An air-source heat pump moves heat energy from the ambient outdoor air into a building. The Energy savings happen due to the moving of heat energy (pumping), rather than generating the heat, such as by Electric resistance or burning a fossil fuel. It is the same technology found in your refrigerator. Air-source heat pumps can either heat or cool a building by reversing the flow of refrigerant. In one way, it transfers heat from outside to inside (heating mode), and in the other direction, it transfers heat from inside to outside (cooling mode). It seems counterintuitive that you can generate heat from temperatures as low as negative 25° C (STEP, 2019). The important physics principle to consider is that even at negative 25° C, there is heat energy left before you get to absolute zero. The system just becomes less efficient, as you need to move more heat energy, which requires more electricity to pump. Many cold-climate heat pumps utilize some form of electric resistance backup heat for those very cold days where the heat pump would not be able to transfer enough heat energy to counter the heat loss.
It's important to understand the ratings used to describe air-source heat pump efficiency and suitability for use in cold climates. Cooling efficiency is represented by the seasonal energy efficiency ratio (SEER). This is the same rating provided to air conditioners. SEER ratings typically range from a minimum of 10 to a maximum of 17. Heating performance is represented by the heating seasonal performance factor (HSPF). The factors typically range from 5.9 to a maximum of 8.6 (NRCAN, 2019). Another rating to consider for air-source heat pumps is the coefficient of performance (COP). This is a number expressed as the number of units of heat energy moved for each unit of energy input. Heat pumps gain their efficiency by moving more energy than what is input into the system. Heat pumps typically have a COP of 2.0 to 3.3 (NRCAN, 2019). This means for each unit of energy input (electricity), the heat pump moves 2.0 to 3.3 units of energy (heat).
Another consideration is the heat pumps defrost cycle. When the outdoor temperature is at, or below, freezing during the heating cycle, moisture in the air will pass over the outside coil, condense, and then freeze on the coil (NRCAN, 2019). This frost build-up reduces the efficiency, and the frost must be removed. Air-source heat pumps have a defrost mode to pass warm air over the coils to remove the frost. Some air source heat pumps will run a defrost cycle at a set schedule, while others perform the defrost cycle only when required (NRCAN, 2019). Energy is wasted when defrosting. The more efficient models will utilize this on-demand defrost cycle.
TYPES OF AIR-SOURCE HEAT PUMPS
split systems, or whole-house systems, most closely resemble a traditional air handler/furnace/air conditioning system you would see in a typical home there is an outdoor fan/coil unit (which closely resembles an outdoor coil from a traditional air conditioning system) matched with an indoor heat pump, which replaces the air handler. The air handler/heat pump combo distributes air through a traditional duct system in the building. The volume of air required is typically higher with a heat pump, so existing duct sizes may not be adequate for retrofit installations.
Mini Split Systems
Mini split systems consist of an outdoor fan/coil unit, packaged with typically up to four indoor fan/coil head units. The most common types are mounted high on a wall. Flush ceiling and wall mounts, floor mounted below-window systems and ceiling four-way systems are also available. These systems can serve one to four zones, depending on the number of heads. They work well when supplementing heat or air conditioning in heating systems with no existing ductwork, or where extending ducts into a zone would be impractical.
COLD-CLIMATE AIR-SOURCE HEAT PUMPS
Air-source heat pump suitable for cold climates have features which enhance the efficiency of the heat pump, specifically in cold climates. Inverter drives, or variable frequency drives (VFD), control the speed of an AC motor by controlling the frequency of electrical power supply to the motor (Ecologix.ca, 2019). This allows the compressor to run at the exact speed required to move the required amount of refrigerant, making the system more efficient, allowing the heat pump to operate at a range of capacities, often from 50% to 110% of full load conditions (Ecologix.ca, 2019). Liquid injection allows the heat pump to operate at lower source temperatures, a key component of the cold-climate air-source heat pump. A small amount of liquid refrigerant leaving the condenser bypasses the expansion valve and outdoor coil (evaporator) and is injected into the low- pressure side of the compressor (Ecologix.ca, 2019). Research has shown the COP is enhanced by 17% and the heating capacity by 25% at -15° C (Jang, Lee, Chin, & Ha, 2010).
APPLICATION TO PART 9 OF THE ONTARIO BUILDING CODE
The Ontario building code, supplementary standard SB-12, 220.127.116.11.(17), requires the following: " except as provided in sentence (18), a building is permitted to be designed and conformance with any of the compliance packages available for the climate zone that the building is located in, if the primary space heating of the building is supplied by (a) a wood burning appliance, (b) on earth energy system, or (c) an air or water source heat pump that does not use electric resistance as a back-up heat source". Further to that, sentence (18) provides the following: "for the purpose of sentence (17), the requirements in the compliance packages for space heating equipment do not apply".
These sentences provide the practitioner the option of utilizing any of the compliance packages for the climate zone that they are in, without needing to consider the efficiency of the space heating appliance. An important item to determine for compliance with these sentences is if the heat pump has electric resistance backup heating. If this is the case, the only option would be compliance with the applicable tables for electric space heating. The definition of electric space heating refers to "air-source heat pumps in combination with electric resistance backup heating... That provides more than 10% of the heating capacity provided for a building. There is an applicable package for air-source heat pumps that have electric resistance backup heating. For Zone 1, it is table 18.104.22.168.C. package C4, and for zone 2, it is table 22.214.171.124.C. package C2.
CASE STUDY ANALYSIS
The Toronto and Region Conservation Authority, through the Sustainable Technologies Evaluation Program, conducted a research project on using air-source heat pumps with multiple heads during 2017 and 2019 in an Ontario rowhouse complex. The units were heated previously with electric baseboard heating, and cooling was provided by window units. Heat pumps were used to heat and cool the rowhouse units for much of the monitoring. Starting in November 2017. Baseline data was obtained by switching off the heat pumps for several weeks and reverting to baseboard heating. Tenant interviews captured the tenants experience with the technology and insured a fair comparison. There were six units in the rowhouse block. Units 1 to 4 received the retrofit and units 5 and 6 did not, serving as a control for the experiment.
Electricity consumption during heating mode for units 3 and 4 was reduced by 32% and 19% respectively. Consumption during cooling mode was compared across units 1 and 6. The heat pump was estimated to save 1,148 kilowatt hours per year in cooling mode when compared to window air conditioners. If the heat pump provided cooling where there was none before, the additional consumption was estimated at $316 KW hours per year, and this increases relatively small in comparison to the heating Energy savings.
Average annual savings for the units were estimated at $868, including both heating and cooling mode operation. Average annual heating mode savings for estimated at $624, and cooling mode savings were estimated at $244. There were other tangible benefits, including increase in property values, marketability of the units, tenant retention and overall satisfaction, as well as improved health, safety, and occupant comfort.
Tenants appreciated the Energy savings, the simplicity of the retrofit process, the user-friendliness of the controls and increased thermal comfort over electric baseboards. The case study concluded that air-source heat pumps were a reliable, efficient, and comfortable source of heating and cooling, and we're highly appreciated by the tenants. However, the installed costs are high compared to maintaining the existing system, making a business case more challenging. A full assessment would help identify cost savings, non-energy savings benefits and any applicable incentives (TRCA,2018).
Justin Massacre is the former director of marketing with RSM building Consultants. He is the co-owner of Ontario Tiny Homes, a service providing stock and custom plans for tiny homes across Ontario. This service is hosted by the parent company, MW Drafting & Design, an architectural design firm from Simcoe, Ontario.