Why efficiency sometimes shows no number

Recovery efficiency is the share of the indoor-to-outdoor temperature difference the exchanger gives back: **(supply − outdoor) ÷ (extract − outdoor)**. It is read before the post-heater, because electric heat is not recovered heat — counting it would flatter the exchanger.

The figure goes blank during bypass and defrost, where there is no recovery to measure; when extract and outdoor air are less than 3 °C apart, where the sensors' own error would swing the answer by tens of percent; and when the result is physically impossible. A blank is honest — a number clamped into range would read as a mediocre exchanger, and that would be a lie.