Smart Immersion with contactor

witnessmenow · 20-03-2026 11:12PM #1

We have a standard immersion switch with Bath/Sink and On/Off and we would like the ability to control it smartly either through phone or home assistant.

When these posts come up a lot of people suggest devices from amazon, but I don't really have a whole lot of faith in no name devices from Amazon controlling a large load like an immersion.

One of the often mentioned solutions as an alternative is using a 20Amp Double Pole contactor (Something like this)to actually do the switching of the immersion, and using a smart device to control the contactor.

But I never see much information around peoples actual setups when they do this. Like are they using a consumer unit style enclosure to house the contactor and smart device? If so, how big of one? Are they wiring it before the existing switch? This would allow for Bath and Sink control to still be available and the on/off could be used as an addtional way of disabling the system.

Any suggestions on the above or anybody using any other methods?

rojito · 22-03-2026 11:22AM

Also investigating this. With the price of heating oil skyrocketing due to everything in the middle east I want to turn off my boiler completely during the summer and use the immersion during night rate electricity hours and a bit in the middle of the day to top up while solar panels are producing. I invested in more battery capacity rather than an eddi, but want more manual control of the immersion with a smart switch.

Climote seemed like a decent but expensive solution, however liquidators were appointed recently and I'm not signing up for something that requires a subscription if the company is going to disappear.

fanhistorii · 22-03-2026 11:33AM

Hey mate, spot on with the 20A double pole contactor (like that Garo)—it's the safe, reliable way to handle immersion loads without sketchy Amazon relays frying; wire it in series before your existing Bath/Sink switch so manual control stays intact and acts as a master isolator. Common setups use a compact DIN-rail enclosure (e.g., 6-12 module IP65 weatherproof consumer unit style, ~£20-40) mounted near the tank or switch—house the contactor on DIN rail, pair it with a low-power smart trigger like Shelly Pro 2PM, Sonoff Basic R2, or Zigbee relay (coil draw is ~5-10W), all wired with 2.5mm² flex; guys on DIYUK/OctopusEnergy forums run this for solar/HA integration with zero issues. Keeps it reversible, code-compliant (Part P friendly if sparky does it), and expandable—add a temp sensor for auto-off at 60°C; my similar rig's been rock-solid for 2yrs controlling via HA app. Pics/wiring diagrams often shared on Reddit r/DIYUK—search "immersion contactor HA". Stay safe!

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greenbin2 · 22-03-2026 11:39AM

The tp-link home plug is 13amp

I got a 3 pin double socket next to the tank and i wired the shower element to a normal plug

I can ask alexa to turn on the immersion even from my watch and the tp-link hub plays a sound and turns the immersion off 1 minute after the stat cuts it off

Sound alerts me that its time to shower

Family use it, i use the electric shower, they are ****, but are cheap to run

emaherx · 22-03-2026 09:43PM

https://www.boards.ie/discussion/comment/124331814#Comment_124331814

You need to consult an electrician. Those smart plugs are not suitable for heating elements even the 3 pin plugs is a bad idea and most likely against regulations.

10-10-20 · 22-03-2026 10:06PM

https://www.boards.ie/discussion/comment/124333331#Comment_124333331

Agreed. It's highly unsafe as the relays in those devices are often knock-off brands, and can't deal with more than around 10A, nevermind 13.

emaherx · 22-03-2026 10:41PM

https://www.boards.ie/discussion/comment/124333399#Comment_124333399

Yes basically the rating on those smart switch relays is for intermittent loads and not intended for continuous heating loads.

Look at the rating on normal immersion switches, they are usually 20A or more, even though most domestic immersions draw less than 13A. Due to the constant draw and long hours of use it's not something you want to be running close to the limits. High current during switching makes them very hard on contacts and going for higher rated devices will increase the life expectancy greatly.

It's not unknown for the extended use of heating elements to melt 3 pin plugs due to heating of the pins as the contact is not tight enough and this effect is worsened by essentially going through an extra adapter like the smart socket.

witnessmenow · 22-03-2026 10:43PM

https://www.boards.ie/discussion/comment/124331781#Comment_124331781

According to Gemini, with keresone prices the way they are, added with the inefficiency of having to heat the hot water via the central heating, we are basically already at the place where it's cheaper to use the immersion, even if it's a day rate, never mind a night or EV rate.

I took a look at my immersion switch today. Consumer unit has a dedicated MCB for the immersion, then the feed comes in from the bottom of the back box in hot press. So I might have to put the new consumer unit below it or extend the wire up above it somehow.

greenbin2 · 22-03-2026 11:59PM

Its drawing 7 amps for about 25mins at a time

No heat on the pins if unplugged while running

10-10-20 · 23-03-2026 12:48AM

https://www.boards.ie/discussion/comment/124333680#Comment_124333680

Ok look, your risk of a fire is higher if you use plug and socket components on a device which is designed to the permanently or hard-wired to the supply in a hot-press (there's a clue in the name), and probably using a cloud-based remote control system where you don't actually have control of the system (the cloud is "someone else's computer").

So I don't get an infraction, I'll address it this way:

People who install these components against good guidance and who may also be immune to taking feedback onboard (ie: not going to listen to good guidance) would be best off taking some simple precautions and educate any people who share the residence with them on how to effect an escape from a burning building in the dark when the main egress is unusable. Simple as that. And don't forget to tell them why.

Clear?

witnessmenow · 23-03-2026 09:41AM

My immersion definitely draws 3kW, which is 13amps, tested it yesterday but seeing what the draw to the house was on the EV charger app and turning on the immersion.

There is basically nothing that you'd reasonably want to automate that is a bigger power draw than a 3kW immersion. Even something like a kettle is only on for a few minutes.

