A dual fuel heat pump pairs an electric heat pump with a furnace that burns another fuel, usually natural gas, propane or oil. The heat pump does the heating in mild weather. The furnace takes over when it gets cold enough that the heat pump is no longer the cheaper or stronger choice. It can make sense in a few Maine homes. It is not the default answer here, and since dual fuel systems do not qualify for Efficiency Maine rebates, the choice affects your budget.
TL;DR
- A dual fuel system is a heat pump plus a fossil-fuel furnace, with a controller that picks which one runs. It is also called a hybrid system or a heat pump with a furnace.
- The switchover usually happens somewhere around 25°F to 45°F in cost-based setups. Cold-climate heat pumps can carry a home far below that.
- Dual fuel heat pump systems do not qualify for Efficiency Maine heat pump rebates. Federal residential heat pump tax credits have also ended.
- Dual fuel pays off most where gas is cheap relative to electricity and you already have a good furnace. Most Maine homes are not in that position.
- Sizing and control settings decide whether any of it works. Ask for a heat-loss calculation before you ask for equipment.
Table of Contents
- What a dual fuel heat pump is
- How the switch between heat pump and furnace works
- Why Maine changes the math
- Dual fuel, auxiliary heat and electric backup: what is different
- Benefits and downsides of dual fuel
- What a dual fuel system costs
- Rebates and incentives in Maine
- When dual fuel is worth it, and when it is not
- Our perspective: size for your house, then decide on backup
- Questions to ask any installer
- How we can help
- FAQ
- Sources
What a dual fuel heat pump is
The Federal Energy Management program's technical FAQ at PNNL describes a dual-fuel heat pump as an electric air-source heat pump that uses another fuel for auxiliary or backup heat instead of electric resistance coils. It can be one integrated unit, or a standard heat pump and a furnace tied together by a controller. The controller chooses which one runs based on outdoor temperature, relative efficiency and fuel cost.
In practice you have an outdoor heat pump unit, an indoor coil and air handler on top of or beside a furnace, and ductwork that serves both. In summer the heat pump cools the house like an air conditioner. In winter it reverses and moves heat from outdoor air into your rooms. Carrier's overview notes the furnace supplies combustion heat only when the system selects it.
The other names readers search for all describe the same idea. "Hybrid heat pump system" and "heat pump with a furnace" mean a heat pump paired with a fuel-burning furnace. "Heat pump with electric backup" is different, and we cover that below.
How the switch between heat pump and furnace works
The logic is simple. When it is mild, the heat pump moves a lot of heat for each unit of electricity. As outdoor air gets colder, a heat pump's capacity and efficiency fall. At some point the furnace is the better tool, so the system hands over, then switches back when temperatures rise. Trane describes this automatic changeover as part of a matched heat pump and furnace system.
A Columbia University study of hybrid systems frames it the same way: the heat pump carries the load at higher outdoor temperatures, and the furnace covers peak demand in the cold.
Do they run at the same time?
Usually not. In the system analyzed in a Department of Energy lab report, the furnace and heat pump did not run at once, because doing so could hurt the heat pump. Some setups behave differently, so ask your installer how your specific controls are configured.
What sets the changeover temperature
There are two ways to set it:
- A fixed outdoor temperature. Simple and common. The furnace takes over below a set point.
- A cost-based balance point. The economic balance point is the outdoor temperature where heating costs the same either way. Modeling summarized by the Minnesota Solar Energy Industries Association's partner article puts it at 25°F to 45°F for natural gas systems.
The Department of Energy lab report adds that whether switching is cost-effective depends on changing energy prices and the heat pump's own performance curve. So a setting that was right when you installed may not be right after fuel prices move.
Why Maine changes the math
Most dual fuel marketing was written for places where natural gas is cheap and everywhere. Maine is different. According to a Maine Governor's Energy Office summary of Census data, natural gas was the primary heating fuel in about 8.1% of Maine homes in 2022. Many homes burn oil or propane, or already have a heat pump.
That matters for two reasons.
- The "gas furnace" half of the typical pitch does not apply. A dual fuel system with oil or propane backup can work, but the savings story is different from the natural gas version in the glossy brochures. The NEEA analysis found that the gap between electricity and gas rates drives the result, and that savings are harder where gas is cheap compared with electricity.
- Cold-climate heat pumps have closed much of the gap. The Department of Energy's building science guidance says cold-climate models can deliver all of a home's heat in single-digit temperatures and perform well below zero. Trane names Maine among the northern states where these units are a solution, though its low-profile models switch to the furnace below 5°F.
We design for capacity at 5°F, not just the nameplate rating, and that capacity number is the one that decides whether you need backup at all.
