Electric boiler (heater for domestic tap water) - what brand for integration with openHAB?

I’m struggling to find a brand of electric boiler that allows wired external controls. There are dumb devices that I’d have to modify to get the water temperature and probably use a power relay to switch on when needed or a fancy wifi + app boilers that are probably overkill and there is no info about integration with any system except promise of “controlling from your phone”.
I’ll be running that boiler in a house with solar panels, so I want to run it when the sun shines, not on a clock scheduler.
I did search the forum - please point me in the right direction if there is a topic about electric boilers.
I’m OK with DIY, but I’m starting from empty space for a boiler, so I’d rather buy a solution than make a solution.

Edit: I’m looking for a device to heat up domestic tap water, not a device to provide heat for central/underfloor heating. Low power 1.5kW and relatively large tank +100 l is preferred because of the planned solar power.


My idea for control of the boiler, if I have to go DIY:
Hardware: thermostat on the boiler, additional temperature sensor for openHAB, power relay to control the boiler externally. The goal is to maximise use of the solar energy, without compromising the comfort.

  1. The thermostat on the boiler set to maximum temperature, probably 70 deg C
  2. Current solar power info available for use needs to be known to openHAB
  3. Current water temperature needs to be known to openHAB
  4. All taps need to be be with thermostat to avoid burns with 70 deg C water (not sure if this is required)


  1. If the water temperature is below comfort temperature (probably 45 deg C) - turn the heating on, no conditions
  2. If the water temperature is above comfort temperature (probably 45 deg C) - turn the heating on if the available solar power is higher than the boiler power (+ a margin of 10-20%).
  3. If there is plenty of solar power the boiler will reach the max. temperature and them the built-in thermostat cuts off the power
  4. Some hysteresis to be added to avoid rapid on-off cycles
  5. Anti-legionella heating to 65 deg C, independent of available solar power to be executed once a week, if the boiler didn’t reach the treatment temperature in the last week. This is probably not required, but I like to use the system to the max :smiley: Time and temperature of treatment to be confirmed.
  6. Guest mode (expected higher water usage): bump comfort temperature to 55 deg D
  7. Holiday mode (expected zero usage): drop comfort temperature to 15 deg D, so effectively only solar power would be used.

I kindly ask for comments from more experienced openHAB users.

Have you looked at pages 12, 13 and 14 of this manual
Specifically, connection A , programmable function input (0V)

(Corrected link)

Viessman Vitotron 100
single phase electric system boiler
(Diagram shows an alternative connection for three phase)

Thank you for the help!
The first link it’s broken, but I checked other models - they seems to allow external control. The problem is that’s it’s a electric boiler for central or floor heating - the water flows in the loop. I need a domestic or tap water, so large (100l or so) tank. also for solar powered it’s better to have low power like 1.5kW, those from Vitotron 100 seems to start at 4kW.

P.S. i just realised that my opening email was not precise enough or even misleading. I’ll try to edit it.

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how much power does your hot water system draw?

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There is an existing water heater with power probably 1.5 - 2 kW, but I’m going to replace it. Water usage for 2 people, so it won’t be very high.

Edit: Ideally I’ll be aiming for large capacity, low power and high quality insulation.


I have one of these under our kitchen sink that works REALLY well.
Enough to fill a good sized sink, by the time the sink water is cold, the tank is hot enough to refill.

It stays hot for many hours.

We have it connected via a Velbus relay, which is activated by a PIR.

No presence in the room - OFF

Presence in the room - Start a “restartable” 20 minute timer. (Velbus Action 0410)

There’s no reason why you couldn’t condition this logic with solar output state or simply “external light level” with a Velbus outdoor PIR (which has light level detection) or a Velbus Weather station, that can disable the relay when it’s dark

and so many more options

I’ll add some monitoring of the relay and post the graph in a day or so, so you can see what is happening


I have added a 10Kohm bead into the thermocouple tube within the unit, to give exact tank temperature monitoring.

A 2Kohm bead could be used if you wanted that data on the Velbus bus, for displaying elsewhere or to condition system logic.

The standard mechanical thermostat is poor at best, I have left it in place to act as an ABSOLUTE maximum limit device.
The unit that this replaced, had a secondary thermo trip device, which never triggered in it’s 20 year life, but good to have.

