[Heating] Controlling a gas fired boiler (thermostat)

Hi community,

I’ve been delving into the HASS community often, doing a bit of DIY (little projects). I’ve recently decided I want to automate my heating with Homematic IP radiator thermostats but have stopped at a snag. Let me start at the beginning.

Current setup: The apartment has a gas fired boiler (radiant, 2002), which is controlled by a central thermostat in the living room (it flips the switch when the hot water starts circulating). The radiators all have dumb radiator valves / heads which open and close when needed.

Desired setup: Homematic IP thermostats on radiator valves and a way to trigger the heat demand.

While the radiator thermostats are trivial, how to start the water circulating is an entirely different matter. The old thermostat is a plain switch (no potential), combining a 24v AC current coming from and to the boiler. Basically, if I would connect the two wires, the boiler would run indefinitelly.

To make things clear, here are some basic pictures

The thermostat in the picture is connected with two 24v AC wires.

My current idea is to use the Shelly Uni, connect the two wires to the potential free. Here’s a (very) rough schematic (next post, since i’m limited):

I have tried to ask something similar on the Homematic forums, but alas, they mostly focus on the German side of the forum, so no luck. I hope that this community proves helpful.


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I have a similar setup with eq-3 max! cube and devices, with the boiler controlled with a zwave relay (and I have seen your message on the homematic forum since I’m considering switching to the HomematicIP devices)

What you are suggesting should work. I don’t know about the shelly connections though, but as long as the shelly is just acting as a simple switch for the boiler thermostat input, it should work

you will then have to interface the thermostats with the shelly via an automation in home assistant (or node red in my case)

A downside that I have encountered with my setup is that if the link between the thermostats and home assistant is lost, or if home assistant crashes, there is no command anymore for the boiler! if you’re lucky the switch was closed and the boiler keeps circulating the water and the thermostats keep doing their job, otherwise, you will have no heating until everything is back up!
To solve this, I’m thinking about using this homematic relay switch HMIP PCBS: HomeMatic IP switch circuit board at reichelt elektronik in order to have a direct link b/w the thermostats and the boiler. However I’m not sure if this is 100% independent of the network or not (I’ve posted in the homematic forum to ask this question)

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The future of the Homematic IP Cloud integration is uncertain, so I don’t want to recommend buying a Homematic IP Access Point + WHS2 if you want to integrate with Home Assistant. From my own testing (link to homematic forum) I think a WHS2 is no longer essential.

@ganfoud suggests using a PCBS to overcome the no-heating-when-ccu-is-down issue by creating a direct link between a room thermostat (WTH, STH, STHD etc). and the PCBS. Unfortunately I can’t tell you if that’s possible or not, because a homematic device specifies with what other devices types it can create a direct link. You definitely can do that with the WHS2 and a CCU, so that might be a safer purchase.

If you don’t care about that, any switch actuator that integrates with Home Assistant should do the trick if you setup the correct automations. Make sure the boiler can dissipate its heat to an open radiator before you turn it on to avoid tripping any overheat protection circuitry, which may need resetting manually.

Thanks for the replies, I’ll try to return the favor by explaining what I did, so others might benefit.

@ganfoud the solution you’re after might be this: homematic forum (Auto translate it). In the last post, the user describes how to use the PCBS + RMS to achieve what you are talking about. For me, the solution seems more complex than using a Shelly 1 > Since posting the above, I’ve switched opinion and decided that I’d rather use the Shelly 1 instead of the Shelly Uni because it’s a bit simpler and the HASS integration is more sure.

I’ve also thought about the wifi / hass crash scenario. I plan to integrate a switch (physical) to manually turn it on. Alternatively, I might just leave the old thermostat where it was, disconnected, and use it when that happens. A more robust solution would be to have another smart switch to switch connection when no wifi but I haven’t gone down that rabbit hole (yet).

@wutr thanks about the post about the homematic IP cloud! I have a raspberryPI lying around, so I’ll switch to raspberrymatic should that happen. I’m unlikely to switch from the homematic solution since I already have 2 thermostats, access point and window sensor. However if you have a great alternative suggestion, I guess I could pawn it off on ebay…

I would recommend caution to anyone who’s reading this about using the WHS2 → that thing will push 240v into the boiler which will fry the circuit. A dry-circuit (potential-free) is absolutely necessary. Trust me, I learnt it the hard way.

