Getting a temp reading from hot water cylinder

Does anyone know of a way to get a temperature reading from an unvented indirect hot water cylinder (tank)? I have a Megaflo for my home hot water system but I haven't found a way of reading the water temperature. The cylinder does have an electric heating element but we use the gas boiler to heat the water as it is more economical.

My use case is that I want to know when we run out of hot water so I can start the boiler to reheat the cylinder. I run the boiler on predefined schedules for our hot water needs and this works fine most of the time. Occasionaly we may have visitors or some members of the family (usually suspects!) will have longer baths or bath too close to each other and deplete the cylinder of hot water. So while this doesn't happen too often it is still annoying when it happens specially since unvented cylinders don't give much warning when they are about to run out of hot water, one minute you have hot water and the next you get cold. This is because the hot water outlet is at the top of the cylinder and the cold inlet at the bottom and as you already know cold water moves to the bottom and hot to the top so you get a strong thermocline effect. I guess I could try to connect to the electric heating element relay but I suspect this is not what I want. The relay will probably just be On when the water tank is below a certain max temperture (which you set in the cylinder) and Off when it reaches it whereas what I want is to know when the cylinder reaches a Min temperture I will define. Thanks!

I use a Secure SES303 with sensors on my hot water tank in combination with the Z-Wave Sensor Logger plugin.

I have four sensors attached in series to the SES303. Something to watch out for is that you can only have one sensor if you have a 4 metre cable or 4 sensors with 1 metre cables.

I have cut into the Hot Water Tank foam covering at three heights and placed the sensors so that they measure the temperature from the metal of the tank. I also have one sensor on the hot water feed pipe.

My SES303 is in the main bathroom with a small hole drilled through the wall into the airing cupboard where the hot water tank is. You have to leave some slack on the sensor cable (in my case in the airing cupboard) as it is impossible to attach the sensor cable to the SES303 once it is mounted. Not a very clever design!

The setup can be a bit problematic. For example, you have to include the SES303 onto the Z-Stick with the sensors already attached. Attaching them after you include the SES303 results in the sensors not being shown.

Also the Z-Wave Sensor logger plugin is essential to send a request for temperature updates to the SES303 for it to return the values at the next wakeup time.

Here is an example graph from a Control Page:

Hot water Tank

Hot Water Tank.png (268.72 KiB) Viewed 1677 times

If I was doing the same today, I might choose a different option (none of which I have investigated as I don't have a need).

Hi Jon, many thanks for the detailed answer! I never thought of using external sensors but it makes total sense now that you mention it. Unfortunately the Megaflo cylinder is enclosed on a steel casing so I don't have easy access to the insulation layer to cut into it. I could try to cut through that but seems a bit dangerous...

I monitor a passive solar setup using two 12-volt powered Shelly 1's and four DS18B20 temperature probes (each Shelly will read upto 3 probes).

I am fortunate having a couple of spare threaded outlets on the tank so have used thermowells to house the sensors. For the solar collectors i have just tucked the probes under the pipe lagging.

In the absence of thermowells it would not be too difficult to slide a sensor under the insulation against the inner tank at whatever level you choose, This could be done from the opening for the tank's electric element/thermostat hatch. My tank has loose fibre insulation - it would be harder with foam but probably still do-able.