Incorrect tank temp

[kron77]

Hello. I have a mitsubishi ecodan R32 with a 200 litre unvented tank (mcdonald not ecodan). I'm seeing a big difference between the temp read on the system vs reality. For example, when it's saying the tank is at 44 degrees it's actually reading 52. This is causing the system to turn on more regularly to keep it at 50. 

 

Anyone seen this before and could anyone let me know where the problem would be?  THank you 

[PhilT]

7 minutes ago, kron77 said:

Hello. I have a mitsubishi ecodan R32 with a 200 litre unvented tank (mcdonald not ecodan). I'm seeing a big difference between the temp read on the system vs reality. For example, when it's saying the tank is at 44 degrees it's actually reading 52. This is causing the system to turn on more regularly to keep it at 50. 

 

Anyone seen this before and could anyone let me know where the problem would be?  THank you 

Need more detail. What exactly is reading 44, and what exactly is reading 52? I have the same as you and there is only one temp on the controller from the tank sensor. The tank sensor is near the bottom so will read a lower temp than near the top due to stratification.

[kron77]

Hi PhilT, 

Sorry! So the ecodan controller unit is reading 44 but when i test it with a thermometer on the closest outbound tap, i'm seeing it coming out at 52 degrees. 

[PhilT]

1 hour ago, kron77 said:

Hi PhilT, 

Sorry! So the ecodan controller unit is reading 44 but when i test it with a thermometer on the closest outbound tap, i'm seeing it coming out at 52 degrees. 

yeah mine's similar. With just the two of us and no visitors we easily get by with 40degC set temp but the sensor position is at around 25% of the height of the tank, so the water coming out of the top feed is much hotter.

[SteamyTea]

Temperature is the mean free path speed of a molecule or atom.

Mean temperature is the mean of the sum of all molecules.

When water is above 4⁰C, the density reduces with temperature rise.

So at the top of the cylinder, the free path speed is going to be higher than lower down the cylinder. 

Some of those top molecules will be above 100⁰C. This is what caused evaporation.

 

Nature is quite wonderful, and often not intuitive.

[JamesPa]

43 minutes ago, PhilT said:

yeah mine's similar. With just the two of us and no visitors we easily get by with 40degC set temp but the sensor position is at around 25% of the height of the tank, so the water coming out of the top feed is much hotter.

@kron77Remember that this is arguably a desirable feature of dhw tanks not related to the heat source!  Stratification means you get some usable dhw more quickly than you otherwise would.  Occurs with immersion heaters and gas sources just the same, depending on tank layout.

 

Of course there are also some downsides, but to overcome it requires a pump so it's rarely done because of the positives.

Edited May 15, 2023 by JamesPa