· by Michael Blair, Nashville's HVAC
Middle Tennessee winters are mild enough that a heat pump works well here for most of the season, and cold enough that the exceptions matter. NOAA’s 1991–2020 climate normals for Nashville International Airport count about 66 nights a year at or below freezing, but only about 14 at or below 20°F and fewer than two at or below 10°F. The decision comes down to your gas availability, your insulation, and how you feel about supply air that is warm rather than hot.
How does a heat pump differ from a gas furnace in winter?
A heat pump moves heat rather than making it, which is why it runs so efficiently in mild weather. The cold still costs it something: the Department of Energy’s training module on heat pump thermostats and dual fuel control says heat pumps lose efficiency and capacity at lower outdoor temperatures and higher heating loads.
How far down a unit stays useful depends on the unit. To be certified as a cold climate heat pump, ENERGY STAR’s 2020 verification webinar says a unit has to keep at least 70% of its 47°F heating capacity at 5°F, with a COP of at least 1.75. In the Bonneville Power Administration’s field study of 57 Pacific Northwest homes, systems sized to a 20°F balance point met the whole heating load in the Puget Sound zone and effectively did away with backup resistance heat. Not every variable-speed unit carries the cold-climate rating, so we look up the model’s rating rather than going by the brochure.
A gas furnace works the other way. Its output does not fall as the weather gets colder, and the DOE module notes that a gas system meets a sudden jump in demand simply by burning more fuel. It also delivers hotter supply air, which some people strongly prefer.
Where the running cost lands
It depends on the ratio between local electricity and gas prices, and that ratio moves. EIA’s monthly Tennessee residential natural gas prices went from $9.26 per thousand cubic feet in January 2025 to $11.23 in January 2026, then reached $15.54 by March 2026. Electricity moved less: EIA’s Electric Power Monthly puts the Tennessee residential average at 13.71 cents per kilowatt-hour in July 2026, against 13.21 cents a year earlier. A heat pump’s cost for each unit of heat is the electricity price divided by how efficiently it is running, so whether it beats gas in a given winter comes down to both numbers.
Real houses do not always match the published ratings. In a 2015 Building America study of seven ductless heat pumps in the Northeast, indoor airflow measured typically 50% to 80% of the manufacturers’ listed rates, and efficiency ranged widely from house to house. Most of those units still heated for less than oil, propane or electric resistance would have; the study did not compare them with natural gas.
The bigger variable is the house. A leaky, poorly insulated house puts a higher heating load on the system every cold night, and higher loads are exactly where the DOE module says a heat pump loses efficiency and capacity. The more often the pump falls short, the more often the auxiliary electric strips come on, and the Bonneville study calls that resistance heat costly.
Auxiliary heat is where bills go wrong
Most ducted heat pumps have backup heat, usually electric resistance strips, though the Building America report notes that many cold-climate units, including the ductless kind we install on our mini-split jobs, are built to run without them. The strips engage when the pump cannot keep up, and the DOE module calls that an inefficient supplemental heat mode that can use more energy. Defrost is the other time they come on, which our post on why a heat pump steams and blows cool air explains.
A big overnight setback is one of the quickest ways to put a house on its strips. In the Bonneville study, setbacks set off resistance heat during the morning recovery, and energy use across 14 monitored centrally ducted homes came out about six percent greater. That is why the setback advice for a heat pump differs from the advice for a gas furnace: the DOE module recommends only small or gradual setbacks, or a thermostat made for heat pumps that uses its own logic to keep the supplemental heat to a minimum.
- Keep overnight setbacks small, or make them gradual
- Dual-fuel systems switch to gas below a set outdoor temperature
- Check the thermostat is heat-pump aware, not a furnace thermostat
- Insulation and air sealing cut the load that sends a heat pump to its strips
What is a dual-fuel heat pump system?
A dual-fuel system pairs a heat pump with a gas furnace and switches between them at a chosen outdoor temperature. The DOE module puts that changeover anywhere from 5°F to 35°F, depending on what the equipment can do. Set near the top of that range, it gives you heat-pump efficiency in the forties and gas heat in the twenties. The module also describes a second control method, called droop, where the furnace is set 3°F below the heat pump’s setpoint and joins in only when the pump cannot hold the temperature on its own.
