Unvented water cylinder smart control

I have a Megaflow DD300HE unvented hot water cylinder.

It is connect to a three-position switch (off, normal, boost):

The property is electric only (no gas), and the electricity is charged at a flat rate. We have been recommended to leave the cylinder constantly set to ‘normal’, as, in theory, the amount of heat loss from the system should be very low, so you effectively have 300 litres of hot water ready to use at any point. When the water is used, cold water goes in and is heated up. However, we have pretty big electricity bills (average £215 per month), and some part of that seems to be heating water. I have noted I can switch the cylinder off for a day and still have hot water the next day.

I was considering replacing the three-way switch with a timer switch, so the unit basically defaults to ‘off’, goes onto ‘normal’ for a couple of hours early morning, and the ‘boost’ functionality (which we have never used so far) would just be there as an option should we not have enough hot water one day.

But of course, I am always seeing what smart options there are. So I wondered if anyone knew of a smart switch I can install? The perfect switch would:

  • Allow me to set on/off times via an app
  • Allow me to ‘boost’ via an app
  • Allow me to ‘boost’ via a button on the switch itself

Any thoughts, inputs, suggestions, etc would be very much welcomed!

The first thing you need to do is identify how many heating elements (immersion heaters) you have. The plate says 2 x 3kw & 2 x 2.7kw - that doesn’t make sense as I doubt very much that you will have 4.
If on the other hand it’s either 2 x 3kw or 2 x 2.7kw that would make more sense. I think this is the case from what I’ve found online.

Next we have to work out how they are wired. For example it might me 1 heater in normal and both heaters on boost.
But even that is some heavy current drain in boost. Using the 2x 3kw @ 240v it equates to a current draw of 12.5a for each heater which would be 25a in boost mode if my assumptions above are correct. And I’m not aware of a smart switch that can cope with that so it would need a separate relay. Unless they are wired independently. And you could do it using 2 separate smart switches.
So first things first, you need to see how many heaters you have and how they are connected.
I can’t find a manual to your model. If you can, post a link and we can advise further.

Thank you for your initial thoughts. I believe it is 2x 3kw, there is a manual here:


It covers multiple models but, for example, on page 15 it has a table with the different models which lists this model with the 2x 3kw information.

I did speak to the manufacturer today, they assumed I would have two seperate switches on the wall, when I said I had a tristate switch they didn’t seem so familiar with that set up, and couldn’t really advise further. So treating it as two separate circuits could be the right train of thought?

If there are 2 x 3KW resistors, it is best to use the smart switches to activate two contactors of 25A each. Similar to these.

I don’t know if the hot water outlet you have installed with a cold water mixing valve. This is what you can save the most energy since it reduces the direct consumption of hot water from the thermo and less energy must be used to heat it up again.
You can have the water in the thermos at 65ºC to avoid legionella and leave the house with the mixing valve at 50ºC.

In my opinion, turning off the thermo for a while the only thing it does is delay the moment in which you are going to use up the energy to heat that water that has cooled down. I don’t think you save energy unless you have cheaper rates at certain times of the day. In my case, I have very cheaper rate from 23:00 H. to 13:00 H. next day.

You can effectively use the second 3KW resistor with a manual switch, since you should only use it for when you have to heat from total cold or with a very intense hot water consumption at a certain time. So you could put a single smart switch controlling a contactor placed near the water heater.

I agree. This is to utilize low rate electricity or maximize usage of renewable energy (usually rooftop solar). With Smart Things, added “bonus” is unreliability.

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It’s strange to have the 3 way switch because that tends to suggest you select 1 or the other immersion heater but that doesn’t really make sense unless you have one on a separate timer (normal) and boost simply turns the other one on totally independent. But this still only has 1 immersion on at a time.
I’d be inclined to do a bit more investigating. Do the immersion have any indicator lights? Or do you feel confident with a multimeter? If so I’d put the switch one way and see which immersion is on and then repeat for boost.
If they are connected independently this could be a simple retrofit. Sonoff do some 16amp smart switches that wire in directly and that is plenty for each immersion so you could simply wire each immersion via one of these and operate them via timers or manually/remotely via the ewelink app, smartthings via cloud integration or alexa via that app or voice.

Aeotec who ironically now also make the Smartthings hub also make or sadly perhaps made a Z-Wave switch specifically for controlling the immersion heater used in a hot water tank.


Aeotec ZWA006-C Smart Boost Timer Switch, Z-Wave Plus S2 with SmartStart, 16…

Aeotec Smart boost timer switch, Z-Wave Plus S2 with smart start, 16 amps, monitor power use, backup battery


Aeotec Help Desk

Smart Boost Timer Switch user guide

Aeotec Smart Boost Timer Switch. Aeotec Smart Boost Timer Switch was developed with Z-Wave Plus. It is powered by Aeotecs’ Gen5 technology and features Z-Wave S2. To see whether Smart Boost Timer Switch is known to be compatible with…

You might be able to find one on eBay.

And has it got a DH to work with SmartThings?

They don’t yet have an officially released one. I did open a support ticket with them and they have emailed me a pre-release DH.

As these forums do not appear to support attachments nor private messages I would suggest anyone needing a DH should do the same and request one.

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@jelockwood you can private message by clicking on the person icon and click message.

Thanks still does not let me attach a file though.
If @GSzabados wants I could paste the loooooong DH file in to a PM message, let me know. (Same for you @Mikeyf79 )

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Thanks, @jelockwood. I was just asking that is there any working DH for it. As my experience, Aeotec can be the manufacturer of the SmartThings Hub, but they might not have DHs updated to their products, as they have/had their own Zwave hub.