Evaluation of ventilation system in very low energy houses.

Since 1985 more than 170 very low energy houses, all of the same type and structure, were built in the Flemish Region, Belgium. Because conduction losses are very low, mean Urn-value 0.30-0.35 W/(m².K), ventilation losses become very important, up to 45% of the heat losses if no heat recovery is utilised. Three of the houses were monitored in detail for energy consumption, energy and ventilation efficiency. All houses are equipped with the same ventilation system: balanced mechanical ventilation with heat recovery. Tracer gas measurements, pressurisation tests, multipoint temperature measurements and on site and laboratory tests of the heat recovery system, give us a complete scope of the ventilation system and its energy and ventilation efficiency. Pressurisation and depressurisation tests revealed the main air leaks in the construction: the different connections wall-floor and wall-roof, the window perimeter, even the sockets. Extra care in construction practice changed the n50-value from an average of 4.5 AC/h to 3.5 AC/h, still high for a house with controlled ventilation. After testing the airtightness, we carried out tracer gas measurements in whole dwellings and between the different zones. Real ventilation rates and interzonal flows were derived. Questions lie: Are the airflows in accordance with the design values? Do they match the requirements? How can we measure interzonal flows with one tracer? were answered. The paper gives a mathematical description of the tracer gas flow patterns (solution of the differential flow equations) and compares the results with the measured data. To complete the evaluation we carried out laboratory tests and field measurements on the heat recovery' system. In laboratory the flat plate cross-flow heat exchanger showed a thermal efficiency up to 65% under specific climate conditions. In the dwellings, insulation and airtightness of the ducts appeared to be very important. The temperature efficiency decreased to values less than 45%. As a conclusion, one may stress that a global evaluation of the ventilation efficiency has to include different tests. The medium or poor airtightness has the greatest impact on the energy efficiency of heat recovery and on the ability to control the system. Detailed testing also showed some flaws in the ventilation system and in the building construction.