Efficiency of air-to-air heat exchangers in occupied houses.

The purpose of the project has been to determine the saving in energy obtained in the practical operation of an FTX-system -that is, a fan-controlled supply and exhaust ventilation system with heat recovery - compared to an F-system, which is solely a fancontrolled exhaust system. The investigation, carried out in a terrace-house district in Skellefteg, showed the following savings for the FTX-systems in comparison with the F-systems: in 1-storey houses (81 m², airtightness approx. 1): appr. 1000 kWh/year in 2-storey houses (99.5 m², airtightness approx. 3) : appr. 1250 kWk/year. This saving is only about 40% of the theoretical saving at the same air exchange in the different experimental groups. This low saving was discovered to be due to the differences in air exchanges in houses during the warming-up period. The cause of this is partly the airtightness of the houses, which influenced infiltration, and partly the draught problem met in the houses with the F-system. Because of the draught problem the windowframe ventilators were closed and thus a lower air exchange ventilation was obtained . Special attention must therefore be given to these draught problems if F ventilation is planned in new construction. A fairer comparison of the saving in energy is to make the comparison at the same air exchanges (0.5 exchanges per hour), but with the corrections for infiltration that occur during operation. In this case the saving in energy becomes: for a I-storey house: appr. 1900 kWh/year, for a 2-storey house: appr. 2700 - 2800 kWh/year.