I am going to purchase Multi-Split air conditioning unit for my apartment that I am fully renovating at the moment. So it is the right time to make wholes in the walls etc.

The supplier is giving an outdoor unit that can sustain 5 indoor units, and I have 3 wall type indoor units and 2 concealed ceiling units (concealed under fake cardboard ceiling with linear diffuser to breath out the air).

One supplier included a separate drain pump for the ceiling units, the other supplier claims to have them built-in to the ceiling units. The wall units don't seem the have drain pumps according to specs.

Now my confusion: What is this drain pump? Is this required so that water doesn't drip from the outdoor unit or is required so that indoor units don't drip water? Am I suppose to get all the drain hoses attached to my plumbing? Is this where the pump is required? How far can I pump the water to reach the plumbing?

So far I have only used a single indoor and single outdoor unit scenario, and some hose seems to hang out from the outer wall dripping water to the street. Is this hose used by the outer unit or indoor unit?

As you can see I am conceptually confused as I don't understand the innerworkings. Sorry for chain of questions. I think the answer to all will be explained once you explain the drainage situation.

Thanks in advance

1 Answer 1


Cold surfaces accumulate water from the air. For an air conditioner, the indoor unit is the cold one that needs a drain line running somewhere to get rid of accumulated condensate (otherwise the indoor unit will drip water). In the case of a heat pump in heating mode, the outdoor unit will accumulate water, but there is no need to pipe that water anywhere else.

Because mini-splits are often installed away from any convenient drains, pumps are frequently used to send the condensate where needed. Depending on the pump used, the condensate can be moved long distances.

With proper installation the condensate can be disposed of into the sewer or often directly outside. Where condensate is disposed and how the piping must be installed is determined by manufacturer specifications and local building codes. Your suppliers should be familiar with the requirements.

  • There is liquid pipe together with the gas pipe that goes from indoor unit to outdoor according to the spec book. Isn't this liquid pipe already removing the condensed liquid from the indoor unit and moving it to the outdoor unit? Nov 18, 2015 at 15:53
  • Those are refrigerant lines. The refrigerant circulates in a sealed loop between the outdoor and indoor units, changing phase between liquid and gas states, transporting heat. The refrigerant flow in those lines is part of the cooling cycle, but has no relationship to getting rid of water that accumulates on the evaporator from the air.
    – user39367
    Nov 18, 2015 at 16:47
  • This is a very interesting point to be confused on. It is the type of error that someone who is not a native English speaker or a very advanced AI algorithm would easily make. Please forgive my presumption, but looking the OP's StackExchange profile I have to ask, is this some type of AI Turing test?
    – user39367
    Nov 18, 2015 at 17:17
  • I don't know how we got to Turing test from refrigerants. Thanks for the answer. I 90% understood the situation. The only part I didn't get is how to actually channel the condensate to my plumbing. Since the wall units don't have pumps water should flow with gravity, hence the pipes should have certain angle. And for those ceiling units that come with a pump, how do I know how far I can go with those? The issue is the plumbing team and the AC team are different guys, and I need to coordinate them. Nov 18, 2015 at 21:02
  • The manufacturer on the ceiling units should have condensate pump specifications (maximum lift, maximum run length). I am glad to know that you are a human. I mentioned the Turing Test because most people do not mistake refrigerant lines for water condensate lines. I can understand how there could be some confusion if you have never seen either one.
    – user39367
    Nov 18, 2015 at 21:59

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