By Francesco Lo Presti
An air-source heat pump (ASHP) takes the available heat from the air and transfers it between the outdoor unit to the indoor unit, or vice versa, depending on the season. During the winter months, the heat pump will take heat from the outside air and use it to heat the building inside. In the summer, this process is reversed, and the heat pump will take the heat from inside the building and release it outside.
As this technology grows in popularity, it is important to understand how best to size and apply this equipment when on the job site. Each home will require its own solution and a “one size fits all” approach just won’t cut it in the field. And when it comes to retrofitting a heat pump into a home, it gets even more complicated. “Some homes may have an existing duct system, and the homeowner may still want to use it. Maybe the homeowner is looking to abandon their ductwork for a zoned solution, or maybe they would like to have a combination of both,” explains Chris Mottinger, Midwest regional sales manager for Fujitsu.
The initial conversation between the contractor and homeowner will help determine the best solution for the home.
Once the heat pump is selected, the next step is determining the load requirements. “This means that a contractor must consider the length, width, height, dimensions, and orientation of the home, along with the insulation and windows. From there, you must complete a manual J calculation,” explains Paul Davis, general manager of Mak Mechanical. “A manual J calculation considers all the above factors to determine the home’s heating and cooling load requirements. This includes the percentage of glass per square footage of a home. If there is a high percentage of glass in a home, then a higher load will be needed, and you can no longer use standard sizing.”
The location of the home is also an important factor when determining the required efficiency of the heat pump. “At a certain point or location, a heat pump will become non-efficient. For example, a heat pump can’t grasp as much heat out of the air when it’s located further north,” explains Davis. The geological location and weather conditions will be added to the heat loss calculations, and “You want to make sure you’re staying within an efficiency rating of around 90 per cent,” shares Alex McIntosh, sales manager of Mak Mechanical.
Proceed with caution
All relevant information needs to be gathered prior to conducting a load calculation. “In the event that information is hard to determine, it is best to proceed with caution and estimate for the worst-case scenario,” explains Mottinger.
This calculation is the essential starting point for any retrofit job because “heat pumps have a lower heating capacity than many older installed furnaces; meaning, it’s important to know the heating capacity truly needed for a home,” says Tom Zinn, a contractor with Affordable Comfort Heating and Cooling. “A manual J calculation can help prevent the drastic oversizing of a unit, which can potentially lead to cooling issues by short cycling or running only at a low speed and having airflow issues on long runs.”
Without a manual J load calculation, there isn’t a proper way to correctly determine that the system will meet the building or home’s requirements. Without it, then the contractor is just guessing, says Mottinger. This can lead to comfort issues, over or under sizing, and reduced system efficiency.
Ductwork
One common challenge for retrofitted heat pumps is ductwork. Should the contractor utilize the existing ducts to install a ducted system or is it best to install a ductless system?
The first step is to talk with the homeowners. “For example, does the home have hot and cold spots? Are they not getting airflow to the upstairs? Is there good ductwork to the main floor? These are part of that initial checklist to determine the best option for the home,” explains McIntosh.
If the existing ductwork is going to be reused, “It is important to inspect the duct system for leakage, especially in an unconditioned space. After it is inspected, a static pressure calculation should be done to ensure the equipment has a fan capable of delivering capacity at determined airflow through the existing ductwork,” explains Mottinger.
A contractor can use a ductulator, an online tool, to calculate and size the air distribution system. “By using a ductulator, a quick check can be done on both the supply and return main trunks to determine the largest equipment that should be run through that ductwork,” explains Zinn. It is also important to remember that “often older systems have small ductwork which can limit equipment sizing. Too small of duct for the equipment can lead to air movement noise, whistling, and poor air distribution throughout the home.”
Future-proof
A contractor should account for future work that might need to be completed down the road when planning out the retrofit. Building materials, windows, and the tightness of the building envelope will all need to be taken into consideration during the planning stage. “You are hoping that the original house was sized correctly. But to play it safe, you will probably have to oversize a unit for about 15 per cent to compensate,” says Davis. “We always recommend doing a proper heat loss calculation, but if a homeowner is looking to save the costs of that, you can include the original equipment size and the ductwork size into the duct size calculation.”
Down the road, the homeowner might want to get some kind of addition to the home which could affect sizing requirements for the heat pump. Access to the ductwork will need to be available in this case to determine if the system can handle the extra square footage. “Then you would do all the calculations based on the equipment, ductwork, sizing, and length of runs to determine whether a second system will be needed,” explains Davis. “Typically, you don’t want to exceed 50 per cent more than the original availability from the original equipment.”
If a home utilizes a multizone system, the contractor can connect indoor capacity greater than what is available from the outdoor unit. The same can be done when connecting indoor unit capacity below what is available from the outdoor unit. “If the future renovation’s load requirement can be figured into the initial selection, you can size and select equipment to be added to the system later if the initial installation and future installation are both allowable combinations by the manufacturer.”
It’s essential to have all this information in the early stages so you can size the equipment for that home. “If a significant upgrade is planned, you may downsize equipment knowing it’ll be the right size once the renovation is completed. This would only be done when the upgrade is a certainty,” says Zinn.