With the summer weather approaching, many of us without central air conditioning (A/C) might want to consider buying a portable air conditioner.   Even if you do have central a/c, it may be advantageous to consider buying a portable a/c because it costs much less in electricity to condition only one room instead of a whole house. 

Portable a/c units are designed to plug into your home’s regular 115 volt electrical system.  They have wheels that allow them to be moved throughout the house as needed.   In moderate climates like here in the SF Bay Area, air conditioning is needed primarily in the late afternoon and evening when the daytime temperatures climb into the 90’s (or higher!).   If it’s not that hot, it’s usually sufficient to open the windows and use natural ventilation to bring in cooler outside air.

So what are the main things to be looking for when choosing a portable a/c unit?  Noise is a primary consideration, especially if the unit will be used in the bedroom.   Check the specifications for a noise rating.  Keep in mind noise is somewhat difficult and subjective to quantify.   Also manufacturers tend to pad their numbers since nobody is checking.  Most of the time, the noise level is listed in dBA.    An average rating for a portable a/c unit would be about 60dBA.  Again, noise is a subjective thing.  While some people may be sensitive to the noise the fan makes, others will find the compressor noise more objectionable.  The dbA scale kind of averages all the frequencies together into a single number rating.

But just as important is the operating efficiency of the a/c unit.  For this reason I strongly advise selecting only those units that have two exhaust hoses.   You will find that most of the models on the market today only use one exhaust hose.  Having an extra hose obviously adds more cost to the unit.

Why are two hoses better than one?  Understanding how the unit functions will shed some light on the importance of using two hoses instead of one.  Every a/c unit for portable use functions the same way.  They work by taking the heat in your room air, and internally rejecting that heat to another air source, usually air outside the home. 

If your a/c unit only has one hose, then this hose is used to route the hot air that was heated up internal to the a/c unit as a result of the heat rejection from the home.  In the situation of a one hose system, the unit forces the air from inside the home to the outside of the home.  This creates a negative pressure inside the home.   That causes air from outside the home to be drawn inside through any opening or cracks.   If the single hose a/c unit is inside a room with the door opened, air from the rest of the house will be forced into the conditioned space, and air from outside the house will be drawn into the rest of the house.  In both cases, this defeats the purpose of cooling the air in the room or house.  The rest of the house will get hotter than it would if we simply kept the windows and doors closed.  

However, if you have a two hose a/c unit, the air flow to the room is balanced and there is not a negative pressure created in the room.  This is because the second duct is bringing in outside air instead of using conditioned room air for the air stream that is needed to reject the heat from the room.   The end result is a much higher efficiency system and that keeps the room pressure from becoming negative relative to the outside of the room.

If you are planning to buy a portable a/c unit, don’t even consider using a single hose design.  It does not work very well at all.  It will feel cool right in front of the unit, but the rest of the room will be just as warm as if there was no a/c at all.