Or leave it connected set at the absolute minimum temp that is acceptable. Should all else fail it would kick in before the house hits freezing temps.

@Mikefila so have two sets of wires, both able to complete the circuit?

I thought the WHS2 used potential-free contacts?

Exactly, you can have multiple closure points. My particular setup I have a smart therm for day to day, an old honeywell round as the fail safe, an aquastat and a stand alone relay that can all close the thermostat circut.

@monsunsi actually the issue I currently have is not really a crash of home assistant or the max! Valves: it is mainly the link between the two, so if the homematic solution is stable the zwave switch I am currently using is perfectly fine and using a good old dumb thermostat as a fail safe solution is a good one!

Hi guys,

following up with my solution that works perfectly now. Hopefully others like me will find this solution much faster and will be able to automate the issue. In the end, this is how it works and it’s very simple.

To make this work you need a relay with a potential-free circuit. I used the Shelly PLUS 1 (plus because it’s 2eur more expensive and it offers more flexibility). You could use a rPI and some arduino relays, but why go through that hassle when Shelly + HA OS makes things much simpler.

DO NOT use anything without testing it first. The WHS2 from Homematic shoots 240V into the furnace effectively frying it. Always test the output with a multimeter and ensure you have an output of 0 DC/AC while the circuit is complete.

The wiring is simple.

I decided to use 12V DC to be on the safe side. You can use 240V AC straight from the plug. Please note that wiring the Shelly Plus 1 and Shelly 1 for DC power is different.

You can discard the 2x two-way switch if you don’t want a fallback but redundancy is always preferable.

Also please note the wires coming from your gas furnace. You can’t mix them up but please be careful. Playing with electricity and gas can be dangerous.

Then it’s a simple thing to control this in HA OS.

I added a few template sensors:

  - binary_sensor:
      - name: "LivingRoom_Demand"
        state: >
          {% if (states('sensor.hmip_hmip_etrv_LivingRoom_metrics_heating')|float) > 15 %}true
          {% else %} false {% endif %}
      - name: "Bedroom_Demand"
        state: >
          {% if (states('sensor.hmip_hmip_etrv_Bedroom_metrics_heating')|float) > 15 %}true
          {% else %} false {% endif %}
      - name: "Other_Demand"
        state: >
          {% if (states('sensor.ble_temperature_xiaomitemp_1_a4c13878d22e')|float) < 15 %}true
          {% else %} false {% endif %}  

Obviously you need to change the entities to whatever you have at home. I used homematic devices and xiaomi aqara thermometers then wrote the logic around it.

In the end, the heat demand is either ON or OFF so I combined the sensors:

    - platform: group
      name: Heat Demand for Appartment
      device_class: power
      - binary_sensor.LivingRoom_demand
      - binary_sensor.Other_demand
      - binary_sensor.Bedroom_demand

A potential upgrade you might do:

  • discard the two way switch and wire the old thermostat directly to the shelly plus 1. But first test if your thermostat can handle the 5V output (mine can handle 3V, so I had to use switches). This would make the whole solution even better and neater

I’ll leave you with that, hopefully no one fries their furnace electronics after reading this rather simple guide.

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Hello all

In september of this year I will be relocating and installing a new heating system for my house.

This solution will comprise of a air to water heat pump a gas boiler and thermal solar panels all heating a 500 lt hot water storage tank. this tank will then be connected to the tecnical hot water and the sanitary hot water distribution system via 2 seperate pumps feeding the whole house.

I currently have 11 hot water radiators and i will install 4 fancoilsspread over 3 floors

youd you please provide me with confirmation if my idea hear below would work.

I would have watet tank always full of water at 65 deg C from sept to may and then the temperature would be reduced to 40 deg C from may to september

In my mind I would substitute the radiator valve with zwave or zibee or wifi (compatible with shelly) thermostatic radiator valves. Since i already have mutisensor sensors on all floors I would use an automation and use as triggers the sensors on each floor to control each individual TRV.

in this case i could also use the motion sensors in the automation as a condition


would it be more advisable to use wifi trvs equipped with thermostatic valves and integrate each valve into HA and use the sensors just for presence detection.

or would anyone have a better solution

thank you all for your response