AIR SOURCE HEAT PUMP

An air-source heat pump (ASHP) is a heat-pump system which transfers the thermal energy from outside ambient air into a building’s heating and/or cooling system. ASHPs operate using a closed thermodynamic refrigeration cycle. ASHPs do not produce heat through combustion, but instead “upgrade” existing environmental heat for use in space heating, cooling, and in some system configurations, domestic hot water production.

ASHPs utilize the same fundamental physical operating principles as other heat-pump technologies. They are considered a separate category due to the external heat source being ambient air, as opposed to ground or water.

This page explains the basic operational theory and function of air-source heat-pumps; however, this page will not cover installation, system sizing, performance, costs, nor project-specific design.

What is an Air-Source Heat-Pump?

An air-source heat-pump is a thermal energy transfer system which utilizes the existing thermal energy contained in outdoor ambient air at low temperature levels and raises the temperature level for indoor use. An electric power input is necessary to drive the compression process which is needed to move thermal energy from a cold source to a warm heating circuit.

Although ambient air at low temperatures has available thermal energy, the heat-pump captures and transports this thermal energy to the building’s heating distribution system. In addition, the same system can be reversed to remove thermal energy from interior spaces and transport the thermal energy to the exterior for cooling.

Air source heat pump operating in heating and cooling mode using reversible heat transfer

Defining Characteristics of an Air-Source Heat-Pump

An air-source heat-pump is characterized by the following system characteristics:

Use of Ambient Air as the Heat Source

Unlike ground or water based systems, an air-source heat-pump requires no underground collectors or water access infrastructure. Ambient air acts as the environmental heat-reservoir.

Outdoor Heat Exchanger Unit

An air-source heat-pump system consists of an external heat exchanger assembly combined with a fan. The fan assembly provides for continuous airflow across the heat-exchange surface to allow for efficient heat absorption or heat rejection dependent upon operating mode.

Separation of Generation and Distribution

The heat-pump unit generates heat through energy-upgrading. The generated heat is then distributed throughout the building through separate system components such as air-handling units, hydronic circuits or storage tanks. The separation of heat-generation and heat-distribution allows for the heat-pump technology to be integrated into various building system configurations.

Air source heat pump characteristics showing ambient air heat source and separated system components

Operational Theory of an Air-Source Heat-Pump System

From a system standpoint, an air-source heat-pump circulates a refrigerant through a closed-loop thermodynamic refrigeration cycle.

The thermodynamic cycle is explained in detail on the Heat Pump Technology page.

The refrigeration cycle is composed of four major components:

Heat Absorption (Evaporation)

Low pressure liquid refrigerant flows through the outdoor heat exchanger assembly. As ambient air is drawn past the heat exchanger coils, thermal energy is transferred to the refrigerant causing it to vaporize into a gas state.

Compression

As the gaseous refrigerant enters the compressor, electrical energy causes an increase in both pressure and temperature of the refrigerant resulting in an increased energy content.

Heat Release (Condensation)

The high temperature refrigerant transfers thermal energy to the building’s heating system through the indoor heat exchanger assembly. At the same time, the refrigerant condenses back into a liquid state.

Expansion

After passing through the indoor heat exchanger, the refrigerant passes through an expansion device, which decreases the pressure and temperature of the refrigerant so it can begin another cycle.

In this manner, the air-source heat-pump continuously cycles heat from the outside environment to the inside heating system. In addition, when the air-source heat-pump is operating in cooling mode, the flow of refrigerant is reversed and heat is removed from the inside spaces and discharged to the outside environment.

Air source heat pump thermodynamic cycle transferring heat from ambient air to indoor heating system

Configuration Types of Air-Source Heat-Pumps

There are many ways in which an air-source heat-pump can be arranged in terms of configuration type. These configuration types indicate how heat is supplied to the building and NOT different technologies.

Air-to-Air Systems

Air-to-air systems transfer heat directly to indoor air using fan-cooled indoor units or ducted systems providing both space heating and cooling.

Air-to-Water Systems

Air-to-water systems transfer heat into a water-based heating system. The heated water supplies either radiators, under-floor heating, buffer storage tanks, or domestic hot water systems.

