Performance analysis of demand controlled ventilation system using relative humidity as sensing element.

This paper evaluates the suitability of humidity-controlled house ventilation system to determine (i) the effectiveness of relative humidity as a sensing element, and (ii) the operating and performance characteristics of such ventilation strategy. The ventilation system consists of continuously running "mechanical" air extractor units and "passive" air inlet units equipped with humidity sensors. The ventilation system was installed in two single storey houses which were monitored during November 1989 to April 1990. Results showed that the changes in the relative humidity did not appear to track the levels of normal human activity accurately. The difference in air in-flow through each passive air inlet due to changes in RH (2 to 10%) varied from 0.8 L/s to a maximum of 1.7 L/s, which was found to be insufticient based on high CO2 levels (> 1200 ppm) in occupied rooms. The humidity-controlled mechanical exhaust system was found satisfactory in maintaining the level of exhaust air-flow with changes in RH. The air in-flow and out-flow analyses showed that the air leakage though the house envelope remained as a predominant form of fresh air supply to the house, thus defeating the purpose of demand controlled ventilation. The energy consumption of these two houses reduced by more than 8% by cutting down the fresh air provided to house during un-occupied periods. Relative humidity as an exclusive sensing element may not be sufficient enough to achieve required quality ventilation in houses.