On energy alone, the heat pump does not need the furnace. Oak Ridge National Laboratory’s 2022 simulation of a dual-fuel heat pump, modeled in a colder climate zone than ours, found the dual-fuel system cut heating energy by 42% against a gas furnace, while the heat pump alone cut it by 53%. The dual-fuel setup showed the larger cost savings once a time-of-use electricity rate was applied.
Our view is that dual fuel makes the most sense where gas is already at the house and the equipment is being replaced anyway, and we would not run a new gas line just to enable it. Our system replacement page covers how we size the equipment to the house, and if you are weighing the options, ask us for a free written estimate on whichever ones you want priced.
Sources
- NOAA NCEI, 1991–2020 Annual/Seasonal Normals, Nashville International Airport (USW00013897) — Average days a year with a minimum at or below 32°F (66.3), 20°F (13.7) and 10°F (1.8)
- US DOE Building Science Education Solution Center, Smart Thermostats and Dual Fuel Control, Proficiency Level 2 — Heat pumps lose efficiency and capacity in the cold and at higher loads; a fossil system burns more fuel; large setbacks trigger inefficient supplemental heat; small or gradual setbacks; heat-pump thermostats; balance point changeover at 5°F to 35°F; droop method 3°F below
- ENERGY STAR, Cold Climate Heat Pump Controls Verification Procedure webinar (2020) — Cold climate criteria: COP of 1.75 and 70% of 47°F heating capacity at 5°F
- Bonneville Power Administration, High-Performance High-Capacity Heat Pump Summary — 57 Pacific Northwest sites; 20°F balance point met the load in Puget Sound without backup resistance heat; setbacks triggered resistance heat in morning recovery; about six percent more energy in 14 ducted homes; resistance heat costly
- EIA, Tennessee Price of Natural Gas Delivered to Residential Consumers — $9.26/Mcf January 2025, $11.23 January 2026, $15.54 March 2026
- EIA, Electric Power Monthly Table 5.6.A — Tennessee residential electricity 13.71 cents/kWh July 2026, 13.21 cents July 2025
- DOE Building America and CARB, Williamson and Aldrich, Field Performance of Inverter-Driven Heat Pumps in Cold Climates (2015) — Many cold-climate units have no backup resistance heat; airflow 50% to 80% of listed rates; wide range of efficiency; cheaper than oil, propane or resistance heat; supply air about 80°F at the end of defrost; a unit defrosting every 10 minutes from refreezing meltwater
- Oak Ridge National Laboratory, Yoon, Li, Im and Lyons, Potential heating energy and cost savings of dual fuel heat pump (SimBuild 2022) — Climate zone 5A simulation: dual fuel saved 42% heating energy over a gas furnace, heat pump alone 53%; dual fuel the larger cost savings under a time-of-use rate
FAQ
Heat Pump or Gas Furnace in Nashville — questions
Do heat pumps work in Tennessee winters?
Yes, and Middle Tennessee is a climate where heat pumps do well. In NOAA’s 1991 to 2020 normals, Nashville averages about 14 nights a year at or below 20°F and fewer than two at or below 10°F, while a heat pump certified under ENERGY STAR’s cold climate criteria keeps at least 70% of its heating capacity at 5°F. The few hard-freeze nights are the ones where backup heat or a dual-fuel setup earns its place.
Why does my heat pump blow cool air?
Heat pump supply air comes out warm rather than hot, so it can feel cool on your hand while the house is still heating. If the air is genuinely cold, the unit may be in defrost, which is normal and lasts a few minutes; in a 2015 Building America field study, the supply air dipped to about 80°F by the end of a defrost cycle. In the same study, one unit going into defrost every 10 minutes was most likely icing up from the meltwater of the unit above it, so a heat pump that defrosts that often needs a service call.
Should I turn the thermostat down at night with a heat pump?
Only slightly. Recovering from a large setback pulls in the electric auxiliary heat, and in a Bonneville Power Administration field study, nighttime setbacks left 14 centrally ducted homes using about six percent more energy. The Department of Energy recommends small or gradual setbacks, or a thermostat designed for heat pumps that keeps the strips to a minimum while it recovers.
Is a heat pump cheaper to run than a gas furnace?
It depends on local gas and electricity prices, and both move. A heat pump’s cost per unit of heat is the electricity price divided by how efficiently it runs. Tennessee residential gas went from $9.26 per thousand cubic feet in January 2025 to $15.54 by March 2026; electricity moved less. A leaky house that leans on backup strips costs more.