Single Hose vs Dual Hose

If you pay close attention to the features of a portable air conditioner you’re considering you might see a reference to “single hose” or “dual hose” configuration. What is that, and which is better?

As usual, there’s a little more to the answer than “pick THIS, never THAT.”

Why An Exhaust Hose At All?

All portable air conditioners take air in one place and exhaust it out another part of the unit. More than that, they’re designed to exhaust it outside the room. You may know that, of course, but have you ever asked yourself why?

It’s because of a basic law of physics. You can never make one place cooler without making another even warmer. In other words, nothing is 100% efficient and any machine will always produce more heat than the amount it cools the air.

That’s why you always feel hot air blowing outside your refrigerator for example and the hot air always has more heat energy than was removed from the inside. So, it’s essential to move that hot air someplace else. Common sense, but now you know why.

Single Hose and Dual Hose – Installation and Costs

Normally, it would be a waste of space to discuss all that but it’s highly relevant to portable air conditioner design and why there are pros and cons to either of the basic hose designs.

The most obvious difference between a single hose portable air conditioner and a dual one is, duh, one has two the other only one. But manufacturers don’t increase the complexity and cost of their products for fun. There’s a reason and it always involves tradeoffs…

A single hose AC unit takes in air from vents. It then cools it by running it through a mechanism similar to the one in your refrigerator. The cool air goes through one set of exhaust vents to, duh again, cool your room. The removed heat is directed out a single hose, by design, through a window kit.

The window kit is a simple metal, plastic, and/or rubber affair intended to be easily placed into a nearby window. Not only is it meant for the installation to be simple and quick but also ‘temporary’. That is, you may choose to leave it there for years but it’s designed to be moved to another room and window if you wish and without a lot of effort.

Naturally, if you have two hoses rather than one, the cost of the AC unit is going to be higher and the installation process more complicated. It can be a little or a lot higher, a little or a lot more complex depending on the design but there will be always a difference.

Functional Pros and Cons

The main operational differences between a single hose and a dual hose configuration are where the rubber really meets the road, though.

A single hose system pulls air from within the room, the compressor/condenser components cool the air, then the fan blows most of that air back into the room. A small amount of the room air however will be used to cool the air conditioning unit itself. That room air is then vented out through the exhaust hose. That design has two potential drawbacks.

Exhausting part of the room air outside tends to create what experts call “negative pressure” in the room. That is, if you withdraw some air from the room, that lowers the pressure in it just a bit. To make up for the lost air other air from adjacent spaces will now attempt to replace it – that is air that infiltrates from underneath doors, from gaps around windows or through small cracks and holes in ceiling, floor, and walls.

That air is almost always, WARMER than the air inside the room (since the idea is to have your room cooler than elsewhere). That undercuts the cooling effect; at the same time the AC unit is generating cool(er) air, it is drawing warmer air into the room. That slows down the rate at which the room is cooled. It also reduces the efficiency of the cooling process. So, it requires more electrical power to cool the room to the desired temperature.

If that efficiency is lowered enough the room may never reach the desired temperature. If, for example, you’re using the portable air conditioner in an attic of a two story house – where a lot of hot air tends to get trapped – your single hose unit may not have a high enough BTU rating to do the job. Having a few computers or other heat generating devices (such as a small refrigerator) in that room only makes the job harder.

By the way, even if your room is so well sealed that practically no air from outside can be drawn in you don’t get a ‘free lunch’. That only tends to make the air conditioner work harder to draw in the air, again requiring more power to do the same effort. That’s beside the fact that the air is going to get pretty stale – oxygen reduced – pretty fast.

By contrast, a dual hose system draws air from outside the room through one of the hoses. That eliminates negative pressure. The other hose blows the exhaust out the window like a single hose unit.

That design has a nice side benefit, though one that’s intentional, of cooling the room quicker than a single hose design, all other things being equal. Balancing that benefit is the fact that a dual hose system is sometimes somewhat less efficient than a single hose one and requires more electricity to run.

But Which to Choose?

In the real world as opposed to the theory of design, all other things are not always equal and “less efficient” can be a lot or a little. So, while it’s very helpful to consider the question “Single Hose or Dual Hose?” there are many other things to consider when selecting a portable air conditioner.

It would be great if it were possible to simply say “always buy a dual hose portable air conditioning unit” and be done with the topic. But differing costs, BTU ratings, EER ratings, manufacturing quality, and other factors always play a large role too.

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