Air-conditioning for tall buildings can be a challenge for a number of reasons.

  • As you build taller you have to contend with variance in ambient temperature and wind which is a particular issue when you exceed 60 storeys
  • With air-conditioning typically accounting for 30 per cent of a building’s energy consumption, by simple virtue of their size, tall buildings use a lot of energy
  • With saleable floor space being crucial for development business cases to stack up, developers don’t want large areas of plant wasting space, yet buyers won’t accept anything but the best internal environment

If you look at how high-rise apartment living has developed over the years, while heating systems would be included historically air-conditioning was not a standard fixture. As our climate has started to change and the demands of building users have increased, the expectation is for both heating and cooling. This is far more efficient to do as a single system.

Traditionally, there have been three main approaches:

  • A central four pipe water system which consists of air cooled chillers and boilers to serve indoor fan coil units located within each apartment
  • A central water cooled system which consists of cooling towers and boilers to serve water sourced heat pumps fan coil units located within each unit
  • Split air cooled air conditioners with outdoor condensers mounted on the balcony of each unit.

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The Pros and Cons of Traditional Systems

Four Pipe System

The central four piped system air conditioning is the one of the most energy efficient systems available. However, it is also the most expensive system to install. Water metering devices will be required to measure energy consumption to each unit and billed via the body corporate which has additional cost and complexity.

Water Cooled System

The running cost of the water cooled central system will be charged to each unit and the indoor unit running cost will be charged through the individual unit electricity retailer. A cooling tower on the top of the tower, however, does have the potential to be noisy, which is not ideal if you are locating expensive penthouses there.

Water sourced heat pump AC systems within the apartments themselves also are noisy and are rather bulky pieces of equipment which reduce the floor to ceiling height within the units.

There are also potential issues with legionella outbreak; it is critical to get the chemical treatment right at the start and then maintain it adequately throughout its service life.

Single Split Units

This is the budget option, which is low cost to install.

It also gives tenants more control over their apartment environment and billing is individual for each apartment.

However, these units can be noisy and visually unappealing, located on the balcony. They compromise your already limited external space and have the tendency to cause noise and uncomfortable balcony environment.

They also struggle to operate effectively when located especially at greater building heights.

From an energy perspective, having lots of smaller units is not very efficient, and the more apartments and the higher you go, the more you are compromising energy usage.

New Technology

Now there are two alternatives beginning to change the way we look at air-conditioning solutions for tall buildings.

Variable refrigerant flow (VRF) air conditioning units are effectively central air cooled outdoor condenser units serving multiple indoor air conditioning units.

There are also hybrid systems consisting of a closed circuit cooler connected to water cooled variable refrigerant condenser units serving multiple indoor air conditioning units.

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The Pros and Cons of New Technology


Variable refrigerant flow air conditioning is more energy efficient than the water cooled heat pump air conditioning system and have the option for heat recovery between the indoor air conditioning units.

The system does allow long runs of refrigerant pipework, but the maximum that this system can services is approximately 15 floors, which means an intermediate plant level is required around every 30 floors

As it is air-cooled, the condenser units on the intermediate floors need to sit on the perimeter of the building to discharge hot air. This means you potentially lose a whole floor of building to plant.


This system negates the need for intermediate plant floors as condenser water can easily service 60-plus storeys

These water-cooled units are small and can sit on every floor or every other floor, if required, in a simple service cabinet, saving space and giving the plant floor back.

They then feed into indoor air conditioning units within the individual units via refrigerant pipework. This system could provide double heat recovery from both the water loop and refrigerant proving it to be very energy efficient.

VRF vs Hybrid: A Desktop Study

We conducted a desktop study on a typical 60-storey building and a typical floor plate of 1,000 square metres.

Giving a conservative estimate, utilising the hybrid system saved at least 600 square metres per floor for saleable space with $10 million worth of value for the developer on top of the energy savings, better air conditioning system offering increasing perceived unit value and internal environment benefits gained for the unit owners.

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This article was written in collaboration with Patrick Lee, Senior ESD Engineer