Dual fuel, auxiliary heat and electric backup: what is different
These terms get mixed up, and the difference affects your bill.
| Setup | Backup source | What it means for you |
|---|---|---|
| Dual fuel (hybrid) | Gas, propane or oil furnace | Backup heat is combustion heat. Needs a furnace, a flue and fuel delivery or a gas line. |
| Heat pump with electric auxiliary heat | Electric resistance coils | Simple and cheap to install. Coils use far more electricity per unit of heat than the heat pump does. |
| Cold-climate heat pump as primary heat | Existing system kept as emergency backup | Heat pump handles the heating season. The old system stays idle unless something goes wrong. |
The PNNL definition draws the line exactly here: dual fuel uses another fuel source in place of electric resistance coils. Heat pump auxiliary heat is usually the electric strips inside the air handler, often called "emergency heat" on the thermostat. If your thermostat shows aux heat running a lot, the heat pump is either undersized, struggling, or set up with poor controls. Our guide on what to set for lockout temperature explains how that setting works in a Maine winter.
Benefits and downsides of dual fuel
What it does well
- Comfort and reliability in extreme cold. The furnace covers the coldest days. The NEEA white paper reports that well-run dual-fuel systems can reach seasonal efficiency of 200% or more.
- Efficiency in shoulder seasons. Natural Resources Canada notes heat pumps can be up to 300% efficient, while furnaces are typically 80% to 95% efficient.
- Possibly a longer furnace life. Using the heat pump in mild weather may extend the existing furnace's life, according to the same industry analysis, which also notes more even temperatures.
- A middle path on emissions. The system relies on efficient electric heat most of the time and keeps fossil backup for extreme cold.
Where it falls short
- Two systems to maintain. You service a heat pump and a furnace, and you carry the cost of both.
- Poor setup wastes the benefit. NEEA's paper warns that bad sizing, setup or control settings can keep a system from reaching its potential.
- No Efficiency Maine rebate. More on that below.
- Fuel exposure. You stay tied to oil, propane or gas prices on the days the furnace runs.
- Higher upfront cost if you are replacing both. There is more equipment, more ductwork work and more labor.
If you want a broader view of how heating options stack up on efficiency, this most efficient way to heat comparison is a helpful companion, though keep in mind the fuel prices and rules there are not Maine's.
What a dual fuel system costs
No single honest number exists, and anyone quoting one without seeing your house is guessing. Carrier says installation cost depends on home size, existing ductwork, the models chosen and local labor rates. It includes the heat pump, the backup furnace, duct changes and labor. Operating savings depend on electricity and gas prices, equipment efficiency, climate, sizing and controls.
The practical way to think about it:
- You already own a good furnace that is under 15 years old. Adding a heat pump to it is the cheapest route into dual fuel.
- Both systems are due for replacement. You pay for two sets of equipment. Compare that against one properly sized cold-climate heat pump system.
- You have a boiler or baseboard heat. Dual fuel needs ducts for the heat pump and furnace to share. Ductless heat pumps are often the better fit, and they do not pair with a furnace the same way.
We do not publish installation prices because they depend on the heat-loss calculation, system type and your home. We give a free assessment and estimate instead.
Rebates and incentives in Maine
This is where many Maine homeowners get surprised. The Efficiency Maine rebate eligibility table lists dual fuel heat pump and furnace combinations as not eligible. Its rules say the heat pump must serve as the primary heating system through the heating season, with existing fossil systems used only as emergency backup. Rebate amounts range up to $9,000 in lifetime rebates per housing unit for qualifying low-income households, with $2,000 per outdoor unit for moderate income and $1,000 per outdoor unit at any income. The program also requires sizing based on your home's peak heating load.
The federal Section 25C and 25D residential credits for air-source heat pumps are no longer available as of 2026, so there is no federal offset to weigh either. Programs change, so check our Maine rebates page and the Efficiency Maine site before you sign anything.
That single rule can swing the decision. A cold-climate system sized to heat the whole house can earn a rebate. The same equipment paired as a dual fuel system does not.
When dual fuel is worth it, and when it is not
Dual fuel can be a sound choice when:
- You have natural gas service, a recent high-efficiency furnace, and a gas price that is low compared with electricity.
- You want a hedge for a home with a very leaky envelope or a poorly insulated space you cannot fix soon.
- Your ductwork is already in place and in good shape.
- You accept that you will not receive an Efficiency Maine rebate.
A cold-climate heat pump as the primary system usually wins when:
- You heat with oil or propane today. Our guide to switching from oil to a heat pump covers who should plan to stay at least seven years to come out ahead.
- You want to qualify for rebates.
