Retrofitting 80% Residential Furnaces for High Efficiency


TMH-report.pngSpace heating is the number one end use for energy in Minnesota buildings, and residential forced air furnaces consume more energy for space heating than any other type of heating device. To date, the only way to increase the efficiency of a natural gas forced air heating has been to replace it with a higher efficiency (90%+) model.

This project explored an alternative to furnace replacement as a way to achieve high efficiency space heating — an upgrade to residential standard efficiency furnaces called the transport membrane humidifier (TMH). The TMH saves energy by increasing the efficiency of induced draft furnaces from 76%-82% to the 90%+ efficiency level typically associated with condensing furnaces. The TMH extracts additional energy from the combustion process by recovering water vapor and waste heat from the furnace flue gas to preheat and humidify the inside (return) air. This study explored the potential energy savings and comfort benefits while also characterizing the risks associated with increasing humidity levels and retrofitting existing equipment. 

Full report:
Retrofitting 80% Residential Furnaces for High Efficiency

This project supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through the Conservation Applied Research and Development (CARD) program.