Field Evaluation of Direct Heating Equipment in California

This report presents a multi-year field evaluation assessment of the real-world performance of direct heating equipment (DHE) across 11 California households. The goal was to obtain empirical data on DHE operation, energy use, thermal comfort provided, and impacts on indoor air quality. We first monitored the performance of existing low-efficiency DHE units over one heating season to establish a baseline. Subsequently, 10 of these units were replaced with high-efficiency direct-vent and upright wall furnaces, which were then monitored over the following two heating seasons. It should be noted that the sample of 11 households is small, and the findings should be interpreted with caution, as they do not represent the broader DHE fleet in California and the United States.

Results from the 10 sites indicate that high-efficiency DHE significantly improved thermal comfort, producing faster space warming and better distributing warm air. Natural gas consumption per heating degree day decreased at six of the monitored sites. However, gas usage increased at four sites, likely due to a "rebound effect" in which users operated their new heaters more frequently or for longer periods to take advantage of improved thermal comfort and enhanced thermostat controls. While the rebound effect may have reduced energy savings, the increased use of the heater has also led to higher consumer welfare, as reflected in better thermal comfort from the additional energy consumption.

Field data also indicate that, in some cases, indoor concentrations of carbon dioxide and carbon monoxide in the direct-heated space increase during heater operation, and that these increases are lower with high-efficiency units than with low-efficiency units. While household behavior and other household sources of carbon dioxide and carbon monoxide likely contributed to the increases, we do not rule out the possibility that the heaters might also have played a role. The data also show some changes in indoor concentrations of particulate matter and total volatile organic compounds, but they do not provide sufficient evidence of an association between heater operation and these changes. Despite increases in pollutant concentrations during heater operation, we did not observe any critical exposure levels under regular operating conditions. However, during two events at different sites, the monitoring system detected that newly installed units were leaking elevated levels of carbon monoxide into living spaces, requiring immediate shutdown and replacement. In both cases, the units were diagnosed as malfunctioning due to manufacturing defects. 

Finally, while user feedback validated the improvements in heating speed and distribution, overall satisfaction was considerably hindered by equipment noise. Multiple households found the new circulation and combustion fans too loud for living spaces. The report concludes with recommendations for future improvements to DHE.