Sense check for contactor and wifi switch for immersion heater

We’re getting central heating installed next week. Being on Octopus Agile I want to take advantage of free/cheap (i.e. less than price of gas) electricity to heat the water in the cylinder. (Yes, gas boiler – long story!)

From my research a normally-open 20A contactor will do the job of switching the supply to the immersion on/off. Then I need something to switch the contactor. Shelly Pro EM looks to me like it does the job, as it mentions driving contactors.

I want DIN mounts as it’s tidy and I’ve got lots of space in the consumer unit. I’ve got a couple of Shellys already and been happy with them.

Am I on the right track? I’d to check before I call my electrician.

I did wonder if I could use the cheaper Shelly Pro 1, but that doesn’t mention contactor control. I’m guessing I would need to use that to switch 240V to the contactor. Reading this document for a slightly different application I wondered if I would need an RC snubber.

Thanks in advance for any help!

I haven’t seen this sort of advice here before on this forum but it occurs frequently on the Octopus Smart Energy forum. You may get better luck there. If you aren’t already on the forum DM me your email address and I’ll try to get you an invitation.


There are others.

Hey Tim. Funnily enough I emailed for an invite last night. I used to work for OE and had an account back then, but it was linked to my work account and I don’t have access any more.

Sounds like a plan, and probably a lot cheaper than getting an Eddi. Just make sure there is a way you can override the Shelly in case it or its connection fails.

I went for the Shelly Pro EM in the end.

I decided to leave it for the electrician to decide on the contactor and figure out the override. Figured we’d need another two RCBOs (one for the Shelly, one for the immersion) as well so he can just get all the bits at once.

I will report back for any one else searching in the future.