Knowledge · Guide

Temperature recovery: what it tells you and what it does not

Temperature recovery describes how much of the possible temperature difference a heat exchanger recovers. Definition, influences and the usual pitfalls.

Temperature recovery efficiency - in German “Rückwärmzahl”, also called the temperature transfer ratio - is the central figure of any air-to-air heat recovery. It describes what share of the maximum possible temperature difference the unit actually transfers.

How is temperature recovery defined?

Referred to the supply air side: the actual heating of the outdoor air divided by the greatest possible heating - that is, the difference between the exhaust air and outdoor air temperatures. If a unit heats outdoor air from -10 °C to 14 °C while the exhaust air enters at 22 °C, that is 24 K out of a possible 32 K: a temperature recovery of 75 %.

What does it depend on?

• Transfer area and plate spacing: more area and narrower gaps raise the recovery - and the pressure drop. The two belong together.
• Mass flow ratio: the characteristics apply to balanced streams. If one side is considerably stronger, the result shifts; the sizing therefore works with the actual mass flows of both sides, not with nominal values.
• Humidity: if water vapour condenses out of the exhaust air, latent heat is released as well - the effective recovery then rises above the dry value. Our sizing therefore reports condensate separately.
• Flow arrangement: counterflow components reach higher values than pure cross-flow of the same area.

Why is “more” not automatically better?

Every percentage point of recovery is bought with area or narrower gaps - both raise the pressure drop and with it the fan power, which is incurred permanently. Beyond a certain point the additional electricity demand eats up the heat gain again. The sensible recovery is the one your heat balance needs and your pressure drop budget allows - not the maximum value on a data sheet.

A second pitfall: manufacturers’ figures apply to one particular operating point (volume flow, temperatures, balanced streams). Values are comparable only at the same point. Our sizing therefore calculates YOUR point instead of carrying over catalogue values from a different one.

What does this mean for your enquiry?

State the permissible pressure drop honestly and, if you wish, let a minimum temperature recovery run along as a limit. The sizing then checks which designs and sizes meet both requirements at once. Our team comes back to you with the result - or explains what it fails on.