Why Do Some Portable ACs Have Two Drains?

If you take a look at the back of your portable AC, you might see two drain ports: one on the top and one on the bottom.

But why are there two drain ports, anyway? Do you need to empty both of them? And which one do you connect the drain hose to?

The upper drain is used to connect a drain hose for continuous draining, while the lower drain is used to empty the condensate collection pan or tank.

In this article, I’ll go over the difference between the two drain ports that are on some portable ACs. You’ll learn why only some portable ACs have two ports, and why you don’t really need to hook up a drain hose to either of them!

Self-Evaporating Portable ACs Have Two Drains

The upper drain on a portable AC is meant to be connected to a drain hose for a continuous drain setup.

A continuous drain hose is only needed in very humid areas, or if you’re running your portable AC in dehumidification mode.

The lower drain port on a portable AC is used to drain all the water out of the AC when moving it around or storing it after you’re done using it.

You shouldn’t need to empty the water out of the lower port unless you’re moving your portable AC, storing it, or if it fills up with water and stops running.

Diagram showing the difference between the two drain ports on a portable AC

The upper and lower drain ports on a portable AC are connected to two separate drain pans – the upper drain pan and the lower drain pan.

  • The upper drain pan collects condensation from the evaporator coil.
  • From the upper drain pan, the condensation flows into the lower drain pan where it pools.

So why does the condensation just flow down to the lower drain pan and pool up?

Self-evaporating portable ACs have a device called a condensate slinger.

The condensate slinger or “splasher” splashes water from the lower condensate pan onto the condenser coil. That’s why some portable ACs sound like a waterfall when they run!

The splashing of water serves two purposes:

  1. The splashing helps cool down the condenser coil and increase the efficiency of the portable AC.
  2. The splashing of condensate on the warm condenser coil will evaporate the condensate and blow the vapor outside through the exhaust hose.
condensate slinger splasher in a self-evaporating portable air conditioner

So that’s why self-evaporating portable ACs always have a little bit of water in them – the splashing is what makes portable ACs self-evaporating!

But that brings us back to the upper drain port – why is it even necessary? Doesn’t all the water from the upper drain pan just flow to the lower drain pan anyway?

The upper drain port serves as a condensate drain in case too much condensation is produced by the evaporator. This usually only happens in humid climates, or if you run your portable AC in dehumidification mode.

That’s why you connect the drain hose to the upper drain port, not the lower one. The portable AC can do its job to remove most of its condensation by evaporating it from the lower drain pan and blowing it outside with the exhaust air.

The upper drain port just serves as a “backup” drain to be used used in humid climates.

Self-evaporating portable ACs need some condensation in the lower drain pan. If you connect your drain hose to the lower drain port, then the self-evaporating feature of your portable AC won’t work!

diagram showing how a self-evaporating portable AC works

Which Drain Port Do You Empty the Portable AC From?

If your portable AC has two drain ports, you only need to empty the bottom drain port when moving or storing the AC.

It won’t hurt to empty your portable AC from the upper drain port as well – but you’ll find that not much water will come out since most of it drains down to the bottom pan.

However, if you live in a humid area or find it necessary to set up a continuous drain hose on your portable AC, then you should empty the upper drain port since your portable AC will collect lots of moisture.

Do You Need Two Drain Hoses for Portable ACs With Two Drain Ports?

You do not need two drain hoses for portable ACs with two drain ports. You only need one drain hose attached to the upper port.

Most times, you won’t need any drain hose at all – unless you live in a humid area.

Portable ACs with two drain ports are self-evaporating – so they automatically drain themselves by evaporating the condensate and blowing the vapor outside through the exhaust hose.

What Happens if You Don’t Use a Drain Hose on Your Portable AC?

If you have a self-evaporating portable AC, then you don’t need to use a drain hose unless you live in a humid area.

If your portable AC is not self-evaporating, then you will eventually need to empty its condensate tank from the drain port on the back of the portable AC.

So how do you know if you have a self-evaporating AC?

The easiest way to tell if your portable AC is self-evaporating is to check if it has two drain ports. The upper drain port is used to connect a drain hose, while the lower drain port is used to empty its lower drain pan while moving or storing the portable AC.

Hi, my name is Trey Lewis and I’m the founder and chief editor at HVAC Training Shop. My goal for this website is to help homeowners troubleshoot and maintain their home’s HVAC systems. Whether it’s changing an air filter, troubleshooting a blower motor, or just buying a new humidifier, I want to make sure that you’re covered.

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