Add temperature probe to cylinder (without pocket)

Our cylinder has sensor pockets at the bottom in the coil area and way up at the top without anything in between. This means that if the hot water sensor for our Ecodan FTC6 is in the lower pocket it tries to reheat any time hot water is used but if it is in the top one we run out mid shower.

I have been heating twice a day on a schedule and most of the time this is fine but occasionally if everyone has a shower in the evening for example, we can run out.

So… I decided to drill a hole in the external metal covering about half way up the cylinder and then create a hole in the insulation to insert the probe. This probe is just to trigger the reheat, there is a separate one used by the heat pump to know when it has reached temperature in the lower pocket. Therefore this one doesn’t need to get a very responsive or completely accurate measurement, just one that is accurate enough but will show when the stratification level has passed it.

However I am getting a very low reading from my probe. I assume these temperature probes can be in contact with the thing they are measuring on the end as well as the sides (the flow and return ones are strapped side on to pipes)?
It could be that some of the loose insulation bits are getting in the way of letting the probe come in contact with the cylinder but I can’t work out a way to stop this in a small long hole.

Anyone go any ideas tips or done something similar sucessfully?

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Not quite the same thing but I did this with my old copper cylinder covered with ~25mm solid foam insulation. to try and work out the tank capacity

I carefully drilled the insulation, gently turning the bit with my fingers until I hit the copper! then inserted some DS18B20’s with a lump of bluetack on the back to stop it falling out. Like you I discovered they were reading much lower than I expected.

The problem was the sensor was not hard against the cylinder, in the end I managed it with the DS18B20 in a small piece of plastic pipe (like a drinking strawr) and a rubber band contraption keeping it under pressure. I hot glued a couple of wire loops to the sides of the foam to make anchors, the pink squares.


I have subsequently learned that heatsink compound can help, I don’t know your cycinder, In my case the foam was solid enough to be “tube”, if you have a big “air gap” with “loose” insulation you could try making a sleeve that pushes against the inner cylinder then push in the DS18B20

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Thanks, I managed to get the sensor pushed in a bit more and it is reading the temp -10C so good enough for what I want.

Currently it’s wedged in with a screwdriver but I might look at making a spring of wire to push behind it.

@ajdunlop I have an older Ecodan pre-plumber cylinder which only has the temperature probe at the bottom (in the immersion pocket). I’ve been considering adding additional probe(s) as I have no idea how much hot water remains once a small amount has been taken from the tank (as you can see in the attached).

Would you mind sharing your experience with the detailed placement of your additional probes i.e. top and middle and how useful your new 3rd probe is in practical terms?

Hi Dan, All I did was to drill (carefully) a hole in the metal outer skin of the cylinder and then using a drill bit in my hand carefully remove a bit of the insulation between that and the actual cylinder inside.
Mine is not an Ecodan cylinder.

I thought most Ecodan cylinders came with 2 temperature sensors, maybe this was something only added in new ones.
It is shown here on this video

As I don’t have an Ecodan cylinder I bought the additional THW5A sensor

I bought ECODAN PAC-TH011TK2E DHW CYL TEMP SENSOR ( and wired this into the same connector as the THW5B sensor.

I found that the only way to get this sensor to be used by the FTC6 was to switch on DIP switch 3-7 so it thinks it is connected to a plate heat exchanger rather than a coil based cylinder.

When this DIP switch is on if you go to the ‘DHW Recharge’ DHW setting you can select between Large and Standard (when the DIP switch is off it is always set to Large).
When set to Large cylinder reheats are triggered by the bottom sensor but when set to Standard it will use the top one.

Our hot water strategy is to schedule the hot water to be on at the start of our 2 off peak periods (with a little bit of off before) but with the regard setting set to Standard so if the hot water starts to get low it will come on based on the top sensor.


I had problems with the hot water until I replaced the mixing valve, the plumber had removed the backflow preventer when installing (at least thats what I think its named) probably for using the water mains for expansion… Looking at DHW usage before and after and how long time there is between reheats now I have wasted atleast a couple thousand KWh over the years because of that missing backflow preventer.

After putting on a proper thermostatic mixing valve with backflow preventer the stratification enables me to takes showers down to almost 20c on the sensor.
I have a pre-plumbed cylinder with FTC5, I think the sensor is about 1/3 up from the bottom of the tank. External 6+3 immersion, 200L tank

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I have a 300L cylinder. Any bigger and it would not go through the door!

The temp pocket is 40% the way up. It is heated to 52C nightly. We can get to about 10C before the hot runs out. Though that temp probe is not a good indicator of when it is going to run cold.

I put a temp pocket in the output. This is far more useful. You see that the water cools really quickly when finally runs out.

Our thermostatic tap is not great as it will always mix in some cold. So when the water is still 38C the water running into the bath is too cool.