Accurate hot water temperature

Can anyone advise on what would be the best way to tell the water temperature from a “dumb” water heater? I have seen solitions where the temp sensor is clamped to the overflow valve. Is there a way to get a more accurate reading? Water heater is electric.


This section is for sharing working projects — not for asking help. Please move your post to the appropriate section.

Moved it for you.

If you are into DIY the DS18B20 would be a good fit:

You can use an ESP device or some Sonoff devices (not the power monitoring ones) running Tasmota.


The water temperature is unlikely to be constant throughout the tank due to stratification.

For example, on my hot water cylinder, I have three sensors (“naked” DS18B20):

  • tank top near the immersion heater (stuffed under the insulation in contact with the tank);
  • coil inlet from the boiler at about 2/3 of the height (cable-tied to the pipe next to the tank);
  • and coil return at about 1/4 of the height (as above).

Just before my morning shower, after a few hours of sitting undisturbed and thus a good indicator of the actual temperatures at those positions, they were reading 54°C, 45°C and 31°C respectively.

The coil sensors are hugely inaccurate when the boiler is running, as they show the temperature of the water running through the coil, but they’re the only accessible bit of tank I have unless I were to drill more holes in the insulation.

I work out a “tank charge” from the three sensors, restricting its rate of climb to work around this issue. See this topic for a description.

Your best bet is probably to do similarly — get as many readings as you can, then work out a measurement that gives you an indication of the heat in the tank.

1 Like

Thanks Tom thats exactly what I plan to use. I know they are waterproof, but can they handle the temperature?

The temperature range of the DS18B20 is -55°C to +125°C. So yeah should be ok for a water boiler.

Thanks @tom_l

I have and had the same need, that’s why I googled it again, as I have recently moved from DomoticZ to HA. I found this old thread, but in hope that someone would fint my thoughts useful, I commented my solution(s).

For that Sonoff sensor mentioned above, it has to be installed an additional pipe to set it up to the centre of the water heater. I’m a user of HA only for a month but with Domoticz I used DS18B20 with case type of TO-92:


It’s small enough to fit on top of the original thermostat brass pipe.

In the original pipe of the thermostat, there is enough space for the three temperature resistant silicon wires to lead the signal out of the heater (I bought all of these from Aliexpress). In every Sonoff Wifi smart switch, there are sufficiently free inputs to solder the wires to, or you can attache the 3,5 mm connector if you have a switch with the socket.

You must use sealing heat shrink tubings for insulate the solders / sensors legs and after all the sensor as well. You have to experiment with you own water heater and it’s thermometer, measuring the exact lenght of the thermometer tube and it’s installation tube to be sure that the sensor fits there freely and the wires wouldn’t be too tight between thermostat and the tube wall and you wouldn’t breake their insulation while inserting everything back to the water heater.

It’s sad that the POW, POW R2 can’t be used with the sensor if one couldn’t use a separating method beween the sensor and an input (opto couplers, i-couplers or the isolators used in ethernet devices). But, again, the new POWR3 has a RJ11 socket for the same sensors mentioned in this thread.

Or, secondly, I use a bunch of Xiaomi Mijia BT temp/hum LCD sensors with custom FW:

The author of the FW has told me that it’s not very complicated to change the FW and add the driver for DS18B20 to connect it to an available controller’s chip input having a smaller battery powered device close to the water heater and showing the room and water temperature changing their results in couple of seconds. As I’m not programmer, there should be enough ineterest for he would make the changements.

I’ve 2 water heaters, that I turn on only when I need them, considering the energy prices these days. thats why I would appreciate the temperature information on HA and close in the also bathroom to see the the temp of water that rests for the next day or after some family member had been using it.