Interesting post! A point to note when comparing defrost efficiencies: it’s key to note that the heat meter’s temperature sensors in a given installation may be positioned far enough from the heat pump to alter the apparent deltaT and thus apparent heat output. This is especially true during the preheat step used by Viessmann, because the temperature during this step rises rapidly, causing the water temperature to drop approximately linearly along the length of pipe from the heat pump outflow to its inflow. As a result, in the Clunbury installation, the heat meter underestimates deltaT by a factor of 1.7, i.e. the actual COP during defrost heating is 1.7 x larger than the heat meter’s estimate. Using your analysis, this shifts the advantage quite strongly in favour of Viessmann’s approach.
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