Air-Source Heat Pumps

Introduction

An air-source heat pump moves heat energy from the ambient outdoor air into a building. The energy savings happens due to moving the heat energy (pumping) rather than generating that heat energy, 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. One way it transfers heat from outside to inside (heating mode) and other direction transfers heat from inside to outside (cooling mode). It seems counterintuitive that you can generate heat from temperatures as low as -25°C, (STEP, 2019) but the important physics principle to consider is that even at -25°C, there is a lot of 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 wouldn’t be able to transfer enough heat energy to counter the heat loss.

An important aspect of air source heat pumps to understand includes the ratings used to describe their 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 with air source heat pumps is the Coefficient of Performance (COP). This is a number expressed as how many units of heat energy is moved for each unit of energy input. Heat pumps gain their efficiency by moving more energy than what was input into the system. Heat pumps typically have a COP of 2.0-3.3 (NRCAN, 2019). This means for each unit of energy input (electricity) the heat pump moves 2.0-3.3 units of energy (heat).

Another consideration is the heat pump’s 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. ASHP’s have a defrost mode to pass warm air over the coils to remove the frost. Some ASHP’s 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, so the more efficient models will utilize this on demand defrost cycle.

Types of Air-Source Heat Pumps

Split Systems

Split systems, or whole house systems, most closely resemble a traditional air handler/furnace/AC system you’d 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 type 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 extending ducts to that zone would be impractical.

The Cold Climate Air Source Heat Pump (ccASHP)

Air source heat pumps suitable for cold climates have features which enhance the efficiency of the heat pump, especially in cold climates. Inverter drives, or variable frequency drives (VFD), control the speed of an AC motor by controlling the frequency of electrical power supplied 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 and allows the heat pump to operate at a range of capacities from, typically from 50% to 110% 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 coefficient of performance (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, 3.1.1.1.(17), requires the following; Except as provided in sentence (18), a building is permitted to be designed in 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) an 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 located 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 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 per cent of the heating capacity provided for a building “. There is an applicable package for air source heat pumps which have electric resistance backup heating. For Zone 1 it is Table 3.1.1.2.C. package C4, and for Zone 2 it is Table 3.1.1.3.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 the majority of the monitoring period, starting in November 2017. Baseline data was obtained by switching off the heat pumps for a several weeks and reverting back to baseboard heating. Tenant interviews captured the tenant’s experience with the technology and ensured a fair comparison. There are 6 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 1,148kWh/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 kWh/year and this increase is 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 were estimated at $624, and cooling mode at $244. There were other tangible benefits, including; increase in property values, increase in marketability of the units, tenant retention and overall satisfaction and 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 there highly appreciated by the tenants. However, the installed costs are large 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)

References

Ecologix.ca. (2019, July 10). Glossary. Retrieved from Ecologix.ca: http://ecologix.ca/resources/glossary/

Jang, Y., Lee, E., Chin, S., & Ha, S. (2010). Effects of Flash and Vapor Injection on the Air-to-Air Heat Pump System. International Refrigeration and Air Conditioning Conference (p. 1). Purdue University, Purdue e-Pubs.

NRCAN. (2019, July 9). Air-Source Heat Pumps. Retrieved from Natural Resources Canada: https://www.nrcan.gc.ca/energy/publications/efficiency/heating-heat-pump/6831

STEP. (2019, July 9). Sustainable Technologies Evaluation Program. Retrieved from Air Source Heat Pumps: https://sustainabletechnologies.ca/home/heating-and-cooling/air-source-heat-pumps/

TRCA. (2018). Assessment of Multi-Split Ductless Air-Source Heat Pump Retrofits in an Ontario Rowhouse: Heating and Cooling. Vaughan: Toronto and Region Conservation Authority.