Study Concludes Exhaust-Only Ventilation Best
The Consortium for Advanced Residential Buildings (CARB) evaluated three whole-house ventilation strategies in a non-profit development of efficient homes in Chicago in 2005. The homes were constructed with structural insulated panels (SIPs), heated with condensing furnaces, and utilized sealed combustion water heaters, and efficient lights and appliances. The blower door tests showed 300-350 CFM50 for 1700 SF of conditioned space.
They compared an ERV interlinked with the air handler fan (it did not sufficiently reduce CO2 without the air handler operating), an AirCyclerâ„¢ supply ventilation system that fed the air handler, and an exhaust only (bath fans on timer control).
They monitored the homes for six winter months, including temperature, humidity, and carbon dioxide concentrations outdoors, in two bedrooms, and in the living room; fresh air supply flow rate, temperature, and humidity; exhaust air flow rate, temperature, and humidity. And they calculated total energy costs including electricity consumption for ventilators and air handlers, and natural gas consumption or savings over an identical house with no central ventilation.
The exhaust fans were the only units that continued to flow at the same rate at which they were commissioned (the ERV averaged higher and the AirCycler averaged considerably lower). The exhaust-only system also maintained the lowest levels of indoor CO2.
Among the systems monitored, it is clear that the exhaust only system is the least costly to operate; costs were less than 50% of those of the other systems. Looking at carbon dioxide concentrations and air distribution, SWA believes that data show that the exhaust system does provide adequate distribution and is the most appropriate system for these homes.
It is worth reiterating that the exorbitant energy costs for the Air Cyclerâ„¢ and ERV systems would be significantly reduced with electrically efficient, well-sized furnaces or especially (in the case of the ERV) with a dedicated duct system for ventilation.
This study certainly does not suggest that exhaust-only systems are appropriate for all applications. These small homes had an efficient exhaust fan in the center of both floors (each approximately 850 ft2). The effectiveness of exhaust strategies in larger homes requires more investigation, and the need for active air distribution may grow as house size increases.
Nevertheless, this study shows that exhaust-only ventilation certainly can be a very effective, low-cost ventilation option.