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  • Writer's pictureMatt Weber

How Do Dual Fuel Heating Systems Work?

Updated: May 20

(guest post by Hazel Hale)

Keeping your home warm in frigid climates poses a significant challenge, with many heating systems falling short in extreme cold. Striking the right balance between efficiency and reliability becomes crucial. That’s where installing a dual fuel system comes in. It is a versatile HVAC solution that merges a gas furnace and an electric heat pump. When temperatures plummet, the gas furnace takes charge, ensuring ample warmth. In milder conditions, the electric heat pump seamlessly steps in, optimizing performance and curbing energy consumption.

This article explores the workings of a dual-fuel system, unraveling its benefits and offering guidance on selecting the perfect fit for your home. Navigating the winter chill becomes simpler when armed with knowledge about this dynamic heating solution.

What is a Dual Fuel System?

A dual fuel system is a type of home heating system that uses two different sources of energy to produce heat. The two sources are typically electricity and natural gas, but they can also be propane or oil. A dual-fuel system consists of two main components: a heat pump and a furnace.

A heat pump is a device that extracts heat from the outside air and transfers it to the inside of the house. It works like an air conditioner in reverse, and it can provide both heating and cooling. A furnace is a device that burns natural gas or another fuel to create heat. It works by igniting the fuel and blowing the hot air through ducts to the rooms.

A dual fuel system combines the advantages of both a heat pump and a furnace. It uses the heat pump as the primary source of heating when the outside temperature is mild, and it switches to the furnace when the temperature drops below a certain point. This way, it can optimize the performance and efficiency of each component and reduce energy consumption and costs.

The heat pump condenser is the part of the AC system that stays outside of the home. Working together with the compressor; when the system is in cooling mode it uses the compressor to pressurize the freon into a hot liquid. When the system is in heating mode the heat pump reverses that process and sends the warm air back into your home as opposed to ejecting the hot air outdoors. Image courtesy Goodman.


Transferring Heat, Not Generating It

You might wonder how a heat pump can provide heating and cooling for your home. The secret is that heat pumps can reverse their operation depending on the season. In the winter, a heat pump can extract heat from the cold outdoor air and transfer it to your indoor air, warming up your home. In the summer, a heat pump can do the opposite: it can take heat from your indoor air and dump it outside, cooling down your home.

Refrigeration Cycle Explained

The operation of a dual-fuel heating system hinges on the refrigeration cycle, a fundamental process underpinning its ability to both heat and cool your home efficiently.

The compressor plays a vital role in this cycle. Contrary to a common misconception, it doesn't directly compress liquid refrigerant. Instead, it elevates gaseous refrigerant to high pressure. This compressed gas, now hot, travels through the outdoor coil (condenser). Here, fans expel heat from the compressed gas into the surrounding air, causing it to condense back into a liquid state.

This is where the magic happens in heating mode. The liquid refrigerant then journeys to the indoor coil (evaporator). Strategically placed valves reduce its pressure, triggering its transformation back into gas through evaporation. This process crucially absorbs heat from the outdoor air, not just the indoor air circulating over the evaporator coils.

The heat-laden gas then travels back to the compressor, completing the cycle and carrying the captured outdoor heat indoors. The reversing valve then directs the hot, compressed gas to the indoor coil, where it releases the heat it absorbed outdoors, warming your home.

During cooling mode, the roles of the coils reverse. The heat absorbed from your indoor air by the evaporator is carried by the refrigerant to the condenser, where it's released into the outdoor air through the fan.

In essence, whether in heating or cooling mode, the dual fuel heating system orchestrates a complex dance of refrigerant state changes and heat transfer within the framework of the refrigeration cycle, ensuring year-round comfort in your home.

When the Furnace Steps In

While heat pumps excel at efficiently extracting warmth from outdoor air, their effectiveness can gradually decrease as temperatures dip below freezing (32°F). This isn't a hard rule, though, as modern heat pump models often maintain impressive efficiency even down to sub-zero temperatures, depending on the specific technology and system design.

To ensure consistent comfort during extreme cold snaps, many HVAC systems integrate a backup heating source like a furnace. This backup kicks in when outdoor temperatures hit a threshold where the heat pump's efficiency drops significantly, typically determined by your thermostat and system settings.

Modern thermostats play a crucial role in this seamless transition, intelligently switching between the heat pump and backup based on real-time outdoor temperature and your desired indoor comfort level. This optimizes both efficiency and comfort.

Mitsubishi Electric’s intelli-HEAT Dual Fuel Heat Pump system consists of an exterior heat pump replacing an existing air conditioning condenser and an interior unit installed on a customer’s existing furnace. (Courtesy of METUS)

It's important to note that gas furnaces are a common backup choice, but other options like electric furnaces, heat strips, or even dual-fuel systems utilizing propane are also available.

Overall, the combination of a heat pump and a backup heating source provides a well-rounded solution for year-round comfort. While efficiency might decrease in harsher climates, modern technology ensures a smooth transition and reliable heating performance across varying temperatures.

Seamless Switching for Optimal Efficiency

While the claim that dual-fuel heat pumps seamlessly switch between electricity and gas/oil is generally accurate for modern, well-maintained systems, it's important to consider a few nuances for a complete picture.

