Ducted reverse-cycle air conditioning buying guide

Want to cool and heat your whole house? Here's what you need to consider.
 
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01 .Introduction

reverse cycle air conditioner

Ducted reverse-cycle air-conditioning can be a very convenient way of cooling and heating your home. 

However, before you spend thousands of dollars to have a system installed (and several hundred dollars a year to run it), consider other cooling and heating options. 

Even if you decide on a central system, you may only need a cooling-only or heating-only model — depending on where you live.

Before you buy any cooling or heating system, first optimise the energy-efficiency of your home — for example by insulating the ceiling and walls, and draught-proofing windows and doors — and then calculate the capacity you need:

  • If you want to cool or heat individual rooms in your home with separate appliances, you can use our cooling and heating capacity calculators.
  • If you decide on installing a central cooling or heating system, the right size and design depend on a range of parameters and have to be determined by the supplier for your individual situation. Central systems shouldn't be bought "off-the-shelf". 
These are also the reasons why we don't test ducted systems:
  • The results would only be valid for the tested scenario (which could be very different from your house).
  • A model that performs well in a test situation can still disappoint you if it's not designed according to your needs, or if it isn't installed properly.

Read about the different parts that make a ducted air conditioner, and what features to look for.

We'd like to thank Sustainability Victoria who supplied much of the information for this article.

For more information on Heating and cooling, see Household.

 
 

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Air conditioners work on the heat pump principle. As the name suggests, they pump heat from one place to another. Appliances like fridges and heat pump water heaters use a similar system.

This is how the heat pump cycle works in an air conditioner:

  • A fan draws hot air from your home over a cold liquid, called a refrigerant.
  • Heat is absorbed from the air, cooling it, then it flows back into your home.
  • The warmed refrigerant evaporates and flows into a compressor, which creates a high-pressure, high temperature gas.
  • The gas is pumped through a heat exchanger outside your home, which allows heat to escape and the refrigerant to cool and liquefy again.
  • The refrigerant flows through an expansion device that lowers its pressure, cooling it further to increase its potential to absorb heat again.

Reverse-cycle air conditioners can reverse this process and be used for cooling and heating.
Air conditioners are very efficient. For every kW of electricity consumed, two or more kW of heating or cooling capacity can be produced. Window and split-system models must carry an energy rating label — the more stars a model has, the more efficient it is, and the lower its running costs. Ducted systems have to meet minimum energy performance requirements, but don't carry the energy rating label. See the government's energy rating website for more info.

The efficiency of larger models (most ducted systems) is expressed as the cooling or heating co-efficient of performance (COP) — the ratio between the cooling or heating capacity (in kW) and the amount of electricity used in the cooling or heating process (in kW).

The size and design of a ducted cooling and/or heating system depend on a range of factors:

  • Your home’s floor plan: How many levels are there? What are the dimensions of the rooms (including ceiling height)? Which direction do the rooms face?
  • The size, position and orientation of windows and doors.
  • The type of construction (for example, weatherboard or full brick).
  • The level of insulation.
  • The number of people living in your home.
  • The main use of the area (for example, sleeping, living, cooking).
  • The ceiling cavity space - slimline systems are available for homes with small ceiling spaces.
  • Outdoor spaces - just as with split system air conditioners, the outdoor compressor unit(s) needs to be installed somewhere where noise won't be an issue (for you or your neighbours).
  • Large systems may require a three-phase power supply, which will be an extra installation cost if you don't already have it.

That’s why you shouldn’t buy such a system "off-the-shelf" — get the supplier to design it for your individual situation.

Features

There are several design features to consider. A supplier may be able to offer a range of options, depending on the design and requirements of your home.

  • Vents come a variety of designs and can be installed in the ceiling or walls.
  • Controls are usually hard-wired and mounted on a wall, unlike the remote controls used for single split-systems. You may have one controller for the entire system, but in a large house you might opt for extra controllers in other parts of the house for more convenience.
  • Sensors are used by the controller to keep the room at the targeted temperature. Large open-plan areas may need multiple sensors.
  • Zones: most systems allow for a home to be divided into zones for convenience and economy, so that you can turn on the air conditioner for only the part of the house you want cooled/heated (say, living areas, bedrooms, or different floors) rather than the whole house. Or, you can set different climate levels for different areas as required.

Ducts

The ducting is a key component of the system. Ducts need to be thermally efficient so that valuable cooling or heating isn't lost between the air conditioner unit and the target room. The ducting industry association, ADMA, is concerned that some suppliers are installing inferior ducting, and recommends that consumers check with their installer that the ducting meets the Australian standard for ductwork, AS 4254. A new draft version of this standard calls for ducting to be labelled with the manufacturer name and the thermal value of the insulation, to help ensure the correct ducts are used.  Check the labels on the ducts or get a written statement of compliance to make sure you get the right quality of ducting.

