Why is the estimated hot water volume on the Dimplex Control app higher than the size of my Quantum hot water cylinder?

Applicable to models:  QWCD125-580RF, QWCD135-580RF, QWCD135-480RF, QWCD150-580RF, QWCD180-480RF, QWCD210-580RF, QWCD250-580RF, QWCD300-580RF


Question:  Why is the available hot water on the app showing as a greater volume than my actual hot water cylinder, ie 321L showing available for a 210L tank?


Answer: The app reports "usable hot water", not simply how many litres are in the tank. Usable hot water is water that is at least 40C in temperature, as that is what is typically needed for hand washing, bathing etc. If the cylinder is hotter, then the available "usable hot water" figure can be greather than the size of the cylinder.

Example showing a 210L cylinder heated to exactly 60C fully top to bottom:

If the entire hot water cylinder was heated to 60C and then you start drawing that water off, you would typically be mixing it with cold at the point of use to bring that 60C water down to around 38-40C. This means you are drawing off less actual water from the tank to achieve a usable bath/shower volume at 38-40C due to mixing with cold.

The Quantum hot water cylinders have temperature sensors along the side of the cylinder at different heights. This allows it to detect the temperature at the top of the tank where the water is drawn off to the bottom, where cold water enters the cylinder every time hot water is drawn off. Every time hot water is used cold water displaces the hot water in the tank. The sensors see the change in temperatures and will show an updated estimate of remaining "usable hot water" volume on the app. Example for the 210L tank as hot water is drawn off and cold displaces it:

When there is no more water left in the tank which is above 40C the app will report 0L of "usable hot water" available, as the water would only be tepid.

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