How Does a Heat Pump Work?

18 August 2020

Have you ever sat in the cozy comfort of your living room thanking your Heat Pump for providing the heat that is warming your toes and thought ‘Exactly how does my Heat Pump work?’

Well we are glad you asked!

In heating mode, Heat Pumps are like a reverse refrigerator - they do not create heat they simply transfer it from one place to another. In the winter the system absorbs the heat from the cold air outside and transfers it inside your home using refrigerant as the heat transfer medium.  It may seem counter-intuitive to use cold air as a heat source, but heat energy is present even at very cold temperatures. In the summer the process is reversed and the pump will act as an air conditioner, extracting heat energy from the surrounding environment within your home and transferring it outside using the same heat transfer medium.

Reverse Cycle Split System Heat Pumps consist of two components – an outdoor unit and an indoor unit. For a reverse cycle split system air conditioner, the outdoor unit contains the compressor, while the indoor unit contains the condenser and evaporator components of the system.  The outdoor and indoor units are connected together by piping that carries the refrigerant needed to heat (or cool) the outside air.

With a split-system air conditioner, the compressor sits outside the home in a box that rejects the heat during cooling & absorbs heat on heating. The indoor unit component of the system distributes the conditioned air into the room that houses that unit.

A Heat Pump in heating mode operates just like cooling mode, except that the flow of refrigerant is reversed by the aptly named reversing valve. This flow reversal means that the heating source becomes the outside air (even when outdoor temperatures are low) and the heat energy is released inside the home. The outside coil now has the function of an evaporator, and the indoor coil now has the role of the condenser.

One of the most important things to understand about Heat Pump operation and the process of transferring heat is that heat energy naturally wants to move to areas with lower temperatures and less pressure. Heat Pumps rely on this physical property, putting heat in contact with cooler and lower pressure environments so that the heat can transfer naturally – this is fundamentally how a Heat Pump works.

Ducted Heat Pumps supply the conditioned air into every room in the house through ducts -  connecting all of the rooms in your home to one central indoor unit, generally located in the roof space.  With a typical ducted system, each vent or duct is capable of heating a room with an area of around 80m2.

In low temperatures, all Heat Pumps have to perform a “defrost cycle” to remove ice build-up on the outdoor coils. This can mean that the Heat Pump will temporarily stop operating for several minutes or may produce slightly cooler air.

Ok, so let’s get into what ‘Fixed Speed’ and ‘Invertor Control’ means….

Fixed Speed

A fixed speed system only has a single speed compressor motor that is either on or off - it switches off when the desired temperature is reached and switches on again when the temperature drops to a set level. With just the one speed that the system can run at, it takes longer to get to the desired temperature which in turn leads to a lot more power usage when compared to inverter driven systems.

Inverter Control

Inverter technology uses a variable speed compressor motor much like that of a car - it slows down and speeds up as needed to hold a selected comfort setting. This technology means the desired room temperature will be reached faster and maintained more efficiently.

Inverter technology also provides a more precise room temperature without the fluctuations and energy requirement of fixed speed systems…also the speed control of the outdoor unit also means quieter operation which is especially important for you and your neighbours.

So we’ve spoken about the functionality of both the indoor and outdoor units and covered off the differences between a fixed speed and an inverter Heat Pump system so let’s delve into what some of the other elements of your Heat Pump Air Conditioning system do.


The refrigerant is the substance that absorbs and rejects heat as it circulates throughout the Heat Pump system.


The compressor pressurizes the refrigerant and moves it throughout the system.

Reversing Valve

The part of the Heat Pump system that reverses the flow of refrigerant, allowing the system to operate in the opposite direction and switch between heating and cooling.

Expansion Valve

The expansion valve acts as a metering device, regulating the flow of the refrigerant as it passes through the system, allowing for a reduction of pressure and temperature of the refrigerant.

So there you have it…a Heat Pump Air Conditioning system is a truly amazing invention that is an integral and much valued addition to any Kiwi home – no matter the season.

When the weather turns, turn to someone you can trust.

Rinnai – Your specialists in home comfort.

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