Figure 1: Heat pump outdoor unit in winter.
In winter, the need for an efficient and reliable heating system is paramount. Among the various options available, air source heat pumps have gained popularity due to their energy efficiency and versatility.
In this article, we will explore how air source heat pumps work during the winter months, highlighting their benefits, components, and operational principles.
An air source heat pump (ASHP) is a heating and cooling system that extracts heat from the outdoor air to warm indoor spaces during winter. It operates on the principle of thermodynamics, utilizing a refrigeration cycle to transfer heat energy.
1. Outdoor unit: The outdoor unit consists of a compressor, a fan, and a heat exchanger. The compressor pressurizes the refrigerant, while the fan draws in outdoor air. The heat exchanger transfers heat from the air to the refrigerant.
2. Indoor unit: The indoor unit includes a heat exchanger and a fan. The heat exchanger releases the absorbed heat into the indoor space, while the fan circulates the warm air.
3. Refrigerant: The refrigerant is a special fluid that absorbs and releases heat as it changes state between liquid and gas. It plays a crucial role in transferring heat from the outdoor air to the indoor space.
Figure 2: The structure of a heat pump outdoor unit.
During the winter, an air source heat pump works by extracting heat from the outside air and transferring it to the inside of the building. Its working process can be divided into the following 3 stages.
A. Absorption of outdoor heat
When the ASHP is in heating mode, the outdoor unit absorbs heat from the ambient air, even at low temperatures. This heat extraction process relies on the refrigerant's ability to absorb heat energy.
B. Transformation of low-grade heat into usable heat
The absorbed heat is transferred to the refrigerant inside the outdoor unit's heat exchanger. And then the refrigerant is compressed by the compressor, changing from a low-pressure gas to a high-pressure gas.
C. Heat distribution through the indoor unit
The high-pressure gas is then transported to the indoor unit, where it passes through another heat exchanger. Here, the heat is released into the indoor space, raising its temperature. The fan inside the indoor unit helps distribute the warm air throughout the room.
Figure 3: Working principle diagram of air source heat pump.
The heat pump also has a defrost cycle, which is activated when the outdoor unit's coils become covered in frost or ice. The defrost cycle temporarily reverses the refrigerant flow to remove the frost or ice buildup, ensuring that the heat pump continues to operate efficiently.
In some areas, a heat pump usually has an auxiliary heating system, a backup heating source to provide additional heating in cold climates. The auxiliary heating system consists of electric heating elements or a gas furnace that automatically activates when the heat pump can't meet the heating demand.
An auxiliary heating system increases overall efficiency by allowing the heat pump to operate at its most efficient level while still providing adequate heating. However, using an auxiliary heating system increases energy costs.
A. Outdoor temperature
Air source heat pumps can effectively extract heat from outdoor air even in temperatures as low as -15°C (5°F). However, their efficiency decreases as the outdoor temperature drops. Supplemental heating systems may be required during extremely cold weather conditions.
B. Insulation and building efficiency
The overall efficiency of an ASHP system depends on the insulation and energy efficiency of the building. Well-insulated homes with minimal air leakage retain heat better, allowing the ASHP to operate more efficiently.
C. System size and capacity
Properly sizing an ASHP system is crucial for optimal performance. Undersized units may struggle to meet heating demands, while oversized units can lead to short cycling, reducing efficiency. Consulting a professional installer is recommended to determine the appropriate size for your home.
Figure 4: Heat pump installation.
Air source heat pumps can still extract heat from outdoor air even in low temperatures, but their efficiency decreases as the temperature drops. Most air source heat pumps can operate efficiently down to temperatures around -10°C to -15°C (14°F to 5°F), but their heating capacity decreases as the temperature drops.
Some advanced heat pump models are designed to operate in even lower temperatures, down to -25°C (-13°F) or lower, but their performance may still be limited. In very cold temperatures, the heat pump may need to use an auxiliary heating source to meet the heating demand of the building.
A. Energy efficiency and cost savings
ASHPs are renowned for their high energy efficiency. They can produce up to three or four units of heat for every unit of electricity consumed, resulting in significant cost savings on heating bills compared to traditional heating systems.
B. Environmental friendliness
By utilizing renewable energy from the outdoor air, ASHPs significantly reduce greenhouse gas emissions compared to fossil fuel-based heating systems. They contribute to a greener environment and help combat climate change.
C. Low maintenance requirements
ASHPs require minimal maintenance, with routine tasks such as cleaning or replacing filters. Compared to combustion-based heating systems, they have fewer components prone to wear and tear, resulting in reduced maintenance costs.
Figure 5: Two heat pumps in winter.
A. Limited functionality in colder climates
While ASHPs experience decreased efficiency in very cold temperatures, they can still provide effective heating even in freezing conditions. With appropriate sizing, auxiliary heating systems, and proper insulation, ASHPs are a reliable choice for winter heating.
B. Noise pollution
Modern ASHP units are designed with noise reduction features such as sound-dampening technology and variable-speed compressors. When installed correctly, ASHPs produce minimal noise, ensuring a peaceful indoor environment.
C. Initial installation costs
Although the upfront cost of installing an ASHP system may be higher than traditional heating systems, the long-term savings on energy bills often outweigh the initial investment. Additionally, various incentives and rebates are available to help offset installation costs.
