Dual-fuel Heating—The Best of Both Worlds for Homeowners
It wasn’t too long ago that most homeowners purchasing a new heating system had to decide between a combustion furnace and an electric, air-source heat pump. These days, however, homeowners can enjoy the advantages – and minimize the limitations – of both, with a hybrid heating system.
Also known as a dual-fuel heat pump, a hybrid system uses a heat pump as the main source of heating, but utilizes a natural-gas combustion furnace as a back-up. Of course, it’s a bit more complicated than that.
Before further explanation, let’s review each heating source that combines to make a hybrid or dual-fuel system.
Combustion furnace
Natural gas remains the most common fuel burned in residential furnaces. It’s relatively cheap and convenient to use, though some homeowners without access to a natural-gas line in their neighborhood use propane or fuel oil to run their furnace. These homes are easy to recognize due to the big horizontal metal storage tank that’s close to the house.
Combustion furnaces, no matter what fuel they use, are relatively simple systems. A pilot light (or more commonly with modern furnaces, an electronic ignition) ignites the natural gas or other fuel emitted from one or more burners.
This heats the air in the furnace’s heat exchanger. A powerful blower fan pushes the heated air out of the furnace and into a network of ducts that connect to every room in the home.
Vents and registers are located at the end of each duct run. Supply ducts deliver the conditioned air, and return ducts bring it back to the furnace to be heated again.
Advantages of a combustion furnace over other heating systems
- Since the boom in deep-shale gas and oil over the past decade, natural gas has been extremely cheap.
- Natural gas is readily available in most non-rural areas, with an extensive system of gas lines connected to most homes. It’s also available in some rural areas.
- A combustion furnace generally will heat a home quickly and comfortably, provided the distribution system is properly designed and installed and the home is relatively airtight.
Disadvantages
The main disadvantage of a gas furnace, aside from the fact that it uses nonrenewable fossil fuels, is that its heating efficiency (as measured by its AFUE – Annualized Fuel Utilization Efficiency) can never exceed 100 percent.
This is much better than a few decades ago, when AFUE ratings were as low as 60 or 70 percent. Yet a gas furnace is still very inefficient compared to a well-maintained electric heat pump. The latter can deliver three or four times more heat energy than the electricity that goes into the system.
Gas furnaces also pose a greater health and safety risk, from carbon monoxide emissions and potential gas leaks as well as fire hazards.
While these risks are exceedingly low in a furnace installed by a reputable HVAC installation company that is well-maintained, they are virtually non-existent in a heat pump.
Electric air-source heat pump
While other types of heat pumps exist in residential settings, including the increasingly-popular geothermal heat pump, the air-source type is the one people typically refer to when they say “heat pump.”
It’s essentially a split-system air conditioner, with one important improvement: It provides heating as well as cooling.
Both heat pumps and ACs employ refrigerant (aka coolant) that runs through special copper or aluminum coils. When the refrigerant is made to convert from a liquid into a gas and then back into a liquid, heat energy is either extracted from inside air (for air conditioning) or released into the interior air (for heating).
In a heat pump, the process can be reversed, with the roles of the inside and outside evaporator and condensing coils switching depending on whether cooling or heating is called upon.
When the temperature goes below freezing
A well-maintained heat pump, as mentioned previously, will provide extremely efficient heating and cooling. However, when the temperature plummets below freezing for an extended period, the heat-pump technology struggles to provide comfortable heat.
The outside unit just can’t extract enough heat energy from that cold outside air. This is why air-source heat pumps used for both cooling and heating are so popular in Southern climates.
It’s also why a heat pump typically comes with an auxiliary or emergency electric heating coil, which kicks into action on cold days. The supplemental electric coil is relatively inefficient and costly to use.
This – finally! – brings us to the main topic of this blog.
Hybrid or dual-fuel systems
These systems combine the energy efficiency of air-source heat pumps with the relatively-low cost of gas heating to provide a reliable and relatively affordable 24/7 heating option.
- When the temperature outside drops below a certain temperature and the heat pump is no longer effectively heating the home, the supplemental gas furnace will kick into action. Its auxiliary heat is much cheaper – and more comfortable – than the electric coil that comes with a typical heat pump.
- Some hybrid systems allow the homeowner to manually switch between the heat pump and furnace, to take advantage of whichever fuel source – electricity or natural gas – happens to be cheaper to use at that time.
- Heat pump technology, however, over the long term likely is a more secure heating source than natural gas. The latter is more at the mercy of volatility in markets and rates.