Both sides of the exchanger

The supply-side figure says how much heat the incoming air picked up; the exhaust-side one says how much the outgoing air gave away. Their ratio — the energy balance — sits near **1.0** on a healthy, balanced unit, because the same heat crosses the plates in both directions. A lasting deviation means sensor drift or fouling on one side only, and the sign says which: above 1.0 the supply side is holding heat back, below 1.0 the exhaust side is.

A single reading proves nothing — a door, a draught, one odd minute. It is the trend over days that diagnoses. All three figures fall silent together, under exactly the conditions that blank the supply-side efficiency, so one can never linger on screen while the others give up.