Air Source Heat Pumps Explained

Figure 1: LG heat pump outdoor unit.

Air source heat pumps (ASHPs) have emerged as a popular and eco-friendly alternative for heating and cooling homes. They harness the ambient air from the environment and convert it into usable energy, making them an efficient and sustainable heating solution.

In this article, we will explore what an air source heat pump is, how it works, and its benefits, etc.

1. What is an Air Source Heat Pump?

An air source heat pump is a heating and cooling system that transfers thermal energy between indoor and outdoor environments. It extracts heat from the outside air during colder months and transfers it inside, while in warmer months, it reverses the process to provide cooling.

An air source heat pump typically consists of two main units: an outdoor unit and an indoor unit.
The outdoor unit contains a fan, a heat exchanger, a compressor, etc. The fan draws air across the heat exchanger. The indoor unit includes a heat exchanger and a fan.

Figure 2: Components of the heat pump outdoor unit.

2. How Does an Air Source Heat Pump Work?

To comprehend the working of an air source heat pump, let's break it down into three main stages:

1. Heat Absorption:
The outdoor unit of an ASHP contains an evaporator coil filled with refrigerant. When the pump is turned on, the refrigerant absorbs heat from the surrounding air, even when the temperature is as low as -15°C (5°F). This absorbed heat causes the refrigerant to vaporize.

2. Heat Transfer:
The vaporized refrigerant is then compressed by the compressor, which increases its temperature significantly. The hot refrigerant gas is passed through a condenser coil inside the indoor unit, where the heat is transferred to the home's heating system or domestic hot water supply.

3. Heat Distribution:
The now-cooled refrigerant condenses back into a liquid state after releasing its heat energy. The liquid refrigerant flows back to the outdoor unit, where it expands again, repeating the cycle. The heat produced is distributed throughout the house via radiators, underfloor heating, or forced air systems.

During the cooling season, the above process is reversed. The heat pump extracts heat from the indoor air and transfers it to the outdoor air, providing cooling. The heat pump achieves this by reversing the refrigerant flow direction using a reversing valve, changing the function of the indoor and outdoor units.

Figure 3: The operation principle of an air source heat pump.

3. Types of Air Source Heat Pumps

As mentioned earlier, there are two main types of air source heat pumps: air-to-water heat pumps and air-to-air heat pumps.

3.1 Air-to-Water Heat Pumps

An air-to-water heat pump extracts heat from the outside air and transfers it to a water-based heating system, such as underfloor heating or radiators. During the cooling season, it works in reverse to extract heat from the indoor water and transfer it to the outdoor air.

3.2 Air-to-Air Heat Pumps

An air-to-air heat pump extracts heat from the outside air and transfers it to the air inside a building, providing heating through a ducted air distribution system. During the cooling season, it works in reverse to extract heat from the indoor air and transfer it to the outdoor air.

In addition to these two main types, there are other variations of air source heat pumps, such as hybrid heat pumps.
Hybrid heat pumps combine an air source heat pump with another heating system, such as a gas boiler, to provide heating and hot water. This can be a good option for homeowners who want to benefit from the energy efficiency of an air source heat pump but also want the reliability of a backup heating system.

Figure 4: Air to water heat pump vs air to air heat pump.

4. Benefits of Air Source Heat Pumps

Air source heat pumps offer several advantages that make them an attractive choice for homeowners:

1. Energy Efficiency:
ASHPs are highly energy-efficient and can provide up to 4 units of heat for every 1 unit of electricity used. This means that they can save up to 50% on energy bills compared to traditional heating systems..

2. Lower Operating Costs:
By utilizing renewable energy from the air, ASHPs reduce reliance on fossil fuels, resulting in lower heating bills. Although the initial installation cost may be higher, the long-term savings and reduced carbon footprint make them financially viable.

3. Versatility:
Aside from their primary function of heating, air source heat pumps also provide cooling capabilities during summer months. This dual functionality makes them a year-round solution for maintaining comfortable indoor temperatures.

4. Environmentally Friendly:
ASHPs contribute to reducing greenhouse gas emissions by operating on electricity rather than burning fossil fuels directly. They help combat climate change and align with sustainable living practices.

5. Government Incentives:
Some country’s governments offer financial incentives for homeowners and businesses to install air source heat pumps, including the Renewable Heat Incentive (RHI) and the Green Homes Grant.

Figure 5: Air source heat pump application.

5. FAQs

5.1 What to Consider before Buying a Heat Pump?

Before buying a heat pump, there are several factors to consider, including:

1. Size: The size of the heat pump should be appropriate for the size of the space you want to heat or cool. An oversized heat pump will be inefficient, while an undersized heat pump will struggle to keep up with demand.

2. Efficiency: Look for a heat pump with a high efficiency rating, as this will save you money on energy bills in the long run.

3. Climate: Consider the climate in your area, as extreme temperatures can affect the efficiency of the heat pump.

4. Cost: The initial cost of the heat pump and installation should be considered, as well as any ongoing maintenance costs.

5. Compatibility: Check that the heat pump is compatible with your existing heating or cooling system.

6. Noise: Consider the noise level of the heat pump, especially if it will be located close to living areas or neighbours.

7. Government incentives: Check if there are any government incentives available for purchasing a heat pump, such as tax credits or rebates.

By considering these factors, you can choose a heat pump that is the right size, efficient, compatible, and cost-effective for your needs.

5.2 Is an Air Source Heat Pump Right for Me?

Whether an air source heat pump is right for you depends on your specific circumstances, such as your heating and cooling needs, budget, and location.

If you live in an area with mild to moderate temperatures and have a well-insulated home, an air source heat pump may be a good option for you, as it can provide efficient heating and cooling.

However, if you live in an area with extreme temperatures, such as very cold winters, or have an older, poorly insulated home, an air source heat pump may not be sufficient to meet your heating needs.

It's also important to consider the initial cost and ongoing maintenance costs of an air source heat pump, as well as any available government incentives. Consulting with a professional HVAC contractor can help you determine if an air source heat pump is right for your specific situation.

5.3 How Much Does an Air Source Heat Pump Cost?

The cost of an air source heat pump can vary depending on several factors, such as the size and capacity of the unit, the brand and model, and the cost of installation.

In general, the cost of a basic air source heat pump unit can range from $3,000 to $8,000 or more, not including installation. The cost of installation can vary depending on the complexity of the installation, the location of the unit, and any modifications that need to be made to the existing heating or cooling system.

Consulting with a professional HVAC contractor can provide a more accurate estimate of the cost for your specific situation.

Figure 6: Air source heat pumps of various sizes.

5.4 Why Do Air-Source Heat Pumps Usually Require Auxiliary Heat?

Air-source heat pumps usually require auxiliary heat because their efficiency decreases as the temperature outside drops. As the temperature drops, the heat pump has to work harder to extract heat from the outside air, which can reduce its efficiency and output.

Most air source heat pumps are designed to operate efficiently in temperatures above 32°F (0°C). As the temperature drops below this point, the efficiency of the heat pump decreases. At very low temperatures, such as below 5°F (-15°C), the heat pump may not be able to provide enough heat to keep the home warm, and auxiliary heating may be needed, such as electric resistance heating or a gas furnace.

However, it's important to note that newer, more advanced air source heat pumps are being developed that can operate efficiently in colder temperatures, providing heat even in extreme weather conditions.

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