You would use the sonof inline with the main supply to the water heater. Then use the aux input to the sonic for your temp sensor. Another way you could do it would be to install a ‘T’ piece inline with the hot water outlet and use a threaded waterproof temp sensor. You can get these from auto parts shops. You would need the ‘T’ right against the tank to get the best reading and it wouldn’t be perfect but at least it is a submerged sensor rather than just strapped to the outside
Could you please provide me with the links to the parts I need to get a bit of noob here.
And if possible a rough schematic of how I need to connect everything up.
Many thanks.
Can you please check the current rating (Amps) of the heater? I would hate to provide links to electrical equipment that is not rated for the current demand.
I’ll knock up a schematic when I get home. (Currently on my phone waiting for a flight)
I’m guessing the bottom heater is for economy 7 and the upper one is the boost. You would connect to the upper one and let the economy 7 heat the water at the bottom during E7 times.
well the breaker used for the immersion heaters is MTN116 Hager (http://www.hager.co.uk/product-catalogue/energy-distribution/protection-devices/mcbs/mcb-6a-63a-6ka-type-b/mtn116/46733.htm?utm_source=print)
The immersion heaters are rated at 2.75-3.0kW 240V 50Hz so that gives roughly 12.5A rating.
I dont have an economy 7 tariff.
EDIT: I had my timer on and surprisingly the top heater is much warmer than the bottom one. The bottom one is not warm at all.
Hi Dave, hope you had a safe flight back. Did you have any chance to look into the schematic and the parts mate?
Hi, sorry for the delay, its been a hectic few days. Take at look at these 16A rated Sonof switches with temp sensor:
Its literally just power in and power out (active and neutral wires, with the earth bridged across. MAKE SURE THE EARTH IS CONTINUOUS!) in line with the feed to the heater element.
Thanks buddy.
Just a quick question. On the left of the drawing I suppose it’s the main incomer, mine is already connected to the timer do I have to remove my timer and connect the sonoff? I would like the timer to be connected in the circuit and not touch it ideally.
Secondly, I have two immersion heaters in my tank and both the cables from the heaters go straight to the timer, do I need 2 sonoff switches or 1 is enough to feed both the heaters? I suppose the wiring in the timer would be different for both the heaters.
Thanks.
Yep, that’s the isolator on the left but you can connect to the output of the timer. If both elements are 2.75 - 3kW then you will need 2 sonof switches, but this is good because it will give you more control. The second one can be the model without a temp sensor since you only need one of them for the tank. If you want to keep the timer in circuit it would be best if it can be set permanently on or in manual override so that all control is done by the sonofs and they never lose power and drop off the network
Thanks bud.
Do I need to follow any guide on how to connect the sonoff to the Internet or?
I have never used Sonof before but what I understand is that you need to flash them with a custom firmware in order to use them with Home Assistant.
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Hi @sparkydave @BertrumUK I have received all the parts and need to attach the sensor to the heater.
I have bought 2 sensors i.e. the AM2301 and DS18B20.
I opened up the immersion heater to see where I could attach the sensor. The photo’s below are a close up view of the heater. I need your advice as to where I fit the sensor (place with a red arrow or blue arrow on the photo) and which one?
Many thanks.
I would use the DS18B20 only. Not sure about the safety of placing where the blue arrow is as that is the heater so would try the red and see what readings you get. Usual H&S rules apply and you do this at your own risk.
Hmm thanks.
Can you please tell me what are your safety concerns if was to place where the blue arrow is?
You are working very near to electricity. I’m just advising caution as I would with anyone doing the same as you.
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The issue with putting it near the blue arrow is more a case of getting an incorrect reading of water temp. Really both locations are too close to the element but you can use a template sensor to offset the readings to account for this i guess. I would keep it away from the element simply to try and get a more accurate reading of water temp rather than element temp. Electrically, the unit is sealed (other than the terminals for the wires) so safety “shouldnt” be an issue (disclaimer: I’m not responsible for fingers going where they shouldnt!) 
so are you saying to insert the temperature sensor in the water in the tank rather than the element?
no, just try not to have it too close to the element as that is likely to be hotter than the water and therefore not give an accurate indication of the water temp, but like i said you can program in an offset to counter that
Hmm fair enough. . Where do you think I should put it then? And also can you please provide me with the offset to counter the inaccurate readings?
Thanks