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Automating water tank with an immersion heater switching

Postby kavelot » Sun Dec 20, 2020 1:19 am

Hi!
I just moved to an apartment that has a water tank with an immersion heater.

If you're not familiar with how it works, basically there are 2 heating elements inside a water tank; one is connected to a live wire (240VAC) and the other is connected to another live wire (also 240VAC), with the difference the latter is only turned on from 11pm-8am to use the night-saver tariff.

Image

I'm trying to automate the way those switches work, since I'm not happy with the current setup. I'm replacing them by smart switches (that can handle 3kW); the issue is that I don't want to allow both heating elements to ever be on at the same time.

Since I don't fully trust smart switches (maybe it got the "on" command, but misses the "off"; or there is a bug) I would like to hardwire a safe guard.

I'd be ok with a fuse blowing up (and stopping both) in case this happens, since it would be a good alert that something very wrong is going on. But alternatively it would be fine if one of them just gets disconnected if this happens.

Any ideas how I can do it using only 1-way smart switches?

Thanks!
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Postby Mr White » Sun Dec 20, 2020 10:29 pm

As you pointed out, the supply for the lower heater will be off most of the time, so installing a smart switch will not turn it on as there is no supply to turn on.

If you are considering running them from the same supply you run the risk of causing a fire, as the supply cable is not designed to run two heaters at the same time.
If you could have a smart switch controlling either of them, what would be the point? since you would only want one heater on at any one time, and the lower one would not work when the "economy tarrif" is not running.
Smart switches are just another way to extract money from you, smart lamps on the other hand can be useful.
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Postby ericmark » Mon Dec 21, 2020 12:06 am

There is a number of ways of using economy 7 tariff, it is a tariff not a system, you can get a completely separate consumer unit/fuse box, or it can come from same consumer unit/fuse box the first has a time switch feeding whole fuse box, second has the time switch built into the control.

A google for "immersion heater time switch" shows many options, and your diagram would result in a borrowed neutral with some systems.

I had a problem with my central heating and I had to use relays, and looking at Energenie and Sonoff it seems neither does a change over relay.

If I wanted to do it, I would use an old consumer unit with din rail to mount the relays and sonoff units in, i.e. plastic.

I have one I have kept ready to work outside lights. But step one is work out what system you have, and step two is work out what you want.
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Postby kavelot » Mon Dec 21, 2020 7:51 pm

I have the night tariff and daily tariff coming to the immersion heaters already, everything is already working.
Currently, it works like this:
The lower heater turns on based on a timer: from 3:30am to 8am
The top heater turns on for a couple of hours just when I manually turn a "boost" dial on.

I don't like the behavior of the top heater - I want it to be on more often, without manual intervention.

Both of the heaters have a thermostat. The top one configured for 55C and the lower one for 70C. The logic is that the top one should guarantee that I always have water close to 55C, while the lower one goes higher so it takes advantage of the night tariff.

I could just leave the top one on all the time, but this would probably interfere with the bottom one (both could be on at the same time for a few minutes).
Also, after reaching the desired temperature, the thermostat switch keeps turning on every 20min for about 3min, to keep the temperature.. I'd prefer keeping it one for longer less often (so my idea would be to turn it on for about 20min every hour).
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Postby Mr White » Mon Dec 21, 2020 10:39 pm

As they are all ready wired separately there would be no harm in leaving the top one on all the time.

Your idea of keeping it on for longer but less often would not work since the thermostat would switch the heater off when the water is at the set temperature, no matter what any form of switch is telling it.

Why not try as I said, leave the top one on all the time, it will not "interfere" with anything if you leave the wiring as it is now, but I do think it would be pointless having it run if you are out, so I would suggest you change the timer settings, so that it comes on say 30 minutes before you get home.
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Postby ericmark » Sat Jan 02, 2021 10:55 pm

Some thing wrong, daughter had central heating set to 75°C and immersion set to 60°C and the immersion did not switch on, to save energy she turned the central heating control down to 65°C and it cost her a fortune with electric instead of gas heating water.

In the winter oil heats our domestic water when ever the central heating is on, but in summer we find 5 x 1/2 hour burns through the week gives us enough hot water, it switches on at 8 am and off at 8:30 am except Monday and Friday, and that is enough. Half hour is not enough to fully heat water at 24 kW in fact it does not get 1/2 hour it turns off after 20 minutes as return water to boiler too hot, but this is enough to have hot water at taps, for a bath we run boiler for longer.

So you simply should not need the top heater 95% of the time, so fact you need the top element points to some thing wrong. 40 gallons of water even with very little insulation stays warm for 47 hours.
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Postby kavelot » Sat Jan 09, 2021 9:40 pm

It indeed stay warm (with my insulation I noticed it loses 1-1.5C per hour), but when 1 person takes a shower, it's like pouring 50L of water (at 18C) in the tank.
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