It’s a piece of industry lore that’s virtually become conventional wisdom: Gas furnaces provide more comfortable heat in the winter than electric air source heat pumps (ASHPs).
But is it true?
In its comparative analysis of residential heating systems, research firm Newport Partners used sophisticated building modeling software to answer the question with real data. “You hear a lot of industry buzz about heat pumps being uncomfortable in the winter season, especially when outdoor temperatures are cold,” says Mike Moore, a consulting engineer with Newport. “We wanted to find a way to try and quantify it.”
The question of comfort
Comfort is an inherently subjective measure, so quantifying it can be difficult. ASHRAE fellow and Kansas State University professor Frederick Rohles discussed a particularly interesting study confirming that subjectivity in “Temperature & Temperament: A Psychologist Looks at Comfort.” The professor had been puzzled by the fact that a study’s participants felt colder in one test chamber than another, even though the two chambers were kept at an identical 74 degrees. But he quickly identified the problem: One of the chambers was a walk-in commercial refrigerator, so the study participants felt psychologically colder there. After outfitting the room with carpeting and furniture, Rohles ran the study again. Not only did participants now feel warmer in the redesigned chamber, but simply adding the embellishments was the equivalent of raising the temperature 2.5 degrees.
In homes heated by forced air systems, the temperature of heated air being delivered through the supply registers can have both a psychological and physical impact on occupant comfort. “If the air temperature [being supplied at the register] is below your skin temperature, then you are going to have the perception of being cooled, especially when there is a velocity of air across your body,” Moore says. So for the study, Newport assumed that when supply air temperatures are at or below typical body temperature — slightly less than 100 degrees Fahrenheit — many people feel cool and uncomfortable.
If the air temperature [being supplied at the register] is below your skin temperature, then you are going to have the perception of being cooled.
Newport’s next step was to quantify how often different heating systems would deliver that cooler, uncomfortable air at the heating register. Moore used two building energy simulation tools supported by the Department of Energy (DOE) to find out.
First, the research team created a model of a new house using Building Energy Optimization (BEopt) software, which is used by the DOE’s Building America program to measure the impact of energy-saving technologies and retrofits. They then used the DOE’s EnergyPlus software to run an annual energy simulation across three climate zones. Moore compared the heating supply temperature provided by standard-efficiency propane forced air furnaces and standard-efficiency ASHPs.
The results confirmed what many contractors and construction professionals had already known anecdotally: propane furnaces are typically warmer than electric heat pumps. In both mixed and cold climates, the supply air from the ASHP feels cool about 60 percent of the time during the heating season. Propane furnaces, by contrast, provide supply air consistently above 115 degrees Fahrenheit, above the comfort threshold.
The difference in supply air temperature is caused by the different ways the two systems generate heat. Electric heat pumps use a refrigerant to scavenge heat from the outdoor air. As outdoor temperatures fall, there is less heat available to transfer into the refrigerant, so the supply air temperature falls. Standard-efficiency heat pumps will eventually turn to electric resistance heating to raise the temperature. But before this supplemental heat comes on, the supply temperature remains cool and uncomfortable. And when it’s active, electric resistance heat uses much more power. “You’re paying for that comfort — a lot more,” Moore says.
Propane furnaces operate independently from outdoor conditions. A propane furnace is simply heating the indoor air with propane combustion, so it supplies a steady, consistent heating temperature of 115 to 125 degrees Fahrenheit, regardless of outdoor temperatures.
So is more comfortable gas heating a myth or a fact? While comfort is subjective, it’s a fact that propane furnaces provide warmer heated air than ASHPs throughout the winter. For many building professionals, that warmer air makes the difference between a contented customer and a dissatisfied one.