The York Hybrid Comfort System, as shown here, offers homeowners the opportunity to switch between the most efficient and cost-effective method of heating their home. These hybrid systems also save money on utility bills, sometimes by as much as 40 percent, the company reports. Additional savings can be recognized when homeowners take advantage of utility incentives and federal tax credits. (Courtesy of Unitary Products Division of Johnson Controls Inc.)

With energy prices on a volatile rollercoaster ride, the need for conserving resources, as well as developing and providing high-efficiency, high-performance applications that are energy and cost conscientious, is more pronounced than perhaps anytime in the last few decades.

The NEWSapproached several manufacturers of dual-fuel HVAC equipment with some questions about this burgeoning technology, such as functionality, challenges, and obstacles in implementing it, and what it means, overall, to the future of the HVAC industry at large.


“The current, most widely used definition of dual-fuel or hybrid systems being used in our industry directly relates to the specific application of a combination of an electric powered heat pump used in conjunction with natural gas, propane, or oil-fired forced air furnaces,” said Bryan K. Rocky, product manager for indoor products, North America, Johnson Controls Building Efficiency Group.

“Dual-fuel or hybrid heat systems rely on the same concept that makes hybrid cars so efficient,” Rocky said. “Dual-fuel technology uses the combination of two different energy sources to provide comfort where each source has its proven benefits.

“There are certain situations being explored that expand this definition (for instance, to include solar power, or for geothermal systems), but the majority of homeowners, contractors, and equipment manufacturers still generally talk about heat pumps with gas furnaces as dual-fuel applications.”

“Having a backup fuel source is important to maintaining daily operations in many types of buildings, such as health care facilities, schools, federal buildings, military operations, etc.,” said Erin Sperry, commercial heating product manager, Fulton.

“If one fuel is not accessible due to natural disaster, or other uncontrolled situations, there is an alternate fuel readily available. It is also possible to manage fuel costs and operate equipment based on whichever fuel is most economical. Many utilities offer discounts or other incentives for interruptible fuel supply programs.”

“Dual-fuel configurations can be very efficient, balancing the efficient use of electricity on mild winter days with efficient use of fossil fuel on more severe cold winter days,” said Bill Cunningham, product manager - cooling, for Lennox.

“The ability of a dual-fuel system to use energy efficiently, while maintaining home comfort, helps to reduce the overall consumption of energy in the home.”

The ability of a dual-fuel system to use energy efficiently, while maintaining home comfort, helps to reduce the overall consumption of energy in the home. Dual-fuel configurations can be exceptionally efficient, balancing the efficient use of electricity with the efficient use of fossil fuel. A properly sized and installed dual-fuel system can provide efficient heating comfort in almost all markets. (Courtesy of Lennox International Inc.)


The performance of any mechanical equipment is contingent and dependent on the design of the system, Sperry said. “Fulton’s condensing boilers are designed with extremely robust heat exchangers, with high mass and high water volume leading to operation and less probability of issues as compared to lightweight, low-volume designs,” Sperry said. “Any interruption in mechanical system function will not have detrimental effects on Fulton equipment. At Fulton, we also have our own product line of boiler room sequencing control systems, where we can design automatic fuel changeover logic as well as other customized programming solutions.”

“A properly sized and installed dual-fuel system can provide efficient heating comfort in almost all markets,” said Cunningham. “Markets with very low heating hours or no access to fossil fuel (natural gas, propane, or oil) may find dual fuel to be a less desirable solution. Factors such as electricity rates relative to gas rates play an important role in balancing the two heating sources.”

“Utilizing today’s energy-efficient heat pumps with the extremely efficient furnaces ensures that a homeowner is efficiently heating their home.”

“The terms dual-fuel or hybrid system usually apply to heating applications where the heat pump is used to provide heat to the home through a specified outdoor temperature range, below which the gas furnace is activated to provide heat to the home,” Rocky said.

“The main benefit to a dual-fuel application comes from two areas - increased comfort and the potential for lower energy costs due to switching from one energy source to another based on weather conditions (outdoor temperatures). Increased comfort comes from the ability to automatically provide that stepped-up heating operation and supply air from an efficient gas furnace when those outdoor temperatures occur in extreme cold weather.

