dual-fuel heat pump
Dual-fuel heat pump. Image credit: Crawford Electric.
A dual-fuel heat pump is a heating appliance that combines an electric heat pump and a gas furnace. Dual-fuel heat pumps are effective in regions where the winter temperature is usually above freezing (allowing the heat pump to be used much of the time) but can occasionally drop below freezing, when the gas furnace provides heat more economically. Combining the two, offers the benefits of both systems. However, you need to make sure that the system is set up so that the heat pump is used whenever possible as it will generally run at a fraction of the cost of the gas furnace.
How a dual-fuel heat pump works
Heat pumps work in winter by moving heat from the atmosphere outside to the air inside the home. Because they move heat rather than create it, they're much more efficient than systems that burn fossil fuel. Heat pumps can work effectively just by moving heat providing the outside temperature stays above about 35°F (1.7°C). At lower temperatures, heat pumps depend on auxiliary heating to make sure the home stays comfortably warm. In the case of most heat pump systems, this supplemental heat comes from electric resistance heat. However, a dual-fuel heat pump uses a gas furnace – hence the name "dual-fuel".
Because there are advantages and disadvantages to both a heat pump and gas furnace based on the outdoor temperature, the dual-fuel solution offers the best of both worlds. It's the most comfortable heating system at any outdoor temperature, as well as one of the most efficient, versatile, and economical heating-and-cooling systems you can buy.
Using a dual-fuel heat pump cost-effectively
Typically, in a dual-fuel heat pump, the heat pump's compressor does about 85% of the work and the gas furnace heats around 15% of the time. In an ideal scenario, each fuel is used when it's most efficient.
It's important to get good advice, especially with the cost of natural and propane gas having risen so much in recent years. For example, some homeowners may be told to switch off the heat pump completely during the winter by moving the thermostat to the emergency heat setting. But this can prove very costly because they end up running the furnace exclusively at the very time when the cost of gas tends to be at its peak. It also means they're losing the benefit of half the heating system they have paid for.
The only time any heat pump, electric or dual-fuel, should be set on emergency heat is when the outdoor unit fails. Emergency heat is designed to provide minimal heating until the repairman arrives.
A similar problem happens when the dual fuel heat pump's outdoor thermostat is improperly set. The job of the outdoor thermostat is to determine when the heat pump switches from electricity to gas. If it's set too high, the system switches from electricity to gas too early. This means the homeowner misses out on some of the economical electric heating.
A good heating and air contractor will calculate the optimum outdoor temperature for the dual heat pump to switch from electricity to gas. This temperature, called the balance point, is usually around the 30s for a properly designed system. Setting the outdoor thermostat higher than the balance point means less-than-optimum use of the dual fuel system and higher energy bills.
A dual-fuel unit costs about $600 to $1,000 more than conventional heating and cooling systems because you're getting essentially two systems in one. But the amount you'll save in the next two to three years from lower heating costs will more than make up the difference you'll spend on a better system.
Choosing a system
There are several different brands and models to choose from. The most noticeable differences between them will be the price, and whether they'll be installed inside or outside your home. There are fewer packaged models available on the market. These systems are also more expensive than split systems.