Air Conditioner Drain Pipe Warning

We had our roof gutters and solar panels professionally cleaned last week, and that night we had around 30mm of rain which resulted in water being all over the kitchen bench tops on either side of the sink and on the lounge room floor.

We expected that it must have been due to the work on the roof and called the business who inspected the roof and thought it must have been caused by the air conditioners, so we called our plumber.

The plumber very promptly resolved that the problem must have been caused by the downpipe that both air coditioners drain pipes connect to backing up and forcing water back up through the aircon drain pipes, and he recommended that we get a Y fitting installed with the cap sitting on it loosely.

A couple of nights later, we had around 75mm of rain and the cap was upside down on the ground but no more flooding in the house.

Whilst building inspectors, pest control operators and air conditioning service people strongly encourage property owners to plumb aircon drains into the storm water, our plumber is strongly opposed to the idea.

Fortunately, all the common areas of our residence are tiled so no water damage.

Unfortunately, the plumber suspects that the stormwater pipe leading out to the gutter must be restricted and we will need to get a camera inspection.


When I had my air con installed they added a breather pipe . This would also handle any water that backs up .


As I see it, the concept of plumbing condensate drains from air con into storm water drains defies logic. The volume of condensate from an air con is really quite small and running this drain into the guttering works well and as a safety feature you can stand close under this drain and listen to make sure that it is clear, thereby avoiding minor flooding in your house. A trick of the trade is to have the pan drain - always installed as a fail safe if the main drain blocks - draining a reasonable distance from the air con unit drain so you can tell which one is draining.


In your photo @Fred123, is it possible that the two air conditioner drains poke across the downpipe (instead of dropping downwards as does for @vax2000 ) causing an obstruction?

Then, if you have leaf and other litter washing down the downpipe, and the protruding drain pipes catch the litter you could have had a nice little blockage happening.

With the Y junction you should be able to see clearly how far the drain pipes extend into the downpipe.


The plumber actually checked the downpipe for blockages and even connected the hose to the bottom of the 2 aircon drain pipes to check that they were clear, which they were, which resulted in the kitchen bench and the lounge room floor being flooded for the second time, much to my wife’s horror

We will just have to get the camera inspection after Easter to find the restriction.


It is seen as an issue in areas with termite problems such as coastal FNQ as moisture near the perimeter of a building apparently encourages them, so it is advised to not have any garden beds adjacent to the buildings and not discharge any water near the buildings.


The volume depends on where you live. If you live in the sub-tropics or tropics as does @Fred123 , there is considerable humidity to be extracted.

On upper floors it is feasible to drain onto roofs, or into drains, but if the air conditioners are on the ground floor in these humid regions, the condensate is channeled to downpipes whenever possible,

Possibly in cooler and less humid climes, where wee gremlins aren’t trying to chew you out of house and home, the condensate can just be allowed to run free?


I’m familiar with ducted systems which have the evaporator in the ceiling space. Hence logic says drain into the guttering for the reasons I listed. I admit that I neglected the split system types which need to be drained below gutter line.

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Last night, we had very heavy rain, and when I checked today, the cap had landed 1 metre from the downpipe.

Without our plumber’s smart thinking, the house would have been a real mess today.

I must follow up on the camera inspection on Monday.


When I got up yesterday morning, I noticed the carpet under our bedroom aircon was wet despite the aircon having been professionally cleaned a few months ago.

Sure enough, the drain pipe from the outlet to the stormwater downpipe was full of the jelly like sunstance which builds up, which I cleaned out with the garden hose.

As they say, it never rains but it pours, And even when it doesn’t, our aircon dramas continue.



We live in a sub tropical area and have had split system aircons for 15 years. Two were replaced just last year. The best piece of advice from an aircon cleaner was to put some liquid bleach in the collection tray in the internal head of a split system. The slim, mould, mildew and gunk builds up around the small water outlet preventing it draining. Ask your cleaner to show you how to access it. It’s true that a major blockage of your drainage system will cause back flow but also check how old your units are. Ageing units tend to produce excess condensation around the condenser pipes due to the type of refrigerant. Those who are reluctant to use bleach because of environmental issues, please search Choice for their advice.