Tech Talk - Drains - How your ducted air conditioning drain works

March 21, 2018

All reverse cycle air conditioning units heat, cool and dehumidify; with this comes water. The water produced by your air conditioner is the condensation build up from the evaporator coil. Reverse cycle units change over between heating and cooling, the condensation can be formed by either the indoor OR outdoor unit. When you unit is cooling, water is formed at the indoor unit and during heating, the unit creates water at the outdoor unit.

The science of condensation water formation is simple; the air conditioning coil drops in temperature in order to absorb heat (making the air cold), this causes the temperature of that coil to fall below the dew point of the air. As the temperature drops below the dew point the water vapour in the air liquefies causing water to form. This is exactly the same as a cold water bottle becoming wet once removed from the fridge.

Where does the water go?

The water that your air conditioning unit produces needs to go somewhere or you will end up with a water feature pouring through your ceiling. The water from the indoor unit needs to be efficiently drained away. These drains are installed from the indoor unit and generally run into the roof gutters. Most drains use gravity to remove water. This means the drain needs to run downhill and conform to the manufacturer’s specifications for fall (the drop the drain needs to make each meter).

Typical types of drains

So it’s simple, install a drain pipe and put it into the gutter, problem solved….well sort of. There are a few different drain configurations and unit requirements that need to be considered when installing the correct drain.

Negative pressure drains - This is the most common drain found on most ducted reverse cycle air conditioning units. The drain connection is installed on the negative pressure of the indoor unit (the return air side of the unit). This means air can actually be sucked up into the unit through the drain while the indoor fan motor is running causing an airlock and a leaking unit. When installing a drain on a negative pressure unit, a P-Trap MUST be installed to prevent an airlock. The P-Trap holds water inside the drain pipe preventing air to be sucked up the drain. P-Traps also prevent smells coming up through the drain into the indoor unit.

Positive pressure drains - Positive pressure drains are less common but there are still plenty around. The positive air pressure from the indoor unit actually pushes water down the drain of the indoor air conditioning unit. These drains do not necessarily require P-Traps but are still recommended.

Integrated drain pumps - Some units are manufactured with integrated drain pumps within the indoor unit. These pumps are designed to pump the water from the indoor unit to a gutter or sometimes onto the roof or a drainage point. There are still restrictions on the abilities of these pumps and care needs to be made to ensure the drain is installed to manufacturing specifications to ensure they work correctly.

Aftermarket drain pumps - Some indoor unit locations have no ability to drain water using gravity. This situation may require an aftermarket drain pump to be installed to remove the water and pump it uphill to a suitable drainage point. These pumps have a maximum height and length they can pump water. These pumps come in different shapes and sizes and need to be matched to the size of the unit and the amount of water it creates per hour. A small pump cannot handle the amount of water a large ducted unit produces and will ultimately fail and leak.

Attic/safety trays - This is simply a metal pan that is installed under the indoor unit. Safety trays are designed to collect water from the indoor unit ONLY IF the main drain fails to work. Safety trays are not designed to constantly hold/drain water and should only be used if something is wrong. The safety tray also needs a drain run to a gutter or drainage point and should be completely separate to the main unit drain to ensure it works independently in an emergency.

Drain installations 101

There are a few common-sense things to remember when installing or repairing air conditioning drains. It is surprising how many drains we come across that have not been installed correctly. Just some planning and care when installing the drain will prevent most leaks from occuring.

  • Water runs downhill! When installing gravity drains (which is most units) the drain pipe needs to be higher at the indoor unit then it is at the gutter or drainage point. This also needs to comply with the manufacturing specifications. All manufacturers will specify how much fall the drain requires to work correctly. If the drain does not have enough fall, it will not work properly and leak.
  • Size the drain pipe correctly. So many drains are not large enough to handle the water produced by the unit. The issues with an undersized drain pipe is obvious and is made worse if the the small diameter drain blocks.
  • Install a P-Trap, and a good one at that. There are lots of ‘pre-made’ P-Traps available. These are designed to make installation quick and easy but if not installed correctly, can cause issues. We opt to create the P-Trap from 4 x 90 Deg elbows and cut lengths of drain pipe. This allows the P-Trap to hold more water and ensures it prevents airlocks.
  • Separate the unit drain and the safety tray drain. So often we see both the indoor unit and the safety tray drain joined into 1 drain which is then run to the drainage point. The safety drain is designed to remove water if the main drain fails so if they are joined together and the pipe blocks, the safety tray will be rendered useless. Running a completely independent safety tray drain will ensure the safety drain serves its purpose well.
  • Drain pumps have limitations. Drain pumps overcome difficult installations that cannot utilise a gravity drain. Installing these drains must be done correctly to ensure they work correctly. The height and length of the drain is limited to the pump used. If the drain exceeds the pumps capabilities, it will not work correctly.

In a nutshell

  • Drains are simple but can cause huge amounts of property damage if not installed correctly.
  • Sizing the drain to the correct application ensures reliability during use.
  • Drain pumps are useful but must be selected based on the application of the pump.
  • Safety trays are designed as a last resort to prevent water leaks. They should be installed completely separate to the main unit drain.

Need Help?

T&K Airpower have been working to keep Adelaide homes comfortable for the last 20 years. We specialise in Temperzone, Hitachi and Mitsubishi Electric air conditioning units and know how they tick. Do you have issue with your air conditioning not draining or leaking? Give us a call and we will be more than happy to help.

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