“Energy savings is provided by utilizing the cost-efficient heat pump technology to warm the home during mild winter conditions,” Rocky said.


According to Sperry, important considerations to be aware of when working with dual-fuel products include:

• Awareness of local codes and laws that may affect installation and operational parameters. “Double check to make sure the installation of the backup fuel selected is legal,” Sperry said. “Are there laws that apply to one fuel and not another?”

• How does the building operator or operation system plan to manage the switch of one fuel to another? “Are the appropriate safety devices and control mechanisms installed to make this happen flawlessly?”

• Make sure the equipment manufacturer thoroughly tests each piece of equipment before it leaves the production factory. “Use factory supplied test fire or setup reports and guidelines to assist with the commissioning of the equipment.”

As with any residential or commercial system, the benefits of the system always can be influenced by the building or mechanical system design, Rocky said. Proper ductwork and airflow distribution throughout the structure are always important for a system to operate correctly.

“Control systems, whether standard thermostats or building energy management systems, will be critical to making sure the equipment functions correctly, reliably, and efficiently,” Rocky said. “So, there are definitely factors that must be taken into account from the building-mechanical system design to make dual-fuel or hybrid systems work.”

According to Rocky, the proper installation of a dual-fuel or hybrid system requires that the installer do a load calculation to determine the balance point, the point at which the heat pump can no longer deliver enough heat to maintain comfort in the home. Therefore, a dual-fuel or hybrid system that is properly installed is actually matched to the particular structure to deliver optimal comfort to the occupants.

In regard to challenges associated with implementing and working with dual-fuel technology, Rocky said that customer expectations for a dual-fuel or hybrid system may be much higher than for a standard system, partially as a result of the sales presentation made to the homeowner or research that the homeowner may have done online or through their utility. In some cases, Rocky said, the expectations may exceed what the equipment is capable of doing, or how it may be set up.

“One example of a challenge is the area of explaining to a homeowner the difference between the balance point setting and the low temperature cut out point,” Rocky said. “The balance point is typically defined as the outdoor temperature, which the heat pump will operate by itself, usually down to something like 35 or 40°F. The low temperature cutoff is the outdoor temperature in which the control will not let the heat pump run at all - it is the temperature where the control will let only the furnace run to provide heat. The gap between the two is the situation where either the heat pump or the furnace may operate.”

According to Rocky, many homeowners may think that this gap allows both the furnace and the heat pump to run concurrently, or both provide heat to the home. However, this really is just that area where the control system will allow some variation on where does the heat pump stop working and when does the furnace start providing heat.

“Most control systems will be more complicated for a dual-fuel or hybrid system than a standard single source system,” Rocky said. “The make and feature sets of the thermostats used can impact how the units are wired together, and what features are available to the installers as well as to the homeowners.”

“Depending upon the furnace design, concurrent heating operation of the heat pump and the furnace may actually happen, but most systems are not designed for the furnace to provide that supplemental heat while the heat pump is running.”

Installers and service providers must understand how the indoor and the outdoor equipment communicate, operate, and work together, Rocky said. Each system has its own controls, operating method, setup requirements, troubleshooting criteria, and potential problems.

“The installer and servicer must understand both products to provide the desired performance to the homeowner. Improper installation and setup, particularly setting the balance point, may result in the homeowner not getting the maximum benefit from the system, either from comfort or from energy savings.”

“Finally, an obvious limitation that can influence this application is that fossil fuel (natural gas or propane) must be available so not everyone has the opportunity to consider a hybrid system,” Rocky said. “There can be a large difference in the operating cost of the furnace, depending upon which fuel is available.”

According to Rocky, rising fuel costs for natural gas and propane, along with relatively stable electricity prices, started a jump in the demand for heat pumps in traditional gas furnace markets. And, that trend has continued through to 2009.

“Using high-efficiency heat pumps along with high-efficiency gas furnaces provides the best of both worlds to the homeowner,” Rocky said. “Typically the secondary, or backup fuel, has some limitations associated with it,” Sperry said. “Propane requires outdoor storage, and typically large cities, for example New York City, have laws preventing the use of propane.

