ASHP Performance in a Cold Climate

1. Introduction

This case study evaluates the performance of a cold-climate air source heat pump (ASHP) in Winnipeg, Manitoba. ASHPs offer a low-carbon alternative to traditional heating systems, but their effectiveness in extreme cold remains a key concern. This study examines seasonal efficiency, auxiliary heating needs, and defrost cycles of a Mitsubishi ASHP system over a full winter season.
 

2. Building and System Overview

The subject is a two-storey, single-detached home in Winnipeg, originally built in 1905. The total conditioned floor area is approximately 130.8 square meters (1,408 square feet), including a conditioned basement and crawl space. The home features light wood-frame construction, oriented east, with major energy retrofits including upgraded insulation and triple-glazed windows.

Duxton triple-glazed, argon-filled windows cover 10.4% of above-grade wall area. Air tightness was improved to 0.68 ACH at 50 Pa. These retrofits significantly reduced heating demand, making the home ideal for ASHP performance evaluation.

2.1 Heating and Cooling System

Heating and cooling are provided by a Mitsubishi ASHP system: PUZ-HA24NHA1 outdoor unit with a PVA-A24AA7 air handler. Heating capacity is 7.62 kW (26,000 BTU/h), and cooling capacity is 7.03 kW (24,000 BTU/h). The air handler delivers airflow from 1,041 to 1,486 m³/h (613 to 875 CFM). Backup heat is provided by a two-stage electric element: 5.0 kW (Stage 1) and 10.0 kW (Stage 2). 
 

3. System Control Strategy

The Mitsubishi ASHP system operates under a sophisticated control strategy to coordinate the air handler fan speed, compressor modulation, auxiliary heating engagement, and defrost cycles based on real-time indoor and outdoor conditions.

Auxiliary Heat Activation:

• The system automatically enables auxiliary electric resistance heat when indoor temperatures drop 1.5°C below the thermostat set point. This prevents discomfort during extreme cold or when the ASHP cannot keep up with heating demands.
• A programmable time delay ensures the auxiliary heat does not activate prematurely, reducing unnecessary energy use.
• Once engaged, the fan speed ramps to its maximum setting to provide rapid heat delivery throughout the conditioned space.

Fan Speed Control:

• The fan speed automatically modulates in response to the compressor's operating status, balancing energy efficiency with occupant comfort.
• During auxiliary heat operation or increased heating demand, the system prioritizes higher fan speeds to distribute heat quickly and uniformly.

Defrost Cycle Operation:
In cold, moist conditions, frost can accumulate on the outdoor heat exchanger coil, reducing system efficiency and performance. The ASHP employs an automated defrost cycle to periodically clear frost and maintain optimal operation.

• During defrost, the outdoor fan shuts off, and the system reverses the refrigeration cycle, shifting into cooling mode to heat the outdoor coil and melt accumulated frost.
• The compressor continues running while auxiliary electric heat is activated to prevent indoor temperature drop. This results in a brief decrease in supply air temperature, although the auxiliary heat partially offsets this effect.
• Once the coil is clear, the system switches back to heating mode, the fan resumes operation, and auxiliary heat disengages.
• Typical defrost events last 2–3 minutes but can vary based on outdoor conditions and system demand.
   

4. Data Collection & Methodology

The ASHP system was monitored over a four-month winter period, from November 1^st, 2024 to March 1^st, 2025. The Vigilant^® monitoring system by GeoFease collected data at 30-second intervals to capture dynamic system behavior and performance trends. Monitoring covered all major components and environmental conditions to provide a comprehensive understanding of system operation.

Collected data points included:

• Outdoor air temperature
• Supply and return air temperatures
• Suction and liquid refrigerant line temperatures
• High and low refrigerant pressures
• Utility voltage
• Indoor and outdoor unit amperages
• Auxiliary electric element amperage

To calculate the coefficient of performance (COP) of the ASHP, the heat output was divided by the electrical input:

• Electrical Input: The combined real power draw of the indoor unit, outdoor unit, and auxiliary heating elements was measured.
• Heat Output: Calculated using airflow (CFM) multiplied by the temperature rise across the air handler and a standard heat transfer factor of 1.08 for air.
  CFM estimates were inferred from indoor unit amperage:
  • ≤ 0.2 A → 1,041 m³/h (613 CFM)
  • 0.2 – 0.9 A → 1,264 m³/h (744 CFM)
  •  > 0.9 A → 1,486 m³/h (875 CFM)

Note: While this approach is based on manufacturer data, future studies should validate airflow using a direct measurement tool such as an anemometer.

5. Winter Performance — Average Measured COP

Over the monitored winter season, the Mitsubishi ASHP system achieved an average heating COP of 1.83. This value represents the ratio of heat energy delivered to the home relative to the electrical energy consumed by the system, including the outdoor compressor, indoor air handler, and auxiliary electric resistance elements.

The system consistently provided heat in outdoor temperatures as low as -31°C (-25°F) while the COP was highest during milder winter days when outdoor temperatures remained above -10°C (14°F), and lower during colder periods when auxiliary heat supplementation was required.

5.1 Measured vs. Modelled COP Comparison

HOT2000 was used to model the home’s post-retrofit energy performance. Inputs included building geometry, insulation specifications, airtightness test results, window performance, and mechanical system details. The model simulated annual space heating demand under Winnipeg’s typical weather conditions.

HOT2000 calculated a seasonal heating COP of 1.55 while the measured seasonal COP of 1.83 indicates that the ASHP performed better in field conditions than predicted by the model. The monthly COP modelled vs. measured comparison is detailed in the table below.

6. Auxiliary Heat Usage

The auxiliary electric heat elements supplemented the ASHP operation during extreme cold events. Over the monitored period, auxiliary heat was engaged for a total of 1,272 minutes (21 hours). Stage 1 (5.0 kW) operated for 677 minutes (11 hours), while Stage 2 (10.0 kW) operated for 585 minutes (9.8 hours).

Auxiliary heating was primarily triggered when outdoor temperatures dropped below -18°C (0°F). The use of auxiliary heat significantly reduced the system COP during these periods, often dropping below 1.0 due to the high-power consumption of resistance heating in conjunction with continued operation of the fan and compressor.

7. Defrost Cycle Analysis

Defrost cycles are a critical function for maintaining ASHP operation in cold climates. Ice accumulation on the outdoor coil reduces heat exchange efficiency and can lead to system malfunctions if not properly managed.

Defrost events were detected by the following conditions:

• Outdoor fan shut off resulting in the outdoor unit amperage dropping below 800 W.
• Activation of auxiliary heat to maintain indoor temperature.
• A temporary drop in supply air temperature at the air handler.

Under normal operating conditions, defrost cycles lasted 2 to 3 minutes and occurred periodically throughout the cold season, depending on outdoor humidity and temperature. An extended defrost event of approximately 4 hours was recorded on January 20^th, 2025, suggesting significant frost accumulation from the cold outdoor air temperatures of -31°C (-25°F).

During defrost cycles, system COPs ranged between 0.2 and 0.6, reflecting the combined energy demands of reversing the refrigeration cycle and activating auxiliary heating. An example of the defrost cycle affecting the performance data can be seen below.

8. Key Takeaways

• Over the monitored winter season, the Mitsubishi ASHP system achieved an average heating COP of 1.83.
• Over the monitored period, auxiliary heat was engaged for a total of 1,272 minutes (21 hours). 
• During defrost cycles, system COPs ranged between 0.2 and 0.6.