Smart System Controls: Indeed, these systems boast intelligent controls that monitor factors like outdoor temperature, energy prices (which can fluctuate significantly depending on location and time), and system performance. This allows them to select the most economical and efficient heat source at any given time, aiming for a seamless transition without compromising comfort. However, it's worth noting that some older systems might experience slight temperature fluctuations during switching, which could be noticeable to sensitive individuals.

Prioritizing Efficiency with Comfort in Mind: While maximizing efficiency is a key benefit, user comfort should also be prioritized. Modern systems prioritize maintaining consistent indoor temperatures during transitions. However, it's good practice to remember that extreme cold might require the backup furnace to run more frequently, potentially impacting energy costs.

Backup Heating for Reliable Comfort: Integrating a backup heating source (gas, oil, or propane) ensures consistent comfort even during extreme cold snaps when the heat pump alone might struggle. This redundancy enhances reliability and offers peace of mind.

Overall, dual-fuel heat pumps offer efficient, cost-effective heating solutions with seamless transitions between energy sources and reliable performance in various weather conditions. However, remembering the potential for slight temperature fluctuations in older systems and the impact of extreme cold on energy costs provides a more balanced perspective for homeowners considering this technology.


If you are looking for a way to heat and cool your home efficiently and effectively, consider a dual fuel heating and cooling system. This type of system combines a heat pump with a gas furnace, giving you the best of both worlds. A heat pump works in conjunction with a central air conditioner, using the same refrigerant cycle to cool your home in the summer and warm your home in the winter.

However, when the temperature drops below a certain point, the heat pump loses its heating capacity and needs a secondary heat source. That's where the gas furnace comes in. A dual fuel system switches from the heat pump to the gas furnace when the outside temperature is too low for the heat pump to operate efficiently. This way, you can maximize efficiency and comfort all year long.

For further information or to make an informed decision, reach out to a professional HVAC expert.

About the author: Hazel Hale is the Content Marketing Strategist of Annette Hale’s Indoor Comfort Systems, the most trusted provider of heating and AC repair services in Huntsville, AL and the surrounding areas. When not working, she enjoys photography, digital art and watching her son's soccer games.



Can a Heat Pump Replace an Air Conditioner?

Yes, a heat pump can effectively replace an air conditioner. A heat pump isn’t an add-on to a home AC system, it is a home AC system. Essentially, the components that make up a heat pump are already in place in a standard central air conditioning system. Both systems utilize outdoor units to transfer heat, but unlike traditional air conditioners, heat pumps can also operate in reverse to provide heating during colder months. This dual functionality makes them versatile and energy-efficient, serving as both a cooling and heating system for your home.


Troubleshooting Heat Pumps

The following are common problems with home heat pump systems that DIY’ers might be able to diagnose themselves.

Blowing Cold Air in Heat Mode

If your heat pump system is blowing cold or room temperature air while the thermostat is set to heat, the first step is to check your thermostat to make sure the system didn’t accidentally get set for cooling. Also check your fan control settings. If you run the fan continuously for air circulation and filtering, you may be feeling room temperature air because the system isn’t actively heating. If this is the problem, re-setting the system to heat mode and the fan to auto mode should solve the issue.


Running Constantly in Mild Weather

A heat pump relies on heating energy from outdoor air, so it can be normal for a unit to run all the time in extreme temperatures as it works to extract enough heat for the home. But if the temperature outside is moderately warm, and the unit is still struggling or constantly running, it’s time to inspect it.


Is the thermostat set on air conditioning mode? If so, changing to the heat setting should fix the problem.


If it’s not a thermostat issue, check the air filter located in or near your indoor air handler unit. A dirty or clogged air filter can cause the system to run longer and less effectively. You should clean or replace it routinely. This is something you can easily do yourself.


Your heat pump may need a tune-up. Having your heat pump serviced regularly by a professional, qualified HVAC technician can provide higher efficiency operation. One service typically offered is cleaning your outdoor coil. If the coil is extremely dirty, your system may have trouble keeping up with heating demand.


If it is mildly cold outside, and the heat pump is running constantly, you may have a deeper issue such as a (1) frozen coil, (2) malfunctioning reversing valve, (3) low refrigerant charge, or (4) a compressor issue. For these situations, it is generally best to contact a professional for diagnosis and repair.


Restricted Airflow

Heat pump systems require unrestricted air flow through the outdoor coil to work efficiently. If the system is not heating effectively or if it is running constantly, check the outdoor unit for excessive debris such as leaves or snow blockages. Remove any blockage or obstruction, then resume monitoring the temperature inside. If you still have issues, it’s time to contact a professional.


Fan Not Activating

If your outdoor unit is running, but you don’t feel any air coming from the registers, your indoor air handler may not be activating. In this case, the circuit breaker may have tripped. Your indoor and outdoor units are on separate breakers. Check your home’s electrical panel for a tripped circuit breaker and follow standard safety precautions to reset the breaker. If it continues to trip, contact a professional.


A malfunctioning air handler may also be the result of a burnt out or disconnected wire. Contact your local dealer to diagnose and repair.


If the blower motor is burnt out, contact a trusted HVAC technician to thoroughly check your system, determine why the motor is blown, and estimate repair and replacement costs.


Heat pump system problems aren’t always related to the outdoor unit. In any troubleshooting situation, fixing a smaller issue soon is better than waiting for a bigger, more costly issue later.

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