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In recent years, there has been a growing trend in the U.S. toward the electrification of homes, which often includes replacing gas furnaces with electric heat pumps. This trend is being driven by a variety of factors, including government mandates to reduce greenhouse gas emissions, as well as advancements in technology that have made heat pumps much more efficient in colder climates.

However, heat pumps operate differently than gas furnaces, and it is not always as simple as replacing one type of equipment with another. The first part of this series will highlight some of the alterations needed for heat pump retrofits, such as modifying ductwork and upgrading electric panels. These modifications can add complexity and expense to the system, so contractors should consider them carefully to ensure that their customers are happy with their new heat pump.

Cold Climates

For homeowners looking to replace their gas furnace with a heat pump, the benefits can be many. For example, if the new heat pump is correctly sized and installed, the homeowner may reduce their carbon footprint, as well as see lower utility bills. That’s because heat pumps, when used efficiently, can reduce heating energy needs by as much as two-thirds, said Mark Reding, ducted systems product manager II at Johnson Controls.

“However, there are many scenarios — especially in colder climates — where it will be more beneficial to install a heat pump with a backup furnace instead of electric resistance backup,” said Reding. “Also known as dual fuel heat pumps, these configurations shift the heating load to the furnace during extremely cold temperatures, which can maximize cost-effectiveness and minimize source emissions.”

Tim Brizendine, director of product management at Lennox, agrees, noting that in colder climates, homeowners should either utilize a dual fuel system or else look for heat pumps designed to provide more heat at colder temperatures.

“Variable-speed heat pumps are going to offer the highest level of comfort and provide the most heat in colder climates,” he said.

The bottom line is that in areas that see wintertime temperatures below freezing, a non-electric backup heat source of some kind is a necessity, said Ben Lipscomb, P.E., director of engineering and utility programs at National Comfort Institute (NCI).

“Cold climate heat pumps can still operate to below 0°F, and some can even provide their full rated capacity down to single-digit temperatures,” said Lipscomb. “But if you throw in the potential for a power outage during a cold snap, the risk is just too high to go 100% electric, unless the homeowner has a large generator or solar with battery storage to see them through a disaster.”

Load Calculations

Of course, homeowners will only receive the expected comfort and energy savings of their new heat pump if it is sized and installed correctly. That is why HVAC contractors need to pay careful attention to load calculations and equipment selection, as well as correct existing duct deficiencies and verify the installed performance of the heat pump system.

RIGHT CHOICE: Contractors should provide homeowners with options to help them choose the right HVAC equipment for their needs. (Courtesy of Lennox)

“Simply stated, you can’t do like-for-like capacity replacements of existing furnaces and expect the heat pumps that are installed to perform well,” said Lipscomb. “The first step is to assess the existing home and duct system, and an ACCA Manual J load calculation should be completed to determine how much heat the home needs. ACCA Manual S should then be used to select equipment that will meet those needs. Heat pump capacities change based on outside air temperature, so the nominal tonnage doesn’t actually tell you if a particular heat pump will heat the home.”

So contractors should not, for example, assume that they can replace a 48,000 Btuh gas furnace with a 4-ton heat pump. That’s because a particular heat pump rated to supply 48,000 Btuh at 47°F might only provide about 25,000 Btuh at a design temperature of 15°F, which is just over half of what is needed to keep the house comfortable, explained Lipscomb.

“Additionally, the 4-ton heat pump requires about 1,600 cfm of airflow to operate properly, whereas the furnace it replaced only required 1,300 cfm,” he said. “The 16-inch duct used for the furnace will not be adequate to deliver 1,600 cfm of air, which will further reduce the performance of that heat pump.”

That’s why when replacing any equipment, it is best practice for contractors to calculate the building load to determine what capacity of equipment is needed and not just assume the existing equipment was sized appropriately, said Brizendine. This is particularly true where furnaces are concerned, as they are often oversized for the building load.

“Once contractors understand the building load, they should select a heat pump and a backup heat source to meet the needs of the building,” he said. “They can also determine if a balance point should be set to switch over from the heat pump to secondary heat. The ductwork should also be reviewed to ensure it is sized properly and provides proper distribution to the house.”

ACCA Manual D can help contractors determine whether the existing ductwork can handle the airflow of the new heat pump without pushing the system total static pressure beyond the capabilities of the equipment, said Lipscomb. Contractors will know the total airflow for the system and for each room after load calculations are complete and equipment is selected.

“NCI also teaches methods for assessing ducts in the field and on the fly,” said Lipscomb. “In most cases, some kind of ductwork modification will be needed in order to achieve peak performance.”

Another point to consider is that in colder climates, right-sizing a heat pump for the heating load will often mean oversizing for the cooling load, said Reding.

“To help reduce compressor cycling during the cooling season, contractors should consider using heat pumps with staged or variable-speed compressors,” he said.

Electrical Upgrades

Besides duct modifications, it may also be necessary to upgrade the electric panel, because heat pumps tend to have a higher minimum circuit ampacity and maximum overcurrent protection than an air conditioner, said Reding. That is why it is important to evaluate a home’s electrical needs to ensure there is proper service, wire, and overcurrent protection for the new equipment being installed, he added.

If the electrical requirements of the selected equipment exceed the panel or service capacity, an alternative to upgrading the electrical panel is to downsize the equipment, said Lipscomb.

“To downsize equipment, adjust design temperatures and make up the difference with non-electric supplemental heating (e.g., furnace or fireplace) when it’s colder than the design temperature. Or preferably, improve the home envelope with better insulation and sealing to reduce the load.”

A home’s electrical service may be adequate in a situation where an outdoor unit is being changed from an air conditioner to a heat pump, said Brizendine. However, if the house did not have air conditioning before, then additional requirements for electrical service will be required.

“In this case, homeowners may be better off keeping their furnace and replacing the air conditioner with a heat pump,” he said. “That way, the system can run the heat pump the majority of the time and the furnace only when absolutely needed.”

It is important for contractors to understand that when replacing a gas furnace with a heat pump, every instance will be unique and require different modifications, said Reding.

“Laying out multiple options for the homeowner, with an accurate picture of the costs and benefits of each, can help contractors take on an ‘advisory’ role for the homeowner, thus building trust and delivering even greater benefits.”

The second part of this article looks at the energy efficiency, comfort, and performance of heat pumps as compared to gas furnaces.