I have a few smart plugs around the place and there are some brands I'd trust more than others, like sonoff, but even with them I wouldn't trust them to their rating, especially for a long running thing like an immersion.

Even with these 20amp contactor, I can buy some on Amazon for less than a tenner, but for an immersion I'm definitely not going to cheap out, to me it's just not worth the risk.

ted1 · 23-03-2026 11:02AM

https://www.boards.ie/discussion/comment/124333680#Comment_124333680

so your immersion only draws 1.6 kW? how does it run if its unplugged?

KaneToad · 23-03-2026 11:08AM

I installed these yokes. It gets away from any worries about electricity safety. Does the job perfectly.

https://amzn.eu/d/0dV31seR

witnessmenow · 25-03-2026 10:50AM

I bought a contactor in my local Electrical wholesaler, It was E24 for a Iskra Double pole 20A Normally Open Contactor.

I bought a 4 module Din rail box off amazon for E6.50

I bought blanking plates to cover the empty slots on the box (these might not even be right, I think they are. The wholesaler were out of stock of them apparently!?)

I had toyed with the idea of adding a din rail mounted fuse for the Sonoff Mini (I already have this), but I couldnt find one that took our standard plug fuse so I would have had to buy a selection of fuses too. If I add a fuse I'll just add a single gang box with a fused spur outside of the Din rail enclosure.

If I was buying again, I would probably just buy this Schneider contactor from amazon, it wasn't much more and I had to add stuff to my amazon order to get me to free delivery anyways.

Will give it a go at the weekend and share my setup. My plan is to cut into the wall below the current immersion and intercept the feed cable and put it into this box and feed a new wire up to the existing switch after the contactor.

10-10-20 · 25-03-2026 11:39AM

https://www.boards.ie/discussion/comment/124339117#Comment_124339117

Positive first-steps. I like it.

Don't forget to boot-lace any stranded cable. 💪

witnessmenow · 25-03-2026 11:44AM

https://www.boards.ie/discussion/comment/124339244#Comment_124339244

is that ferrules? I have a crimping kit I use on all stranded cable. Takes a bit longer but I do feel better Everytime I use it 😅

I assume it uses solid 2.5mm T&E for the immersion, I'll use the same again for the new piece

10-10-20 · 25-03-2026 11:54AM

https://www.boards.ie/discussion/comment/124339259#Comment_124339259

Yep, ferrules. Making Electrical Connections Great Again. 😆

ptogher14 · 25-03-2026 12:16PM

Witnessnenow you are getting some good advice on here and some very questionable advice. Don't just go with something because it's easy and cheap. Do your homework before you start automating your immersion. It draws a decent load. I've always been a fan of switching an immersion through a correctly rated contactor installed in the DB. Even manually set immersion timers will be fail over time (Flash, apt etc.). A good quality contactor is a much more robust solution.

Once installed, the contactor can then be controlled by a smart device. Many out there on the market. My own two cents would be if you intend on switching the 230V control circuit is make it sure it is CE stamped. A good starting point would be the shelly range.

I started looking at similar last year and it brought me into the whole world of home assistant and smart devices out there. I am using an ESP 01 relay card to switch the contactor, bought for a couple of euro on aliexpress. I'm not a fan of putting 230V on anything bought on aliexpress for peanuts so have a 12vDC relay in between but the principal remains the same.

Once integrated with home assistant you have al sorts available to you. Switch on during night window, switch on during export, load shed if required, monitor what the immersion is using during the various windows(night, solar), if tied to a temp sensor get alerts to your phone if temp drops and give you the option to boost the immersion.

witnessmenow · 29-03-2026 02:52PM

Installed this today, took a few hours! I have the immersion on at the moment just to test the setup during the day. We haven't used the immersion in a while either

What I used:

Used everything I linked to above, the blanks did need to be trimmed a bit to fit in the consumer unit though.
I used a sonoff mini as the smart device as I had one spare. I liked sonoff stuff, seems well made and has a good reputation in the diy iot world. If I was buying something I'd probably buy a sonoff basic with a din rail mounting bracket. I cable tied mine to the rail.
As well as the above I used a single surface mount box, a fused spur with a switch and a 3A fuse.
For cables I used: 2.5mm flex from the contactor to the existing immersion switch, 1.5mm flex to my fused spur and 0.75mm flex from spur to sonoff. Ferrules on everything!

Cliff notes of steps.

Cut into the wall below immersion. Used a hole saw, put the drill into reverse when it bit into the wall and tried to only go as far as the paper of the plasterboard. Glad I took these steps cause I couldn't have hit the cable more direct if I tried! I originally tried further to the left but I hit a stud.
Disconnected 2.5mm feed from immersion, taped a 2.5mm flex to it and pulled it down to my new hole. I had planned to use solid core but the consumer unit was tight enough for space and the solid core is not very bendable!
I did need to add a 5 way wago for connecting the earths behind the immersion. With ferrules I couldn't fit anything into the existing switch and back box earth terminals with the 2.5mm flex.
Old immersion feed into the new consumer unit, used wagos to distribute it to the contactor and to the fused spur for powering the sonoff mini.
New feed to the immersion switch at the top of the contractor, 2.5mm flex cable to the wagos at the bottom.
Left the earth cable to the immersion switch long to connect it directly to earth wago.

Thanks for the advice! This was a job on my todo list for a while so nice to get it done.

10-10-20 · 29-03-2026 05:44PM

https://www.boards.ie/discussion/comment/124349011#Comment_124349011

Sweet!

Does it make a satisfying >thunk< when it's energised?

witnessmenow · 29-03-2026 06:14PM

https://www.boards.ie/discussion/comment/124349425#Comment_124349425

Yeah it does, I'm not sure what you was expecting from it but it was a surprise the first I heard it!

Smart Immersion with contactor

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