- You would rather maintain one main system and keep the old furnace or boiler as a quiet emergency backup.
- You want to cut fossil fuel use as far as practical.
A Department of Energy guide notes that controls can be set so the heat pump is always the priority and the furnace engages only when the heat pump cannot hold the target temperature. That carbon-first setup is close to how we like to run a backup.
Our perspective: size for your house, then decide on backup
We install and maintain cold-climate heat pumps in Maine, so we are not neutral, and we will say so. We see the same pattern often. A homeowner is told they need dual fuel because a heat pump "can't handle Maine." Sometimes that is true for the specific unit that was quoted, which was sized by square footage or chosen from a catalog without a calculation. The unit was the problem, not the technology.
Our approach starts with a heat-loss calculation for the actual house. From that we pick equipment that holds its capacity when temperatures plunge, down to −22°F and lower. If the numbers show a heat pump alone will not cover the coldest hours, we say so, and we talk about keeping your existing system as backup. If the numbers show it will, we avoid selling you a second system you do not need.
We also think maintenance decides more than people expect. A dirty coil, a blocked outdoor unit or a bad control setting can make any system, dual fuel or not, act undersized. Our no-heat checklist walks through what to check before you call anyone, and our guide to heat pump lifespan in Maine shows what upkeep does for longevity.
Before you commit to any system, ask for the sizing worksheet. If a contractor cannot show it, keep looking.
— John
Questions to ask any installer
- What is the design-temperature capacity of the heat pump at 5°F and at the coldest temperature in my area?
- Did you run a heat-loss calculation for my house, and can I see it?
- What changeover temperature will you set, and how will it be tuned to current fuel prices?
- Will the furnace and heat pump ever run together on this equipment?
- Will this configuration qualify for Efficiency Maine rebates? If not, what would I need to change?
- What does yearly service cost for each piece of equipment?
Our installation process guide shows what a good Maine install looks like step by step, and our installation planning page explains how we plan a project.
How we can help
If you are weighing a heat pump against a dual fuel setup, we start with a free assessment and estimate. We look at your home's heat loss, your existing heating, your ZIP code for service confirmation, and the rebate rules, then we tell you plainly what we would do. We install new heat pumps and replace existing ones, and you can see the full list on our services page.
If you already have a heat pump and want it to keep performing, our care plans are priced publicly. The Annual Cleaning Plan is $219 per year. The Maine Heat Pump Care Plan is $319 per year. A one-time deep cleaning starts from $249. Details are on our plans page, with the terms on the care plan terms page. Service is confirmed by ZIP code before any appointment or payment.
FAQ
Is a dual fuel heat pump worth it?
It can be, if you have a good furnace, gas is cheap relative to electricity, and the system is sized and controlled correctly. NEEA's analysis shows the benefit depends on the rate gap between the two fuels. In Maine, a cold-climate heat pump sized for the whole house often costs less to own, partly because it can qualify for rebates.
What are the downsides of dual fuel heat pumps?
You maintain two systems, you stay exposed to fuel prices, and a poor setup can waste the savings. Dual fuel combinations also do not qualify for Efficiency Maine heat pump rebates. Upfront cost is higher when both pieces of equipment are new.
At what temperature does a dual fuel system switch to the furnace?
It depends on the settings and the heat pump. Cost-based balance points are modeled at 25°F to 45°F for natural gas systems, and some low-profile models switch below 5°F. A cold-climate heat pump may not need to switch until much colder, so ask your installer for the setting and the reason behind it.
Can I add a heat pump to my existing furnace?
Often yes, if the furnace is in good shape and the ductwork suits it. A technician needs to confirm the coil, air handler and controls are compatible. Rebate rules change if you keep the furnace as a co-primary heat source, so check eligibility before installing.
Sources
- What is a dual-fuel heat pump? (FEDS, PNNL)
- What is a Dual Fuel Heating System? (Carrier)
- Heat pump and furnace matched system (Trane)
- What Is a Dual Fuel Heat Pump System? (Trane)
- Cost-Optimal Sizing and Operation of a Hybrid Heat Pump (Columbia University)
- Optimized heating controls report (OSTI)
- Controls for a heat pump with secondary heating (U.S. DOE BSESC)
- System design with existing heating (U.S. DOE BSESC)
- How a dual fuel heat pump system with natural gas backup balances emissions reduction and savings
- Heat pump and gas hybrid basics (Natural Resources Canada)
- Advancing Dual-Fuel Residential HVAC (NEEA)
- Dual Fuel Systems Analysis (NEEA)
- Residential Heat Pump Rebates (Efficiency Maine)
- Maine heating fuels summary (Maine Governor's Energy Office)
- Most efficient way to heat a house: 2026 guide