why installers have to issue certificates of electrical safety for such work.

If the owner is willing to consider a controller with RF / IR functionality. it increases your options.

I use a couple of RF-controlled power sockets to turn my AC on and off. OpenMQTTGateway lets me send RF codes from a Wemos D1 Mini via MQTT. But nothing custom-made is every wired to the appliances.

I’m not touching the actual gas appliance itself, just the thermostat, which is only connected to the gas appliance by two wires. If there’s a circuit, the heating is on. If there isn’t, the heating is off.

But I have to say, if anyone knows of a tasmotizale ready-made drop-in thermostat, that would be much easier!

The thermostat / control unit is part of the appliance. You are replacing a component of the appliance. It may be a cheap component. It may have limited functionality. But it is still a part of the whole. It may be possible to argue a grey area when installing a new control panel yourself. But the DIY nature of your replacement component is well outside of the regulations.

Even if you were using an approved component, the simple fact that the “two wires” disappear into the wall can be enough to require a licensed installer.

Have to admit, discovering that it’s an AC circuit rather than DC, and seeing a video where an almost identical thermostat is replaced by a Nest, which gets its power from the two wires, has me reconsidering the project. I thought I understood how it worked, but it seems more complex. I might have to put up with one of the cloud based wifi thermostats.

Hi Politas,

Michael’s on the right track with highlighting the potential legal and safety issues. There are a lot of potential traps here. For your first IoT project this is probably too far in the deep end.

I’d recommend staying well away from mains voltage electrical engineering, and sticking with commercial AC-DC switch mode supplies so you can work with safe voltages and power levels.

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