Extracted ventilation air rated heat recovery efficiency is defined under standard rating conditions as 'the efficiency of separate heat recovery equipment'. Real heat recovery efficiency depends not only on single unit efficiency but also on a building's heat gain/loss ratio. Maximum efficiency of heat recovery becomes unnecessary when heat gain compensates heat demand from a heat generator. The demand of recovered heat gradually lessens when the outdoor temperature rises and heat recovery is switched off if the outdoor temperature exceeds the thermal balance temperature of a premise. Thus, the mean annual heat recovery's efficiency is less than a technically possible one. This phenomenon is hardly perceptible in poorly insulated buildings where transmission loss plays the main role. The decrease of heat recovery's efficiency becomes significant in large, well-insulated buildings where ventilation heat loss is many times more than its transmission loss. (C) 2005 Elsevier B.V. All rights reserved.