How to make isolated bathroom towel radiators smart?

Hi there,

I’ve had good success with Sonoff S26 plugs with Tasmota and Hassio on standalone electric radiators.

I’d like to now automate electric bathroom towel radiators.

They are currently all isolated in a different room with something that looks like:


What’s the best way of making this “smart”?

I could try to replace with a Sonoff T1: but judging from the reviews it’s quite deep so would be hard to retrofit. I do like a manual override as there will be some users wanting an “old fashioned” way of controlling.

My other idea is to try and stuff a Sonoff R3 inside:

Not sure it will fit?

Third option is to roll my own MQTT device but less keen on this.

Any advice appreciated - yet another option would be to “intercept” the wiring outside the isolator switch but that would be in a wet environment so again not keen.


You can probably use a Sonoff mini.
It also does not need any “real” switch so you can just have a faceplate and a switch.
This switch does not have any electricity so any switch will do that leaves enough room behind for the mini.

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After a slightly painful process of flashing Tasmota, I now have the Mini working. I like the additional switch feature so people can manually turn the radiator on/off (not sure if you need to program this or it automatically overrides).

However, the casing doesn’t have room for the device, and there is a 5A fuse built-in:

The fuse is hard-wired into the switch, so I can’t keep that functionality. I’m thinking of replacing the box with:

  1. Regular (non fused) wall switch as S1/S2 like this one
  2. A non-switched spur like this one

Which will hopefully give me room to put the Mini in one of the above boxes.

So two boxes, one fuse, one switch and one Sonoff mini.


I’m completely amazed with the position of a mains switch. On floor level? In a bathroom?
Wow. I’m not an electrician, but that is not allowed here.

Anyways. What I would do is buy a new case to replace the old one. That way you can restore it at any time and you can make it look and work the way you want.
Just pick any one that is enough size to fit all the parts in there:

and S1 and S2, pull out the cables from below and have them terminate in bare copper wires, off course one red and one black :slight_smile:
Jokes aside. A switch of any kind since S1 and S2 does not have any electricity.

Or, if I understand Tasmota correct, you can even move the switching part to a different device.
Say you have another switch with Tasmota, that switch can trigger the towel radiator.

Valid concern - I can add that this is all outside the bathroom so within regulation as far as I can see :slight_smile:

Good idea on project box - I was thinking of sticking with existing mains boxes as they would all be rated/tested?

Not sure about the tested/rated thing.
I don’t think they need to be. But as I said not an electrician, and we are not even in the same country.
I suggest you look it up.

But I don’t understand what that rating should be. It’s not waterproof and there is no other boxes I have seen that is (in houses).
If it’s hardness of the case, that could be a valid thing. But as long as it’s hard enough that you can’t poke your fingers through the plastic then it’s just like any other case made for the task.

I don’t know. Don’t trust me on anything I say in about the case :slight_smile:

I’ve done similar (with metal flush box though).

Replace you single backbox with something like this:

And fit an unswitched fused spur in one side

and a switch and sonoff in the other side

Make sure the sonoff is rated for the load:

Finally finished this:

  1. Two deep (44mm) backboxes
  2. Sonoff Mini
  3. Fused 5A spur with indicator
  4. Bell press switch

The towel radiator is now on a timer and has a “boost” button for 30 minutes of activation ad hoc.

Think you need a connector on the earth cables !

Also, I think you’d be better having the sonoff after the fused spur, then that would protected by the fuse too.

Or at least some electrical tape.

But why is there a cut in the earth cables just by the wall?
Or are they threaded without isolation through the walls?

I’m guessing supply cable and then load cable for the radiator

Yes, earth is supply and load. I inherited the current “wiring” and will fix with a connector.

Re: putting the Sonoff Mini behind the 5A spur yes, I agree, but the indicator would then always be on. I like having the power indicator only active when the radiator is active. Ideally the 5A would sit in front of the mini and the indicator would be wired to the load but no such option with my equipment.