Theoretical and real ventilation heat losses and energy performance in low energy buildings

Ventilation in low energy refurbished buildings is the cause of a big part of energy losses. In order to reduce this impact, some energy regulations prescribe a solution (such as the Swiss energy Law, prescribing heat recovery) and others prescribe a system global performance (such as the EU delegated regulations No 1254 and 1253 /  2004 determining a global energy performance label of the ventilation system). In this article we analyse global theoretical performance of 6 ventilation systems in an apartment residential building and we compare real performance of a demand controlled ventilation system, incompatible with the Swiss prescriptions, with the performance of heat recovery system, compatible to the prescriptions, in two case studies of low  energy refurbishment. Theoretical performance comparison shows that different energy saving strategies may produce similar effects. Very low fan electricity consumption combined with reduced demand-controlled airflow rate may produce energy savings comparable to heat recovery. This phenomenon is accentuated with high performance heat production systems, with low embodied primary energy (such as high performance heat pumps, renewable district heating or wood boilers). In these cases, primary energy for electricity, necessary to recover heat, added to system embodied energy, may be higher than the recovered non renewable primary energy. The comparison of real energy consumption of the systems in the case studies shows that the small differences between theoretical energy performances between heat recovery and demand-controlled ventilation, is  egligible compared to the energy losses due to occupant behaviour (window opening, high internal temperature), while the investment cost of the two systems may vary by > 450% and the life cycle cost by >150%. This comparative studies show that it is preferable to prescribe an energy performance of the global system, instead of prescribing a specific system or strategy, especially in an energy environment where primary energy for heating and cooling production is continually becoming better and ventilation technology is continuously evolving.