Smart switch for immersion

Just Get an analogue or a digital timer. It would very wired just before the immersion switch using the sane 2 5 cable going to it. Badically thet are in series the timer first then the immersion switch. Otherwise no idea. Maybe someone else in here might know.

Take note of what poster AMKC said above, that timer does not replace the Sink/Bath switch because the timer is a single channel switch i.e. you will still need to manually switch between Sink and Bath. The timer needs to go between the mains feed and the S/B switch. That existing switch looks very old and the LED light probably doesn't (or never did) work, I would replace it with a new switch because, if there is a problem and you're not getting hot water, you need a S/B switch with a working indicator to be able to see if it's getting power from the timer.

As with most Chinese timers, there is no earth connection.

Not strictly true as some start timers come with an isolation switch.

Like this one…. https://www.timeguard.com/products/wi-fi/wi-fi-switches/fstwifitu-wi-fi-controlled-fused-spur-tuya-edition

But having a timer of any kind as your only control means that you would lose the ability to switch between the bath and sink elements. That and not safety is the reason to keep the Sink/Bath switch.

That control is not suitable for 3kw immersion in case anyone thinking of getting it

You dont fuse them at 13A. Overheating and possibly fire risk will likely result

https://sonoff.tech/en-eu/products/sonoff-powr3-high-power-smart-switch?srsltid=AfmBOooWZEMLZ1hLMD49KqlrBsni7Rii5uWoffsLq4DEpLW1QauTQkeD

This is rated up to 25A. It's what I have installed. As someone said, it goes in before the switch and means you still have to manually toggle between sink and bath if that's your thing. I would not use a TH16 - I use them on other controllers but I wouldn't use it for an immersion so hence using the 25A rated one. Note you will need a electrical junction box for it as well as the screw terminals are exposed.

I use it to kick the immersion in during the 3 hour cheap EV window on our electricity plan - it's cheaper to heat water on the immersion that window than to use gas.

I take it you're not a spark

Any rec will tell you not to fuse 2.8/3 KW immersion at 13A

Its simply not done as it's bad practice and bad advise

no, I’m not a spark. But I got the timer installed by a rec. And he raised no issues with it.

My electric kettle is Kenwood, the plate underneath says 2520-3000W, why it it ok to use that with a 13A fused plug but it’s not ok to run my bath element (2,800W) with a 13A fuse?

I may have come across as smart alecky it wasn't particularly intended

You were right on the first point you made that some smart controls have the isolation inbuilt so would be permissible if swopping out a switch

On the second point you may get a spark to say a 13a fuse is ok but I would say they are wrong

What actually causes overheating, I can understand the fuse blowing after a few hours if running right on the blood at 13A at 230V, say 3.0kW, I can't immagine a awful lot of heat being transferred from the fuse itself, maybe its the pins contacts?.

Ok, I did what I suggested I would do in post #19 above and here are the results.

1. The immersion ran on 'Bath' (2.8 kW) for 45 minutes this morning. At the end of the period, I took out the 13A fuse and it was hot to the touch. Not so hot so that I had to pass it from one hand to the next, I could hold it in one hand but it was pretty hot.
2. For comparison, I filled my electric kettle to the max , plugged it in via a smart plug with energy monitor and tested that setup. After 3m 30s and at a consistent power consumption of 2.67 kW, the water was boiled and I took out the 13A fuse. It was just barely warm. So that setup gave no cause for concern as I would very rarely boil a full load more than once in an hour.

I have this immersion timer installed now since 2018. I run the immersion on Bath for a couple of hours over the morning and afternoon every Sunday when I have free electricity but I think the fuse was just a bit too hot for comfort. Thankfully, there was no sign of heat damage around the fuse holder or on the fuse caps so I don't think there's any danger of a fire in the near future

I have been looking at alternatives because the Timeguard timer relies on a cloud server in the UK whch goes down about twice a year (usually on weekends!) and I'd prefer to have a standalone setup. But one issue I have with the 16A and higher rated timers that are on the internet - a lot of them are made by unknown (Chinese) brands and a lot of them have no earth connection.

I think this is the timer I will change to, it's rated for 16A resistive and has been mentioned in a few threads here as doing what it says on the tin. In the meantime, I'm going to stop using the bath element.

https://www.tfc-group.co.uk/product/op-sbwf01-wi-fi-socket-box-timer/