Monday, May 17, 2010

Should You Buy a Portable Air Conditioner?

Portable Air Conditioners can be useful, but they are limited in their capacity and efficiency.

I bought two portable air conditioners several years ago. We bought the lake house, and it had no central A/C, just one split system unit. In that area, you rarely need air conditioning, but that year, there was a heat wave, and temperatures soared into the high 80's. In the later afternoon, temperatures could soar in the house to nearly 100.

As I noted in my Hydronic Heating post, adding central A/C to an existing house that does not have a forced-air furnace can be a nightmare. Running duct work after the fact is expensive and cumbersome and moreover cuts into closet space or other living space. In most cases, it simply is impossible.

I used split system air conditioners for most of the house. But on the top floor, adding split systems would not have been easy. Running power to that level of the house would be tricky (without tearing into a lot of the sheet rock) and placing the condenser units would be hard as well, without resorting to roof or wall mounts.

Window units were out of the question. Too ugly, to begin with, and hard to adapt to casement windows.

Since the top floor was used only for guest rooms, I decided to use portable Air Conditioners.

What are portable air conditioners and should you use one? Online websites are full of complaints about these devices, mostly from users who do not install or use them properly. Many folks buy these units and think they will cool down a whole house, and then get upset when they barely cool a small room.

But if you read the fine print on these things, you'll realize that they are not designed for much more than cooling a bedroom. And yes, they can be noisy, due to their design.

These units, which look like the R2-D2 units from Star Wars, usually sit on casters. They have a compressor built in, along with a fan and evaporator and condenser coils. A flexible duct is used to discharge the warm air from the condenser coils to the outside, so you can reject heat from the room. This flexible duct may be routed through a window with an adapter kit, or through a hole in the wall using a dryer-type vent (more on that later).

The first mistake people make is not venting these units. Some folks have no clue how A/C works, and they assume the machine will just "make cold" for them. But what they fail to realize is that an air conditioner is a heat pump - it moves heat from one place to another. In order to cool a room down, you have to reject heat. So you need to have that pipe vented to the outside, or the room will not cool down. In fact, the heat from the compressor will make the room warmer than normal, if it not vented to the outside.

Some units are advertised as non-venting or having a non-venting option. They basically cool down a room by heating a corresponding amount of water. You then have to drain the water from the unit. This is not a practical solution and the amount of cooling such units provide is pathetic. Without a duct to the outside world, to reject heat, no real cooling can take place.

Second, you have to look at the ratings on these units. Most are about 5,000 BTUs or so. This is not a lot of cooling capacity. While these units may be physically large and heavy, they do not provide a lot of cooling capacity. 5,000 BTUs is enough to cool a small bedroom, not a living room or family room, etc. So many folks buy one of these units, plug it in, and wonder why it won't cool the main room in their house. It just isn't large enough.

For my guest bedroom, it works OK, provided you leave it on all day, if you are expecting guests. Like any A/C unit, it has its limits, and trying to turn it on in an 85-degree room and expect the temperature to drop to 70 degrees in 5 minutes is unrealistic. By the way, the same is true for central air. People invite 30 guests to their home for a summer party and then wonder why, after the room heats to 90 degrees, the A/C won't cool the house. It simply can't keep up with the load. Cold soaking a room in advance is the best solution for both situations.

Third, you have to install them properly. I used a dryer vent on both of mine. The instructions warn against doing this, as it could constrict the flow of air. However, I have a short run of tubing (less than a foot) and it seems to work OK. If you try to run 10 feet of tubing to vent these units, it probably will not work well. The less restriction to air flow, the better.

And in that vein, most of these units only discharge hot air. The higher priced units have an intake as well. If your house is "tight", the fan may have a hard time blowing out hot air if no air can come back into the room. So ironically, you may have to crack open a window slightly to let in some air for the unit to blow back out again. Otherwise, you are trying to pull a vacuum on your bedroom, and that isn't going to work.

Fourth, these units are not very efficient. Unlike a window unit, they don't have the free flow of air across the coils, so the efficiency suffers. As a temporary solution for renters, or for occasional use (like my guest room) or for supplemental cooling, they may be a solution. But they will use more energy than even a window unit and certainly more (per BTU) than central air or a split system.

So, if you are looking for a permanent, full-time solution to your A/C needs, I would not recommend one of these units, but instead look to something more substantial.

Fifth, these units have no condensate drain. All A/C units generate condensate, unless you live an a dry desert climate. As the air passes over the evaporator coil, it falls below dew point, and water condenses on the coil. For a window unit, this is not a problem, the water merely drips outside. For central air, the condensate either drains into a basement drain or is pumped outside by a condensate pump.

Portable Air Conditioners generally have a drip tray. Older units had to be emptied, like a dehumidifier, once the tray or tank was full. In fact, portable A/C units and dehumidifiers are close cousins. An A/C unit is just a vented version of the dehumidifier. And some portable A/C units are sold as combination units (as dehumidifiers when un-vented).

Newer units exhaust the condensate water through the vent by using heat from the compressor to evaporate water prior to discharge. It is a pretty clever invention, and for the most part, it works. When you first cool down a room, particularly on a humid day, it may fill the drip tray (which will shut off the unit until it is emptied). But once a room is "pulled down" in terms of temperature and humidity, the units generally do a good job of exhausting the condensate as humid warm air out the discharge duct.

So there are limitations to these units, and they do operate in a slightly different manner than traditional A/C units.

Many of these units are used by IT professionals to cool computer rooms. In many office buildings, A/C controls are inaccurate and allow smaller rooms to get warm. You can duct a portable A/C unit through the ceiling (blowing the hot air into the crawl space there, which is usually a return air flow) and thus keep a critical computer room cool. Not horribly efficient, but when you are renting office space, trying to get the central chilling plant to keep one room at a critical temperature is often difficult, if not impossible.

So these units have their uses. For a small room that is only occasionally cooled, where a window unit won't work, physically or aesthetically, these portable units can work well. If you keep their limitations in mind, they will work well for you. If you don't understand the nature of these units, however, you may end up being disappointed.

1 comment:

  1. Note: I get a number of hits on this posting on two topics:

    1. Can I vent my portable air conditioner up a chimney?

    Answer: Maybe, but I doubt it. The fan on these is not strong, and it may not vent the air out that far. However, since it is hot air, it may rise of its own accord.

    2. Can I vent my portable air conditioner into a crawl space?

    Answer: Yes, but unless the crawl space is vented, it will get very warm in there and heat up the floor of the room above, perhaps defeating the purpose. Make sure the crawl space vents are open and even then, well, it might not work very well.

    Again, these units need to reject heat, and their fans are not designed to handle long duct runs or resistance. The most direct venting is the best.

    ReplyDelete

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