Report Released: Retrofitting Residential Furnace for High Efficiency

Mar 13, 2018

CEE recently released a CARD funded research report, Retrofitting 80% Residential Furnace for High Efficiency. Researchers investigated 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).

TMH-install-news-item.jpgSpace 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 system has been to replace it with a higher efficiency (90%+) model.

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.

TMH devices were installed and instrumented in four sites. Performance was continuously monitored to facilitate a comparison between baseline furnace operation and TMH operation. During bypass (baseline) mode, flue gases escaped via the regular induced draft vertical vent and the TMH unit and its controls were disabled. The ambient temperature and relative humidity were monitored at three locations per site to determine the effect of TMH operation on the indoor environment.

Main research conclusions:

  • Across all four sites, the average furnace output increased by 10,500 Btu/hr, and steady state furnace efficiency improved from 79% to 93%, yielding an average improvement (or gas savings) of 18%. Without humidification, the improvement in space heating was 9%.

  • Cost effectiveness of TMH retrofits varies substantially, with the end-user simple payback periods ranging from 7 to 15 years.

  • One desirable characteristic of the TMH is that humidity output is proportional to the humidification demand; the TMH outputs more humidity during cold, dry weather.

  • Excess humidification was not observed in sites experiencing a wide range of baseline relative humidity, from less than 20% to over 60%.

The TMH is a viable technology for achieving large natural gas savings for space heating in single-family homes. It is the only viable alternative to a condensing furnace upgrade and it is the cheaper option, particularly for the vast number of standard efficiency furnaces that will remain operational over the coming decades, which number in the hundreds of thousands.

This project was 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. The project was co-funded by CEE in support of its nonprofit mission to advance research, knowledge dissemination, and program design in the field of energy efficiency.

Related Links:
Final Report: Retrofitting 80% Residential Furnace for High Efficiency
Archived Webinar summarizing research findings
Project summary page