HVAC System: Ultimate Guide to Choosing the Most Efficient in 2020

By Rod Kington, National Manager – Sustainability

What is HVAC?

A HVAC system creates a comfortable space to live and work in the built environment. This system moves heat in and out of internal areas to ensure habitable spaces. When designed correctly, significant energy savings and efficiencies can be achieved to help buildings attain carbon neutral status. 

What does HVAC stand for?

  • H: Heating: ensuring the room is not too cold.
  • V: Ventilation: providing fresh air to prevent a space feeling stale.
  • AC: Air conditioning: keeping spaces from becoming too hot or humid.

On average, HVAC systems contribute approximately 70% to a building’s energy consumption. Chillers, compressors, fans and pumps are the highest users of energy. Heat loads are primarily from roofs, windows, outside air, people, lighting and infiltration.

How does a HVAC system work?

A HVAC system is designed to move heat to maintain comfortable conditions within the built environment. Heat is moved out of a building in three stages:

  • Collection: heat exchange in the conditioned space using air
  • Transfer: moving heat using water or refrigerant
  • Rejection: using air, water or the ground as a heat sink

Each step is completed in different ways depending on the system which is installed.

1) COLLECTION: Five main methods are considered. Four can be designed to deliver an efficient solution to suit the space except for constant volume system, which is not an efficient long-term solution. They include:

  • Variable air volume
  • Variable refrigeration volume 
  • Fan coil units
  • Split cassettes 
  • Constant air volume

2) TRANSFER: Compressors are the main facilitator for heat transfer. The key types of compressors are: 

  • Screw
  • Reciprocating
  • Centrifugal 
  • Magnetic bearing
  • Scroll 

3) REJECTION: Heat can be rejected via:

  • Water evaporation: cooling towers
  • Air / air heat exchange: air cooled packaged units
  • Hybrid air/water: air cooling chillers with evaporative pads fitted
  • Ground source: running water/refrigerant through the ground 

Different types of non-centralised HVAC systems

There are three common types of non-centralised HVAC systems:

1. Rooftop Packaged Unit

A stand-alone air conditioning system located on a roof which takes hot air from the building and returns cool air. They are easy to access and maintain with all parts in one place. Newer units are relatively efficient and can incorporate an economy cycle.

2. Split System

They are called split due to separated indoor and outdoor components. Split systems are common in the home, very efficient and predominantly used in smaller sized units.

3. Ducted Systems

Similar to a split system but instead of the indoor unit mounted on the wall it is located in the ceiling space with air ducted to and from it. Multiple rooms can be fed and they are generally used in larger room spaces. 

What are the most efficient HVAC systems for different sized buildings?

Robust high-rise HVAC designs should feature:

  • Oversized water-cooled magnetic bearing chillers
  • Multiple large cooling towers
  • 6 x EC fans air handling units (Serving north, south, east and west plus two centre zones)
  • Variable air volume terminals:  Max 1 per 80 sqm
  • Heat pumps rejecting heat into a condenser
  • Building Management System (BMS) controls via an analytics package

Low rise building (between 3,000 – 8,000 sqm)

The requirements that consider whole of life efficiency including maintenance, servicing energy and water consumption should include:

  • 5 x EC fans (Serving north, south, east and west and one centre zone) plus water cooled DX package units
  • Electronic expansion valve, interlaced coils, 100% economy cycle capability
  • Roof top cooling tower
  • Roof top heat pump and insulated buffer tank
  • Variable air volume terminals: Max 1 per 80 sqm
  • BMS controls via an analytics package

Energy Efficiency Considerations

Many different types of equipment can be used within HVAC systems to create efficiencies. Four are identified below.

1. Fans and Pumps

These are HVAC systems’ heat movers. They move the heat around the occupied space and out to the heat rejection system via a compressor. They run longer hours and are considerably less efficient than a chiller.

Pump Affinity Laws highlight that if pumps are reduced from 1,500 rpm (50 Hz) to 1,450 rpm (48.3 Hz) a 10% energy consumption saving can be achieved. Installing variable speed drives (VSD) on cooling tower fans (>4kW), controlled by a temperature sensor can pay back within three years.

2. Filters and Coils

These are often overlooked. A dirty filter and coil will slow down air flow and make the system require cooler air to maintain the same conditions which wastes energy. Clean filters also prevent dust and dirt from building up on the heat exchange surfaces (coils) and ductwork, saving expensive maintenance and help extend asset life.

3. Car park ventilation

It removes the fumes emitted by cars. A carbon monoxide sensor system including VSDs on ventilation fans is the reduced energy and risk. It will only turn on when required and the payback period is less than three years.

4. Building Management Systems

Systems are installed in most medium and large commercial and retail buildings. They are primarily for the control of the HVAC systems but can also control lighting and monitor other services. BMS can have direct control of between 50 – 80% of a building’s electricity requirements. Installing an analytics package to monitor and maintain the BMS is important.