I have a single immersion heater that supplies hot water to my bath and shower only. Currently it's wired to a simple on/off switch on the wall. For obvious reasons I'd like to replace that switch with a timer. Ideally I'd prefer to replace the switch with a normal plug, so I have the choice of what sort of timer I can plug it into. Is is simply a case of swapping the switch faceplate to a standard outlet and taking the wire from the boiler and fitting a 13A plug on the end? (assuming all safety precautions are taken of course!). If so, would I then be able the plug the boiler into a normal mechanical timer? Pics attached. Boiler is in cupboard in Bathroom.
Some pro's and con's but in theroy you could use a socket and a plug. First problem is any 13A fuse produces heat and so you need to get rid of the heat produced with a FCU (Fused connection unit) it has a large surface area and is screwed to other items able to assist in removing the heat.
With a plug it must be in free air to remove the heat and being so much smaller it has a harder job to get rid of enough heat, in a cupboard of any kind there is a problem that over an extended time and it does take a long time to heat up a tank of water from cold the plug may over heat. Once this happens once if anneals the metal so the grip on the plug is not as good so next time it heats up even faster.
Using the old 15A plug which does not contain a fuse was common with immersion heaters it allowed the plumber to swap the immersion heater without calling the electrician.
In theroy using the German Schuko plug and socket would remove the problem of the fuse heating up. However the Schuko plug can be inserted either way around so the line and neutral can be swapped so British regulations do not allow their use. The French version has the earth pin sticking out of the socket so would be allowed but getting one would be hard as most of Europe has moved to the Schuko plug.
I would say all you need is more insulation around the tank. I brew beer and 40 pints takes a lot of cooling and heating, with 40 gallons you have 8 times the problem and to heat the water with 3kW heater will take a couple of hours and the cooling even longer so if switched off for say 8 hours it will only cool down around 10 degs (if no water used) the amount of heat (energy) lost is quite low.
My beer needs around 5W of power to keep at 20 deg C when garage at 5 deg C in an old fridge as an insulated box. So with reasonable insulation you may need an average of 15W to maintain the temperature which is less than a Sky box uses on stand-by.
The only time a timer really helps is with off peak power or when there is some other heating source. If you have for example solar panels then having the mains electric switched off until 5 pm makes sense you don't want to use electric when solar may do the job for you.
But with your set up I would just add extra insulation and leave it at that.
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