The Furnace Isn’t Dead — It Has a New Role

At the same time, dual fuel can make sense when a home already has a gas furnace, local energy costs favor using both fuels, or replacing one component creates an opportunity to add heat-pump technology. 

The result is a more application-specific approach to dual fuel, with contractors considering the home’s heating load, climate, existing infrastructure, equipment capabilities, utility costs, and homeowner priorities. 

The Replacement-Cycle Opportunity 

Dual fuel is not always a fresh design decision. Sometimes, the replacement cycle creates the opportunity. 

King Tong, senior director, product marketing, Daikin Comfort Technologies North America Inc., said the outdoor unit and furnace often do not need to be replaced on the same schedule. 

A homeowner may have a furnace that is still operating properly while the outdoor unit has reached the end of its life. In that situation, the contractor can replace the outdoor unit with a variable-speed heat pump while retaining the existing furnace. 

That creates an opportunity to introduce inverter heat-pump technology without prematurely replacing a functioning furnace. 

Daikin’s D24V gateway accessory for its FIT system, for example, allows a FIT inverter heat pump to integrate with an existing furnace to create an inverter-driven dual-fuel system. 

Tong noted that the indoor evaporator coil will most likely also need to be replaced to match the new outdoor heat pump. 

Midea sees existing gas equipment as another reason to consider dual fuel. 

“Dual fuel systems especially lend themselves well to homes that already have a gas furnace, by adding a path for heat pump curious homeowners without needing to make modifications to the home,” said David Rames, senior manager of product promotion and development, Midea America Research Center. 

Cold-Climate Heat Pumps Change the Equation 

The traditional argument for dual fuel has often centered on cold weather: Let the heat pump handle moderate temperatures and use the furnace when temperatures fall. But the OEMs say that distinction is becoming less straightforward. 

Tong said the range of conditions in which dual fuel is necessary has narrowed as inverter-driven heat pumps have become more capable at lower temperatures. 

With a variable-speed, inverter-driven heat pump, he said, the heat pump can maintain capacity well into single-digit temperatures. 

“They’re changing its purpose more than its existence,” Tong said. 

“In many markets the heat pump is now the primary heating system, and the gas furnace is there for the handful of extremely cold temperatures and backup,” he said. 

Rames offers a similar caution against assuming that cold climates automatically require dual fuel. 

“However, modern heat pumps now offer heating capacity at much lower outdoor temperatures than previous generations, meaning dual-fuel systems aren’t necessarily required for those living in colder climates,” he said. 

Nicholas Orth, director, product management – outdoor equipment, Lennox Home Comfort Solutions, said cold-climate heat pumps can support both approaches. 

“In certain markets, however, CCHPs could drive consumers away from any backup heat,” Orth said. “I’d encourage dealers to consider CCHPs as all-climate solutions.” 

No Universal Switchover Temperature 

As heat pumps become capable of operating at lower temperatures, contractors also need to reconsider how they establish the point at which a dual-fuel system switches to gas. 

The OEM responses point to an application-specific approach rather than a universal switchover temperature. 

Rames said contractors should consider the home’s heating load, the heat pump’s low-temperature performance, and local energy costs. 

“The capacity balance point is the point at which the home’s heating load begins to exceed the heat pump’s available output,” he said. 

That balance point can vary by climate and equipment. Rames also cautioned against applying rules of thumb developed around older heat pumps to today’s inverter-driven systems. 

Orth approaches the decision from two perspectives: economics and comfort. 

A homeowner prioritizing comfort may choose a higher balance point, while an economically driven strategy would depend on local gas and electric rates. Variable electric rates can make the calculation more complicated, particularly when electricity costs more during peak hours. 

Rheem’s Jeff Goss, director of product management, Residential Systems, U.S. Air, Rheem North America, likewise said the switchover temperature should be based on the individual application. 

“The easiest way to determine the switchover temperature is to evaluate the home’s heating load and choose the most cost-effective temperature for that system,” Goss said. 

The common thread is that contractors should evaluate the equipment and the home rather than simply applying a predetermined temperature. 

Don’t Let the Furnace Take Over Too Soon 

As heat pumps become more capable, setting the switchover point too conservatively can undermine the system’s intended operation. 

