Why evaporative cooling is bad for energy efficient homes

There is a widespread belief that evaporative cooling is a more energy efficient option than a conventional air conditioner - it's completely wrong. Evaporative cooling is a disaster for energy efficient and lower carbon homes.

Are you contemplating buying an evaporative cooler this summer – well, my advice is don’t because these deliver terribly poor performing and inefficient cooling and they’re expensive to uninstall.

Evaporative cooling is primitive, outdated and has been superseded by advances in modern, fast and efficient reverse cycle air-conditioning which is based on a variation of the same technology that runs our kitchen fridge, reliably keeping our food from spoiling day in and day out.

Unfortunately evaporative cooling is quite common due to heavy marketing by the two or three companies that are heavily vested in it, with about 20% of households in Victoria having an existing unit and around 60,000 units sold each year. Unfortunately for new home owners they are included in off-the-plan large volume standardised homes built in new developments around our Australian urban fringe and particularly popular in Victoria. 

Although the performance of evaporative coolers varies, depending on daily fluctuations in temperature and humidity and level of maintenance, they can sometimes make occupants feel a bit cooler, while most of the time they’ll make you feel muggy, delivering high humidity conditions that are outside the accepted comfort zone.

How it works and why it doesn’t work

Evaporative cooling works by adding lots of moisture to the outside air and then pumping that air at high volume through your house. Generally, for the effect to be perceivable, the humidity in the outdoor air needs to be below 30% RH (Relative Humidity). RH is the amount of moisture that is in the air at a given temperature. As air gets warmer it can hold more moisture and conversely as it gets cooler it can’t hold nearly as much moisture. The relative humidity is the difference in the readings between two mercury filled glass thermometers. one that is exposed to the normal open air called the 'dry bulb' and the other which is wrapped in a wet wick called the 'wet bulb'.

An evaporative cooler can only effectively operate when household windows and doors are open. This is not only a security risk but it also means that any work that has been done to upgrade your building envelope is compromised in an extreme way. Insulation and air sealing will be wasted and instead of consistent insulation over an area on the ceiling where the evaporative cooler register is located, it’s now been replaced with a big thermal chimney in each room. You are now directly exposed to the varying temperature and humidity combination that is external to your home, this is particularly important in southern states where seasonal winter heating requirements are large and significant heat is lost through this evaporative cooling ducting induced thermal chimney effect.

In Melbourne, where evaporative coolers are quite common, according to Sustainability Victoria, humidity is normally in the range of 40-50% on hot afternoons. This is outside the effective range for using evaporative coolers. In places such as Mildura, which has slightly lower humidity within the range of 20-30% on a hot afternoon, it’s still hit and miss as often outside humidity coupled with high temperatures is well above 50% and the evaporative cooling fails to do anything appreciable for the occupants.

The use of evaporative coolers also has other associated risks, discussed below:

1) Fire risk

According to Western Australia’s Fire and Emergency Services Authority (FESA) 35 of 70 houses that burnt down in a catastrophic bushfire were burnt from a fire that started in the evaporative cooler unit on the roof. It’s estimated that a significant proportion of houses burnt in Victoria’s Black Saturday fires, where hundreds of houses were destroyed, were due to evaporative coolers that ignited under ember attack.

2) Costly Servicing

A unit needs to be serviced at least every two years and the pads replaced every 2-7 years.  The cost of servicing starts at $200 and usually parts are required at an additional cost.

3) Physical home security

It’s difficult to secure your home with all the doors and windows open, as this is mandatory for operating these units. Many households shut up the house in the evening hoping their evaporative cooler will work, but without free airflow it will not.

4) Water leaks

Water leaks often occur in these systems due to pad failure or failure to replace the pads early, which results in water damage. There is also significant risk in having additional plumbing connections above your lounge room.

5) Mould and damp

Mould and damp can also result from the use of an evaporative cooler as the humidity in the indoor air rises to 80%, a similar scenario to mould growth in your bathroom or shower when you haven’t turned on the exhaust fan.  Furthermore, according to the Victorian Government, evaporative coolers can cause swelling to doors, doorframes and furniture.

6) Installations are undertaken with little care to seal the perimeters of ceiling vents

This coupled with a lack of effective damper or no damper at all causes destruction of the building thermal envelope. You are paying for your evaporative cooler through much higher winter heating requirements. An air pressurisation test of 12 houses by building performance company Efficiency Matrix showed 40% of total building air leakage can be attributed to conventional evaporative coolers and 20% to more expensive units with a damper. So if you are someone who has installed or wanted to install insulation and air-seal your building then all that good work is undone by evaporative cooling.

7) Evaporative coolers are big and bulky and take up valuable space on your rooftop, which would be far better allocated to installing solar panels

A badly placed evaporative cooler can block out upto 3kW of potential continuous solar panel array, this is enough to supply the equivalent of 30-50% of a household’s annual electricity needs.

A modern solution that works

Purchasing a quality Japanese reverse cycle air-conditioner can pay itself off because it not only cools but it heats too. Heating with a reverse cycle air-conditioner can be as much as 22 times cheaper to run (the best case is using your own solar without exporting it to the grid) than heating with gas, which results in around half of its heat loss to the atmosphere prior to contributing to your home.

Claims from the US Department of Energy, California Energy Commission and Sustainability Victoria estimate the cost of running an evaporative cooler at half that of a reverse cycle air-conditioner running in cooling mode. Unfortunately, these claims are misleading as they are comparing evaporative coolers against poor performing air-conditioners rated at half the efficiency of the best modern units. When you compare an evaporative cooler to a Daikin Ururu Sarara or other 6 or 7 star air-conditioners, the energy consumption is around the same but the cooling performance is far superior. It delivers the desired temperature set point that you dial-up on the remote and gives you the required level of humidity for optimum comfort.

Matthew Wright is executive director of Zero Emissions Australia, technical director at Efficiency Matrix and resident columnist at Climate Spectator.

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