Have you noticed dripping water or water stains surrounding your ducted air conditioner vents?

Well, you’re not alone, many other Brisbane homeowners have the same problem during the most humid months of the year! It can be an extremely frustrating problem but it helps to know the facts. We’ll explain what causes air conditioner sweating, why it hasn’t happened before, and the methods of prevention so you can feel at ease and say goodbye to those pesky water stains.

What Causes Air Conditioner Sweating?


1. Humidity Causes Ducted Air Conditioner Grilles to Sweat


Air conditioner sweating occurs when humid air in the home makes contact with the cold surface of the ducted air conditioner grille. Sweating is likely when the air temperature is below the dew point temperature.

The dew point temperature is the temperature at which water vapour will begin to form and turn into liquid. Simply put, if the surface of your air conditioner grille is below the dew point of the room then moisture will begin to form.

This holds true for your roof space as well where it’s much hotter, meaning that in your roof the dew point temperature is much higher. This means that sweating of the ducts and other air conditioning equipment is more likely to occur.



Let’s go through an example, at our office today we have just closed all the windows & turned the air conditioner ON. Take a look at our indoor observations below after we closed all the windows.

Indoor temperature: 22.1°C
Relative humidity: 98%
Dew point: 21.8°C.

So, if the external surface of the ducted aircon grilles in our office gets colder than 21.8°C, we could expect water vapour to form and turn into a liquid. As our air conditioner is set to 22°C and won’t be running for a long time, the grilles don’t have the chance to sweat.

February is the most humid month in Brisbane and also the month when most homeowners experience air conditioner sweating problems. Let’s say it’s an average February day and take a look at the dew point temperature.

Indoor temperature: 28.5°C
Relative humidity: 70%
Dew point: 22.5°C

Now we understand that if the external surface of the ducted aircon grilles in our home gets colder than 22.5°C, we can expect water vapour to form and turn into a liquid.

To put this into perspective, a ducted air conditioner normally provides a 10°C difference in temperature between the air getting sucked through the return air filter grille and the supply air temperature coming out of the grille in your living room.

In this example, the standard temperature difference would lead us to a supply air temperature of 18.5°C. As this temperature is lower than the dew point, air conditioner sweating may occur once the metal/plastic grille drops below this temperature.

The material of the grilles can also impact the amount of condensation/ dripping water that comes off your air conditioner. Metal grilles are more likely to cause sweating. Plastic grilles is a better alternative as they are less conductive than metal.


Checklist to Avoid Air Conditioner Sweating

If you are using your system correctly sweating should not be a problem, if it is we need to do some investigating!

  • Are all of the windows and doors shut properly (including the rooms you are not using)
  • Do you have a minimum of 2-3 zones ON & OPEN? This is normally set up by your air conditioning contractor and referred to as constant zones. Smarter Air sets 2 constant zones for all systems less than 14kW and 3 for larger systems as often recommended by manufacturers.
  • Is the unit set to a reasonable and achievable desired temperature for the day such as the recommended 24°C? This gives the unit a time to cycle OFF for a period allowing the grille temps to warm back up and prevent them from getting too cold.
  • Are the filters clean? Your filters should be cleaned every 3 months and more regularly during the summer months. Dirty filters mean a colder temperature at the grille and an increased chance of your air conditioner sweating.  Filters inside air conditioners are designed to catch dust particles to avoid you breathing it in. If not cleaned regularly, your filter can clog causing it to collect moisture and eventually grow mould. Having a clean air conditioner filter allows your family to breathe clean air all year round.
  • Has your ducted air conditioner system been serviced? The indoor coils of your system might be blocked or dirty. Dirty cooling coils means the airflow is restricted so you’ll end up with a colder temperature at the grille and again increasing the chance of sweating.
  • Is your roof properly ventilated? The cooler your roof space, the lower the dew point in your roof. This results in a lessened chance of air conditioner sweating occurring of the duct and other equipment in the roof. To correct this, contact Smarter Air to arrange the installation of whirlybirds, and cool down your roof once and for all.


2. Why Hasn’t This Sweating Happened Before?

If your air conditioner has been installed for a few years there are a few reasons you may not have experienced the issue until now.

  • The cooling coils may be blocked, meaning your indoor unit may need a deep clean. It’s recommended to get this done at least every three years. This is an additional service on top of your regular annual service and will ensure that any dirt or grime is removed from the indoor coil. Doing this ensures that your coils will get the correct airflow across them. It also prevents your family from breathing in any mould that may be growing on the coils.
  • Your usage habits may have changed, maybe you are setting the temperature lower than 24°C? On warm days, it’s recommended that you set your air conditioner temperature to no more than 10 degrees lower than the outdoor temperature. For example, if it’s 32°C outside, your minimum cooling temperature should be at 22°C.
  • Someone may be leaving an outside window or door open resulting in your system sucking in warm humid outside air?
  • If you haven’t installed exhaust fans your system may be drawing hot humid air from your roof space! You can check this by looking at the fan when it is turned off, is it spinning in the wrong direction? In this case, you should consider installing a DraftStoppa. Smarter Air can also provide and install this product, so get in touch to arrange installation and reduce air conditioner sweating!
  • You may have added ceiling insulation and while this is a good thing for your home it means that the hot air is now trapped in the roof where your ducted air conditioner is located. This means your ducted air conditioner system now has to work harder. Best practice here is to get in touch with Smarter Air, who specialises in roof ventilation using Whirlybirds. Make sure you get some vents installed along with your new roof ventilators.


3. How Do I Prevent My Ducted Air Conditioner Sweating?

Quite often homeowners look at their ducted air conditioner installation when dealing with sweating or condensation issues. And while it’s important to ensure you have a quality installation the fact is that most of the time sweating is due to improper use or maintenance of the air conditioner.

Your air conditioning professionals are however the best people to speak to help you get to the bottom of the problem, using specialised tools they will be able to help you pinpoint the problem and suggest ways to help prevent or at least minimise sweating/condensation issues.

Before getting an air conditioning professional involved though it is worth running through the below checklist as you may be able to solve the issue yourself quickly & easily.

  • Ensure all outside windows and doors are CLOSED. One window is all it takes for your system to begin sucking in warm humid outside air which will lead to sweating!
  • Ducted air conditioning systems are normally designed to cool up to 70% of your home at any one time. So, if you have too many zones turned ON your system will work too hard and not cycle off to allow the componentry and grilles to return to room temperature.
  • Make sure you have your desired temperature set to a reasonable temperature, we suggest 24°C. If it is particularly hot outside you should consider raising the desired temperature a little more whilst sticking to a limit of 10°C lower than the outside temperature. For example, if it’s 36°C outside, ensure your thermostat is set no lower than 26°C. This will reduce your energy consumption and also prevent air conditioner sweating.
  • Take a look at your zone control system and make sure you have at least 2 constant zones for all systems less than 14kW and 3 zones for larger systems.
  • Make you have had your system serviced in the last 12 months.
  • Book in for a deep clean with Smarter Air by calling 07 3067 2396.

If you have any further questions about air conditioner sweating, installation, or repairs, get in touch with our team today.


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