Split and Monoblock Configurations

Split systems separate the indoor and outdoor components and connect them with a refrigerant pipe. Monoblock systems have the entire refrigeration circuit enclosed in one unit and heat is exchanged with the building through water connections. Both split and monoblock configurations use the same fundamental thermodynamic operating principle.

Air source heat pump system configurations including air-to-air and air-to-water arrangements

Function of an Air-Source Heat-Pump in a Building Heating System

Within a total building heating system, the air-source heat-pump serves as the thermal energy-generating component. It supplies thermal energy that has been elevated in temperature to downstream system components, which perform the distribution and control functions.

Some examples of these downstream system components include:

  • Air-distribution systems
  • Hydronic heating-circuits
  • Thermal-energy storage tanks
  • Control-and-regulation units

Because the heat-generation function is separated from the heat-distribution function, the same air-source heat-pump technology can be integrated into diverse building system configurations without altering the underlying fundamental operating principle of the technology.

Air source heat pump supplying thermal energy to distribution, storage and control components

Comparison with Other Heat-Pump Technologies

Air-source heat-pumps are differentiated from other types of heat-pumps based on their external heat source:

  • Air-source heat-pumps capture energy from ambient outdoor air
  • Ground-source heat-pumps capture energy from the earth and rock formations
  • Water-source heat-pumps capture energy from groundwater or surface-water

While all heat-pump technologies use the same thermodynamic refrigeration cycle, the differentiation among these technologies is based on the environmental heat source utilized.

For a comprehensive view of the categorization of all types of heat-pumps, see the Heat-Pump Types.

Purpose and Scope of this Document

This document outlines the foundational concepts and system-level theory for air-source heat-pump technology.

The following topics are specifically excluded from this document:

  • The practice of installing air-source heat-pumps
  • The metrics of performance and efficiency of air-source heat-pumps
  • The economic considerations associated with air-source heat-pumps
  • The regulatory requirements related to air-source heat-pumps
  • The application specific design of air-source heat-pumps
  • The comparison of products

The exclusion of these topics is due to the fact that they are determined by local conditions and engineering design decisions. The topics mentioned above are covered in detail in supportive documents.

With a focus on the theoretical and system level basis of air-source heat-pump technology, this document provides a consistent conceptual framework for the understanding of how air-source heat-pumps interact with modern heating and cooling systems.

Frequently Asked Questions (FAQs)

An air source heat pump is a type of heating and cooling system which uses a closed loop refrigeration cycle to transfer thermal energy from outside air into a building. Unlike a gas boiler, an air source heat pump doesn’t create heat by burning fuel; instead, it harvests existing environmental heat and increases it to a suitable temperature for space heating, cooling or hot water.

An air source heat pump functions by passing a refrigerant through a series of processes in a thermodynamic cycle. The heat pump extracts heat from the outside environment, then compresses the refrigerant increasing the refrigerant’s temperature; subsequently releasing heat into the building and lowering the refrigerant’s pressure to begin another cycle of heat transfer.

An air source heat pump obtains heat from the ambient temperature of the outdoor environment. Cold air still has thermal energy within it; the heat exchanger in the outdoor component of the heat pump extracts this low-grade environmental heat and sends it to the rest of the heat pump system to be upgraded in terms of temperature.

Yes, an air source heat pump also provides cooling. During operation in cooling mode, the heat pump reverses the thermodynamic cycle, removing heat from inside the building and transferring it to the outdoors (similarly to how a traditional air conditioning unit works) but utilizing the same base heat pump components.

Yes, air source heat pumps function in the winter. They remove thermal energy from the cold outdoor air and transfer it into the interior of a building. While efficiency will vary based on the ambient temperature of the outside environment, most modern air source heat pump designs are intended to function effectively in a wide range of ambient temperatures, including those found in low-ambient temperature environments.

In a heating system, the air source heat pump serves as the heat producing unit. It generates the thermal energy required for the downstream heat distribution components (such as radiators, buffer tanks and domestic hot water boilers).

Yes, air source heat pumps can generate hot water in air-to-water configurations. These air-to-water configurations involve transferring the heat generated by the air source heat pump into a water circuit which is utilized for both space heating and the generation of hot water for domestic purposes. Air-to-air configurations, however, only supply space heating and cannot generate hot water.

The different classifications of air source heat pumps depending upon the configuration of the system and the method of heat delivery are air-to-air systems, air-to-water systems, split systems, and monoblock systems.