“Heavy oils, such as No. 6 fuel oil, require storage and must be kept at a minimum temperature to keep the viscosity at an appropriate level so that the fluid can be adequately pumped, as necessary.

“It will be important for the operational and storage requirements for any fuel to be carefully evaluated before equipment installation.”

Aside from the obvious requirement of fossil fuel in the home, most dual-fuel systems can be installed as simply as any other ducted configuration, Cunningham said. “Considerations will need to be taken in the selection of the indoor thermostat and system setup to ensure the proper balance between efficient heat pump and gas operation are maintained.”

Cunningham also pointed out that dual-fuel systems are not easily installed in homes that don’t already have fossil fuel access, as this is a requirement of the dual-fuel system.

“In some applications, adding fossil fuel may be required and could limit the installations,” Cunningham said.

Fulton, a global manufacturer of steam, hydronic, and thermal fluid heat transfer products and provider of energy management solutions, has expanded the Vantage hydronic boiler product line with the addition of a 4 million Btuh model (shown). (Courtesy of Fulton.)


The most important developments in the dual-fuel sector of the commercial HVAC market are coming for renewable fuels and other alternate energy sources, Sperry said. According to Sperry, sometimes the dual-fuel capabilities can come from two types of equipment, for example a hybrid boiler system consisting of a condensing natural gas boiler as one fuel source and a bio-mass (wood fired) boiler as a second fuel source. “Fulton has the ability to design custom heating solutions, including the controls, for bio mass boilers, geothermal and air source gas absorption heat pumps, and bio diesels.”

Fulton offers a Vantage boiler configured for dual fuel with natural gas and B-100 bio-diesel. Utilizing bio-diesel, or burning other waste products, gives customers an opportunity to explore alternate energy sources instead of petroleum-based fuels or other natural resources.

“Dual fuel contributes not only to the ability of a facility to be able to operate even if its primary fuel supply suddenly becomes unavailable, but also the ability of a building owner to evaluate what fuel(s) they are using, and their respective costs. When you consider this with the ability to incorporate new heating technologies and advanced control systems, new, advanced solutions become feasible and appropriate for many applications,” Sperry said.

According to Rocky, York’s Affinity Hybrid Comfort System combines the cost savings of the company’s Affinity Series heat pumps with the power and performance of the Affinity Series of modulating gas furnaces.

“With the performance of the heat pump system ranging from 15 SEER to 18 SEER levels, and the highest rated gas furnaces ever designed and built in the range of 97 to 98 percent AFUE, the York Affinity Hybrid System offers the homeowner the best of both worlds,” Rocky said.


With utility costs fluctuating, contractors should educate themselves on the savings a dual-fuel system could provide their customers, Cunningham said.

“Dual-fuel installations for commercial heating systems in North America today typically are natural gas/propane or natural gas/No. 2 oil,” Sperry said. “Safety awareness - not only of the operation of these fuels, but also the storage, transportation, and monitoring of their usage - contributes to an effective and efficient system. Advanced systems with alternate fuels are an exciting trend in the HVAC industry, so awareness of these options will be very important to being able to manage custom HVAC system designs.”

“The knowledge and experience of hybrid cars and their technology is a good example that can carry over to the dual-fuel heat pump application,” Rocky said. “Most people have a basic understanding of how hybrid cars work, even if they do not know the exact mechanisms.”

“Evolving control systems such as the controls in a furnace or heat pump, or the thermostat on the wall, are becoming more complex with new features and options every day. This goes along with an increased awareness, capability, and technical knowledge of the typical homeowner. The impact of cell phones, PDAs, Internet-connected appliances, LCD/plasma TVs, and many more examples show that most people are becoming more familiar and desire more control over the things that impact their daily life.

“I think people not only want, but expect, enhanced technology in their heating and cooling systems going forward. It still has to be easy-to-use, provide real benefits, and meet their expectations,” Rocky said.

“Change continues to be the driver for everything we do.”

Publication date:11/09/2009