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How Ceiling Fans Work, Thermally Speaking

In hotter climates, ceiling fans can provide a big increase in a building’s energy efficiency rating. But how does the National Construction Code (NCC) & National House Energy Rating Scheme (NatHERS) Software calculate their exact benefit? Time to get technical!

All NatHERS-approved software follows the NatHERS Software Accreditation Protocol. This protocol came into effect on the 26th of June 2012 and all complying software packages meet its requirements. They all share the same underlying CHENATH engine (number cruncher). This engine calculates the combined effect of air and radiant temperatures, allows for the impact of humidity and calculates the air speed in each room from cross ventilation and ceiling fans. Basically, calculating the effects of ceiling fans can quickly become incredibly complicated, so the engine makes some assumptions that massively reduces the simulation time.

Air flow around a ceiling fan

Here’s the basics of the assumptions used by the CHENATH engine:

  • Ceiling fans of the same swept diameter have the same airflow, regardless of fan construction, blade design, the distance from fan blades to ceiling, power, and fan speed.
  • The calculated cooling benefit assumes the room is large with no furniture. The circulating air is assumed to be humidity-controlled & cooled by an air-conditioner that’s being used with a ceiling fan. Other effects on the performance of ceiling fans have been simplified.
  • The maximum cooling benefit is 1.6°C for a 900mm fan (swept diameter), 2.4°C for a 1200mm fan, and 2.8°C for a 1400mm fan.
  • The airflow from a ceiling fan is uniformly radial at floor level, at a diameter of twice the swept diameter (what they call the Target area). This means the Target area is 2.54m² for a 900mm fan, 4.52m² for a 1200mm fan and 6.16m² for a 1400mm fan.

As for the actual formula calculated per room?

The benefit of ceiling fans = (Maximum benefit) × (Target area) × (Number of fans in Room) ÷ (Room floor area)

Now for an example: imagine a single 1200mm ceiling fan in a 4m by 4.5m room. The benefit of this ceiling fan is (2.4°C) × (4.52) × (1) ÷ (4 × 4.5), which is 0.6°C.

You may have noticed that this is only a quarter of the maximum benefit. A 4m by 4.5m room needs
four ceiling fans to receive the maximum benefit – this is impractical, so basically all rooms with ceiling fans will receive only a fraction of the maximum benefit.

Combined with any impact of natural breezes, the NatHERS software uses this cooling benefit to increase the Cooling Thermostat setting in a room with ceiling fans. This reduces the total Cooling Energy, and naturally benefits hotter climates more than it does colder climates. When ceiling fans aren’t enough to keep the building cool, the software assumes that both the ceiling fans & the air conditioner are used together to provide the lowest possible energy consumption.

But wait – don’t ceiling fans require electricity to operate? They sure do, and this has been taken into account in the assumption figures. While the average small-sized air conditioner might consume 1000 Watts, the average ceiling fan consumes just 50 Watts. The whole point of the NatHERS scheme is to reduce the total energy consumption of buildings. There’s an assumption here that all buildings have an air conditioner (or a future occupant will install one). A ceiling fan consumes only one twentieth the power of an air conditioner to provide an equivalent cooling benefit of up to 2.8°C.

When it comes to the Deemed-To-Satisfy assessment method, the assumptions have been simplified further. According to the NCC 2012 Part, it’s assumed that a 900mm ceiling fan can serve up to 15m² floor area & a 1200mm ceiling fan can serve up to 25m² floor area. Ceiling fans reduce the ventilation requirement as per Table In the Glazing Calculator, more ventilation from openings & ceiling fans increases the glazing allowance. In simple terms: under DTS, adding ceiling fans means you can have more glass.

As a final point, while it isn’t mentioned in the Energy Efficiency sections of the NCC, don’t forget that ceiling fans should satisfy other local requirements, including minimum floor-to-fan height requirements.

References: Angelo Delsante (2008) (formally of the CSIRO), personal communication, Steven V. Szokolay and Peter G Hope (2005), “Ceiling fans, their effect on energy rating by the AccuRate program, Stage 1 Report”, The Australian Greenhouse Office.

Whether you are interested in a full thermal simulation of your house to find some suggested renovation improvements, making your new building extra-efficient, or even just seeking some help to make your dream design comply with the building code, contact us for a competitive fee proposal.

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