Device switched on or current detection - how to do it?

I’m looking for a way to detect if a electric water heater is switched on. It’s controlled by a velbus relay but also by a temperature switch, so the velbus relay is effectively an “enable” signal, not necessarily “start” signal (automation terminology, I hope it’s clear enough).
The heater takes probably 8 amps when switched on - any ideas hot to detect it? There are plenty solutions for measuring the current, but I’d like to use VMB7IN - digital input module. And all I need is get the signal when the heater is on.

Fron the VMB1RYS documentation:
- standalone mode: in standalone mode a push-button connected to the input operates the relay momentary or toggle mode can be selected by toggling the slide switch on the back
- Velbus mode: in Velbus mode the module is connected to a Velbus domotics system the relay and input are used as Velbus output and input channels

Does it mean the the input on the module can be used separately from the output? Ir the input always trigger the relay?

Edit2: Unipolar Hall-effect switch + a transistor might be able to do the job.

How brave do you feel?

(Or what is the warranty on the heater?)

Personally, I would connect an interface relay after the thermostat within the heater and just use the contact closure as a status signal

Yes… :smile:

That’s exactly what it means.

I laugh in the face of the danger (read: installation by company “Family, friends and me” voids the warranty anyway). There is a light that indicates operation - I need to get to it and measure the voltage to pick the relay. Or check if there is a free contact on the factory relay.

Thank you for the idea! It’s much easier to implement than mine :slight_smile:

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You can also add a Kwh meter on the water heater power line. Then you connect the pulse output to a VMB7IN and set an alarm. You will have a state of your heater and also its consumption and no need of changing someting when replacing the heater


Thanks for the idea, but installing a kW meter on a fixed load to detect it’s switched on is an overkill for me. Id would work, but I can calculate the energy from time (always 1500 W of power) and ithe heater is almost brand new - I expect it to last a good few years.

I’ll open the cover, maybe I can tap a relay somewhere into the wiring.

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Is there a chance your heater itself is smart to some extent? They rarely make “dumb” devices that plug into an outlet anymore…

e.g. my heat pump has a modbus interface, which allows me a great deal of introspection not only into whether it is working, but also why it is doing so… Which is really useful information sometimes (in my case it has helped to make my case for a warranty claim.)

It’s dumb as a rock - I wanted it that way. I was unable to find anything that allows external control, domestic grade. There are fancy models, wifi, cloud, vendor app - I stay away form that kind. it’s always a pain in the long run. The heater has a double digit 7 segment display showing the temperature, light indication the heating is on and a knob to set the temperature. That’s it.

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That’s Fancy

The Ariston under cabinet water heater I recently purchased has…

  • A red neon to show it is on / heating
  • An uncalibrated rotary dial to set the target temperature.


There was enough room in the capillary tube to put in a 10K bead for my own digital thermostat.

(Using the mechanical one as a upper limit)

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Thank you for keeping the topic alive! That pushed me to search for the elec. schematic. Now I know I need a small 230 V relay and tapping ii into the bulb wiring should solve the issue.Relay will close the contacts on VMB!RYS and I’ll have info in the Velbus network is the heater is on. :- :heart_eyes:

Edit: I got this one: Flush mounted electromagnetic relay 10A 230V AC TYP: PEP-01/230

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