Cooling capacity

The calculator below is for determining the required cooling/heating capacity for a single room: use this for each room to be air-conditioned, as a guide to the overall cooling capacity your what you need.

You’ll be hard-pressed to find a ducted reverse-cycle air conditioner for less than $6000 (including installation), and the cost can easily reach twice that amount, depending on the size of your home.

The running costs mainly depend on:

  • The type and size of your system
  • The energy-efficiency of your system
  • The time you’re operating the system for
  • The construction of your home (floor plan, level of insulation, size of windows, etc.)
  • The electricity tariff you’re paying
  • The temperature you choose on the thermostat: each degree Celsius cooler in summer, and each degree warmer in winter will increase the running costs by 10-15%. Find the temperature you’re just comfortable with — try 25 degrees in summer, and 20 degrees in winter.

But by following some easy rules in day-to-day operation, you can further reduce running costs:

  • Close all external windows and doors when your system is running.
  • Shade your windows during hot summer days (to keep the heat out) and during cold nights (to keep the heat in).
  • When you expect a hot day, turn on the air conditioner early, rather than wait until your home is hot. Similarly, start heating early when expecting a cold day.

Sustainability Victoria estimated the average running costs in 2002 for a 150 square metre Melbourne home at 87c per hour, assuming 15c/kWh electricity cost. Prices have of course risen since then; at 27c/kWh this would be closer to $1.57 per hour, and at 35c/kWh it would be $2.03 per hour. On the other hand, air conditioner efficiency has improved since 2002 so that would tend to reduce the cost. Nevertheless the figures show that air conditioning a whole house can be costly.

Note that the figures may be quite different in other climates, or for a different size house.

Depending on where you live, you may only need a central cooling or a central heating system. In this case, a ducted reverse-cycle air conditioner may be overkill and a specialised system may be more appropriate.

All central systems need expert advice regarding the capacity and installation required for your individual situation. Your state energy authority may have more information on these systems.

Central cooling-only systems

Evaporative cooling

An evaporative air cooler consists of a motor driven fan, a dust filter, a water tank and a wetting medium. The hot air is drawn across the wetting medium, which is saturated with water from the tank. The water evaporates, absorbing heat from the air, and cooler, moist air is blown into the room.  

Evaporative air coolers provide relief from dry heat. If you live in a humid climate, such as parts of coastal Australia, they won’t do you much good — you’ll need an air conditioner. And if you live in a place where temperatures get very high, you may not find one satisfactory either. If you're trying to cool a room down from the low forties, you're unlikely to get it much below 30 degrees Celsius - and unfortunately, it'll be a pretty humid 30 degrees because of the moist air created.

Evaporative coolers are cheaper to buy and run than air conditioners.

With ducted systems, the main unit is installed in the roof, and the cooled air is ducted into the rooms via ceiling outlets. It can use up to 25 L of water per hour, so this type of cooling may not be the best choice if you live in a water-restricted area.

Central heating-only systems:

Ducted gas air heating

Air is heated in a central gas heater (look for an efficient model with a high gas star rating) and distributed through insulated ducts to ceiling, wall or floor panels throughout the house (similar to ducted air conditioning). Different areas can be zoned, each with its own individual thermostat.
This type of heating can circulate a lot of dust (requiring a filter system that adds to the running costs and maintenance requirements), and tends to dry the air.

Hydronic heating

With this system, water is heated in a central boiler, then circulated around the house to panels that radiate and convect the heat to the air. The boiler can be fuelled by natural gas, LPG, wood or off-peak electricity.

The panels are usually individually controlled, so you can adjust the temperature of each room according to your needs. Look for quick-response panels made from mild steel and with a relatively small volume.

This type of heating is very quiet and circulates only a little dust.

In-slab heating

With this type of heating, the concrete floor slab is heated by internal electric cables or hot water pipes. It’s not recommended for suspended concrete floors where the space underneath isn't occupied, or for slab-on-ground in areas with a high watertable. And you’d probably only choose it if you’re building a new home or an extension.

Electric systems run on off-peak electricity, hot water systems can be fuelled by natural gas, LPG or wood.

This type of heating takes a long time to respond to changes in the thermostat setting, so it’s often left running 24 hours a day on an appropriate setting — making zoning (separate thermostatic controls for different parts of your house) very important.

Running costs and greenhouse gases

With all central heating systems, running costs depend very much on what fuel type you’re using and how much you pay for it. Compared to ducted reverse-cycle air conditioning, all types of natural gas heating are likely to have lower running costs, while using LPG and electric hydronic heating are likely to be more expensive to run.

Using natural gas or LPG produces much less carbon dioxide than reverse-cycle air conditioning, while electric heating produces considerably more.

For more detailed info, check the Sustainability Victoria factsheets.

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