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My water heater draws 800W (270L tank) and it runs for 2-3 hours a day, set to 60°C. It’s not worth for me to worry about its power draw. I just set the timer so it is restricted to 9am and 5pm and usually by noon it no longer runs.

If you get one that uses heat pump instead of resistive heating element, it would draw less power.


	String	KitchenGP2_Dark					"Kitchen Dark"										{channel="velbus:vmbgp4pir:VelbusNetworkBridge:32:input#CH5"}
	String	KitchenGP2_Motion				"Kitchen Motion"									{channel="velbus:vmbgp4pir:VelbusNetworkBridge:32:input#CH6"}
	String	KitchenGP2_MotionLD				"Kitchen Motion LD"									{channel="velbus:vmbgp4pir:VelbusNetworkBridge:32:input#CH7"}
	String	KitchenGP2_Absence				"Kitchen Absence"									{channel="velbus:vmbgp4pir:VelbusNetworkBridge:32:input#CH8"}

	Switch	KitchenWaterHeater	"Kitchen under cabinet heater"		{channel="velbus:vmb1ryno:VelbusNetworkBridge:69:CH1"}

Logic is

Any motion on channel 6 “motion” starts a “restartable” 20 minute timer on the water heater, this will reset whenever motion is detected, in effect the water heater is active whenever anyone is in the kitchen.
The tank takes only a few minutes to heat from stoney cold, but also stores heat very well, so even overnight or during working hours, it retains its heat.

Channel 7, “motion dependant on light”, triggers when there is motion detected, but ONLY if the ambient light level is low, in which case it starts a restartable 5 minute timer for the lights.

I’ll upload a graph after a few days so you can see the effects

Ha Ha

Don’t tell the wife what time I got home this morning


I’ve noticed something interesting since creating this graph.

It seems that our under cabinet water heater holds its heat for a lot longer than I had given it credit for and the energy use to recover to the target is minimal.

Note the peak in the energy consumption when the PIR triggers the heater and how quickly it rolls back.

and that it doesn’t even use any energy when it’s triggered again two hours later.

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I like this A LOT

Would you be kind enough to share the make and model number?

A lot of the brands here in Australia are rebadged, i.e. a company established a local brand, and imported rebadged systems from China. So the same stuff might exist in your country but in a different brand with slightly different looks.

One “international” brand might be Sanden.

The one I have is from this company: https://www.hydrothermhotwatersystems.com.au/
It costs me AU$1600 including installation + removal of my old one.


Ariston have them: Ariston Group

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My solution is a add-on to the existing gas heater. The 180L warm water storage tank was fitted with a 3kW resistive heater and a thermal couple to monitor the temperature in OpenHAB. The power of the resistive heater is regulated by a reasonable large dimmer (0-10V controlled thyristor) that is driven by a KNX analog actor. So if solar excess power is available a rule calculates the setpoint for the actor and heats the water till desired maximum (60°C in my case). The resistive heater has a dump bi-metal switch for safety that cuts power at 70°C.
Advantage is that it is not restricted to simple on/off but cn make use of any surplus energy.


Hi, (my first post ever) I converted my 200L (3x0.88kW) 2.88kW electric Ariston boiler where I can switch the heater elements separatly via a Sonoff 4CH PRO. The original boiler thermostat protection is kept.
The aim is to heat with only the surplus of solar energy I have. Therefore I opted to put two elements in serie so that I can finer control the power.
Additional heating is forseen during the night period and via the 4th Sonoff button, I can force it to manual heating.

The only issue I see with the Sonoff are qty mechanical operations (perhaps in due time I’ll foresee a proportinal control). Electrical wise, switching a resistive load is not so harsh.
More detailed information (in Dutch) can be found at Boiler als battery - power shaving.

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I’m not sure where you’re based and hence how valid this will be, but my 2c.

I have a Tesla T-Smart, which is a thermostat probe and a smart controller for a 3kW immersion element for what I think are standard UK hot water cylinders. I don’t know how common or available this is outside the UK.

It has a local API which is documented, I’ve written a binding for this which I need to finish off, but it’s been stable in my system for almost a year. https://github.com/openhab/openhab-addons/pull/14247.

My water cylinder’s main heat source is my gas boiler which also supplies the central heating, so I currently just use this through openHAB to monitor the temperature and control when the water’s heated by the gas circuit, but I can also use it as a backup in the case of boiler issues, and might in the future use electricity to heat the water off peak if I switch to a dynamic tariff.