“The most common miss is setting the switchover temperature too high and letting the furnace take over before it needs to, reducing much of the efficiency the homeowner paid for,” Tong said. 

Orth described a related issue. Contractors familiar with traditional heat pumps may use a more conservative balance point with cold-climate equipment, partly because they want to avoid callbacks. 

He said he has seen cases in which a balance point appropriate for a standard heat pump is used with a cold-climate model, undermining the reason for paying more for the cold-climate equipment. 

For contractors, that makes equipment-specific performance data important. A balance point that made sense for an older heat pump may not be appropriate for a newer inverter-driven model. 

Making the Decision More Dynamic 

The controls that manage dual-fuel systems also are evolving. 

Rames outlined three basic approaches: an outdoor-temperature sensor can trigger the switch to gas below a predetermined set point; a thermostat timer can initiate the switch if the heat pump runs for a specified period without reaching the set point; or the system can compare the thermostat set point with actual room temperature and switch to gas when the room falls below a predetermined amount. 

Goss described a more data-driven approach in which advanced controls and connectivity can incorporate indoor and outdoor temperatures, humidity, weather information, and anticipated load requirements. 

“The increased availability of data allows the system to make more intelligent operating decisions by continuously evaluating current and anticipated load requirements,” Goss said. 

Tong said Daikin sees communicating controls paired with modulating heat pumps taking the concept further. He said those systems can consider outdoor conditions, the heat pump’s actual delivered capacity, and increasingly, local utility economics. 

“The dual fuel system will use either gas or electricity depending on whichever fuel is most efficient and cost-effective at that moment rather than a one-time setting established at install,” he said. 

That creates a progression from a fixed outdoor-temperature switchover toward a system that can make a more dynamic decision based on operating conditions. 

Design the System as a Whole 

The flexibility of dual fuel also means contractors need to look beyond individual components. 

Goss said accurate heating-load calculations are critical, as is properly sizing the heat pump and accounting for the furnace’s capacity, local temperatures, and the selected switchover strategy. 

“It is also important to look at the system as a whole rather than treating the heat pump and furnace as two independent pieces of equipment,” he said. 

Rames similarly emphasized compatibility when pairing a heat pump with an existing gas furnace. 

“Contractors need to pay close attention to compatibility when pairing a heat pump with an existing gas furnace to ensure that the indoor and outdoor equipment and controls work together seamlessly as a complete system,” he said. 

As inverter-driven heat pumps become more common, Rames also said contractors need to understand how the equipment operates so they can configure the system correctly and deliver its intended performance. 

That education component becomes particularly important because homeowners often rely on contractors to explain their heating options. 

Selling the Application 

The OEMs also point to different ways contractors can approach the sales conversation. 

Rames pointed to existing gas furnaces as one opportunity for dual fuel. 

“Dual fuel systems especially lend themselves well to homes that already have a gas furnace, by adding a path for heat pump curious homeowners without needing to make modifications to the home,” he said. 

That replacement scenario can give contractors an opportunity to introduce heat-pump technology while retaining a functioning gas furnace. 

Rheem similarly emphasizes homeowner priorities, including energy costs, comfort, existing infrastructure, and system expectations. 

“Contractors should start by understanding the homeowner’s priorities, including energy costs, comfort preferences, existing infrastructure and system expectations,” Goss said. 

Rather than treating dual fuel as the answer for every home, contractors can use those factors to determine whether the combination makes sense for the particular application. 

As heat pumps become more capable and controls become more sophisticated, dual fuel is becoming less about the limitations of either technology and more about how the system responds to the conditions in a particular home. 

Tong said communicating controls can increasingly consider outdoor conditions, the heat pump’s actual delivered capacity, and local utility economics when determining which heat source to use. 

“The dual fuel system will use either gas or electricity depending on whichever fuel is most efficient and cost-effective at that moment rather than a one-time setting established at install,” he said. 

That makes the contractor’s role less about choosing a fixed switchover point and more about understanding the equipment, the home, and the economics behind the system. 

Rames said that requires contractors to understand how today’s heat pumps perform compared with previous generations. 

“Contractors need to understand the improved performance and heating capacity of today’s heat pumps, as well as have a breadth of knowledge about local utility rates and the home itself,” he said. “This understanding helps the industry as a whole determine when dual-fuel makes the most sense.”