A More Efficient Air-Con Solution for Tall Buildings 7

Monday, February 23rd, 2015
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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
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  1. Vidyadhar Joshi

    Thanks for the article and sharing. Although my discipline is I & C but I am very much interested in archetechure and buildings as well. I have worked in several buildings in Perth CBD. A lot of things do annoy me specially with respect to the huge amount of energy used by these buildings. Air conditioning is required both cool and hot as well. My suggestion to you is to consider the "Chimney Effect" in the tall buildings and benefit from it. The cool air enters from ground floors ( need openings at various points ) and hot air exit from top floors. This is a natural phenomenon which I strongly feel should be used in the design. I am sure this will result in significant savings. Use of Double Entry doors ( Two Doors about 2-3 m apart in line ) is great where there is large diff (10+) between inside temp and outside temp.

  2. Vidyadhar Joshi

    Further to my earlier comments, I feel there is a lot of scope to utilise the heat and cold that is generated within such big buildings. Everyone needs refridgerator and hot water. The compressor of Fridge generates heat. Consider the no of these units in the building and see how much of this heat can be utilised.

    • Patrick Lee

      Glad you liked the article Vidyadhar.

      The chimney effect is definitely a source of energy which can be reclaimed and reused if the floors are interconnected well enough throughout the building. With high pressure on saleable floor areas for return on investment, this can be dificult to achieve especially on pencil thin tall buildings.

      The harnessing of heat generated from refrigerator compressors is good in theory but may not be a practical solution to combine or harness the amount of heat from each unit.

  3. Kenneth Thor

    Excellent, easy to understand article with broad brushstroke comparisons of various systems. Have you given much thought to the total installed cost difference between water sourced heat pumps vs "hybrid" water cooled VRV systems? Remembering to take into account separate metering infrastructure requirement for centralised VRV plant of course. Thanks. Kenneth

    • Patrick Lee

      Glad you liked the article Kenneth!

      There would still be some form of metering infrastructure for the water sourced unit albeit it would be much less complicated as most buildings will include this into the body corporate fee due to the low running cost of the central system. This is because the refirgeration cycle occurs within the water sourced heat pumps and the running cost are paid for by each individual unit according to their usage.

      The "hybrid" water cooled VRV system is definitely a more superior product when compared against the water sourced heat pump for marginal cost difference!

  4. Nigel Grier

    Great and thanks but what about detailing the energy performance for each of the comparisons and the ongoing maintenance and overhaul/replacement costs over the lifecycle of the building and/or plant. Article seems very focused on initial project development and CAPEX only.

  5. Marc Oliveras González

    Hybrid systems look nice but I think that are 'halfway' solutions to the HVAC efficency issues. A building rework plan must start with the study of how to reduce the HVAC energy demand. Planning a good isolation system for the whole building will reduce this demand (E.T.I.C.S. systems for externat isolation or gypsum boards and MW for internal isolation will work nice), and then, think about how many HVAC equipment you need.

    It is common sense that using one hybrid system per floor will be more efficent than using two of them. Regading to efficent HVAC solutions for building, I think the study must start with the isolation systems in order to reduce the needs.