The question of how to size an ASHP will inevitably face us all at some point in our careers. Unfortunately, this is not a one-size-fits-all endeavor. Designers and owners need to maintain a balance between equipment capital cost, carbon emissions targets, and system reliability to do it right for each application.
Here are a few key factors which I evaluate when sizing an ASHP:
Minimum Ambient Temperature drives the temperature at which the ASHP will be evaporating refrigerant. Evaporating temperature is one key part of the “lift” at which the refrigeration system runs. As the evaporating temperature falls with constant condensing temperature (such as in a building heating application), the lift will increase. The lift the ASHP must achieve drives the choice of refrigerant, compressor technology, efficiency and equipment capital cost. Increased lift causes a reduction in overall capacity and coefficient of performance (COP). In short, the choice of minimum ambient temperature needs careful consideration to ensure decarbonization goals are met and job-specific limitations are respected.
The minimum ambient temperature sets the evaporating temperature for an ASHP. The heating supply water temperature sets the condensing temperature for the ASHP. These two temperatures define the lift the refrigeration system must achieve. I always recommend a detailed analysis of the lower limits a building's heating water temperature can support. In new construction, forced convection perimeter heaters permit large first cost and operating savings in ASHP systems. In retrofit applications, attention should be paid to breaking the hot water system up into different temperature loops, so the lowest-temperature water is always used to satisfy a load. These strategies allow meaningful reductions in lift and, therefore, increases in available COP/ Capacity at a given ambient temperature.
The primary goal of installing an ASHP heating system is to reduce carbon emissions. It is essential to recognize that the elimination of carbon consumption may carry a large cost penalty in colder climates like Ontario and Quebec. I recommend that clients define a carbon consumption target for the facility from which ASHP operating hours can be derived. The balance of the hours not captured by the ASHP can be picked up with a secondary heat source such as electric or gas boilers. ASHPs are significantly more energy-efficient than traditional heating systems, and by using electricity (often generated from renewable sources), you can significantly reduce your carbon footprint. For example, an ASHP sized for approximately 35% of the peak heating load can achieve a 65-75% reduction in annualized emissions. Note that grid carbon emissions associated with electricity are extremely variable and location-dependent. 
To address extreme cold spells and ensure consistent heating, I recommend incorporating a backup heating system typically sized for peak heating load. This can be in the form of an electric resistance heater or more commonly a hybrid system that switches between the ASHP and a conventional heating source when needed. This approach ensures that building temperature is maintained during extreme weather while achieving a cost-sensitive reduction in overall carbon emissions.
Electrical infrastructure is commonly a limiting factor in the application of ASHPs for new construction and retrofit applications. In these situations, large annualized reductions in carbon consumption can still be achieved. I typically evaluate the maximum capacity of ASHP at a given ambient that the electrical infrastructure permits. That annualized reduction of carbon consumption can then be compared to the targets and help with decision-making. In cases where the targets aren’t met. I commonly look at supplementing with onsite power generation (Solar/ Wind) or gas-driven ASHPs.
As with most purchases, the implementation of an ASHP system needs to make good financial sense and have a reasonable capital cost. The first part of making the financial case for an ASHP is identifying the minimum capacity required to achieve the annualized carbon reduction target. Then, available financial incentives should be reviewed often so they can fund a larger annualized reduction in carbon emissions. Typically, the available capital drives the size of the ASHP and the associated annualized emissions reductions.
Air Source Heat Pumps are a viable primary heating source in Canada today. Strategic design allows significant reductions in carbon emissions for the right cost. It is crucial to balance the aforementioned considerations to get the right ASHP for your application. Reach out to your HTS contact or fill out the form below, and we would be happy to assist you in identifying an ASHP system for your applications.