When a heat pump stops being the right choice
A heat pump works well until the outdoor temperature drops far enough that it can no longer pull heat from the air efficiently. At that point, the system switches to electric resistance heating — essentially a space heater — which costs far more to run than burning gas. If your area regularly hits temperatures below 25°F, or if your electric bills spike during winter, you may save money by switching to a gas furnace for the coldest months or by replacing the heat pump entirely.
The decision depends on three things: how cold it gets where you live, what you pay for electricity versus gas, and whether your heat pump is aging or failing. A heat pump that works fine at 35°F may become a liability at 0°F. A furnace does not care about outdoor temperature — it burns fuel and produces heat at the same cost whether it is 50°F or minus 20°F.
Key Takeaways
- Heat pumps lose efficiency below 25°F to 35°F depending on the model, and switch to expensive electric resistance heating below that threshold.
- If your winter electric bills are significantly higher than your summer bills, your heat pump is likely running its backup heater frequently.
- A gas furnace costs less to operate in climates with sustained freezing temperatures, even accounting for the cost of the furnace itself.
- Replacing a heat pump with a furnace makes sense if the heat pump is near the end of its life or if you live where winter temperatures regularly drop below 20°F.
- A dual-fuel system — heat pump plus furnace — lets you use the cheaper option depending on the season, but costs more upfront.
How cold does it get in your area
Check your local winter temperatures over the past five years. If your area regularly dips below 25°F, a heat pump alone will trigger its backup heater often. Below 15°F, most heat pumps are almost entirely reliant on that backup system. The colder the winter, the more you pay.
Look at the coldest weeks of the past three winters, not just the coldest day. A heat pump that sees one week of 0°F weather is different from one that sees eight weeks of it. The longer the cold spell, the more the backup heater runs, and the more your electric bill climbs.
Compare your winter and summer electric bills
Pull your electric bills from December through February and compare them to June through August. If winter bills are 50 percent higher or more, your heat pump is running its electric resistance heater regularly. If they are double the summer bills, the backup heater is running almost constantly during cold snaps.
Call your utility company and ask what you pay per kilowatt-hour for electricity. Then ask what your neighbors with gas furnaces pay per therm for natural gas. A furnace burning gas at $1.50 per therm will almost always cost less than electric resistance heating at $0.15 per kilowatt-hour, especially in sustained cold.
The cost of switching versus the cost of running
A new gas furnace costs $3,000 to $6,000 installed, depending on your region and the size of your home. A heat pump replacement runs $4,000 to $8,000. If you are replacing a heat pump that is failing, the furnace may cost less and save money on heating bills — a double win. If your heat pump is relatively new and working, the furnace pays for itself only if you will live in the house long enough for the fuel savings to cover the installation cost.
Use this rough math: take your annual heating bill, subtract what you would pay with a furnace, and divide the furnace cost by that difference. If a furnace costs $5,000 and saves you $1,000 per year on heating, it pays for itself in five years. If it saves you $300 per year, it takes 17 years. That matters only if you plan to stay.
When your heat pump is aging or failing
A heat pump typically lasts 10 to 15 years. If yours is 12 years old and the compressor is starting to fail, replacing it with another heat pump does not make sense in a cold climate. A furnace will outlast the heat pump and cost less to run in winter. If the heat pump is only five years old and working well, replacing it with a furnace is harder to justify unless you live somewhere that gets sustained freezing temperatures.
Ask an HVAC technician to check the refrigerant charge and the compressor. If either is failing, you are facing a $1,500 to $3,000 repair on a system that may only have a few years left. That is the moment to consider a furnace instead.
Dual-fuel systems: heat pump plus furnace
Some homes run both a heat pump and a furnace. The heat pump handles spring, fall, and mild winter days. When the temperature drops below a set point — usually 35°F — the furnace takes over automatically. This setup costs more upfront but uses the cheapest fuel for each season.
A dual-fuel system makes sense if you are already replacing your furnace or heat pump and you live in a climate with both mild and cold periods. It does not make sense if you are adding a furnace to an existing heat pump just to save money — the installation cost is too high to recover through fuel savings alone.
What happens if you keep the heat pump in a cold climate
Your heat pump will keep your house warm, but the electric resistance heater will run frequently, and your winter electric bills will be high. You will not be in danger of freezing — the backup heater ensures that — but you will pay more per BTU of heat than a furnace would cost. Over a decade, that adds up.
If you can afford the higher bills and you like the heat pump for other reasons — quieter operation, cooling in summer, lower maintenance — keeping it is a valid choice. But if cost is the concern, a furnace is the answer in a cold climate.
Frequently Asked Questions
At what temperature should I switch my heat pump to a furnace?
Most heat pumps lose efficiency below 25°F to 35°F and rely heavily on backup heating below 15°F. If your area regularly sees temperatures below 25°F for weeks at a time, a furnace will cost less to operate. If cold snaps are brief and rare, the heat pump may still be the cheaper option overall.
Can I run both a heat pump and furnace at the same time?
No. A dual-fuel system is designed to switch between the two automatically based on outdoor temperature. The furnace does not run while the heat pump is running. The system chooses whichever is more efficient for the current conditions.
How much will I save by switching to a furnace?
Savings depend on your local gas and electricity rates, how cold your winters are, and how old your heat pump is. In a climate with sustained freezing temperatures, a furnace can cut heating costs by 30 to 50 percent. In a mild climate, the savings may be minimal or nonexistent.
Is it worth replacing a working heat pump with a furnace?
Only if your heat pump is near the end of its life, your winters are consistently cold, and the fuel savings will cover the furnace cost within a reasonable time. If the heat pump is relatively new and working well, keeping it usually makes more financial sense.
What if I have a heat pump but no existing furnace ductwork?
Installing ductwork for a furnace is expensive — often $2,000 to $4,000 or more. In this case, a ductless mini-split furnace or a dual-fuel system with a heat pump may be more practical than tearing into walls to run ducts.