So you have that all too-common problem of your top floor not cooling well, even with the central air conditioner on, or, you simply don't have air conditioning. Most cities in North America get very hot during the summer, even my own Toronto. We just had a streak of 35/36 degree days (95-100 F). Combine that with the humidity, and it's enough to make anyone sweat.
Well, the first thing you should do is decide what size of air conditioner you need. By size, I don't mean the physical size, but rather, the cooling capacity of the air conditioner. They come rated typically for 5000 BTUs, 7000 BTUs, 9000 BTUs and 12000 BTUs. BTUs are a measure of heat removal capacity (BTU stands for British Thermal Unit), but you need not concern yourself with the intracacies of fluid dynamics, rather only what size you'll be needing.
When it comes to air conditioners, I like to recommend that you buy more powerful than you need. HVAC people recommend that when it comes to central air conditioning, you buy the "right" size rather than have something that is oversized. The reason they say this is because if you have a overpowered AC, it'll cool down your house faster, and so will cycle more. That means there will be more time that the AC is not running, and your HVAC system is recycling air that is more humid. In other words, they think that your air conditioner should run for a longer period of time so that it dehumidifies the air more.
The problem with that is that there is more to AC needs calculation than simply square footage or even volume. There are so many variables in the equation, like the type of siding or brick, vapour barrier effectiveness, R-value of insulation, distribution of vents, airflow and others, that these guys almost never accurately predict the correct size of air conditioner. Which is why so many of us are left with upper floors that are positively hot.
When looking for a portable AC, remember that you are not only battling the heat, but also crazy powerful UV rays from the sun. So buy a more powerful one when you can. Right away, I can tell you, I advise not to buy a 5000 BTU or 7000 BTU unit. They are simply not powerful enough for anything but the smallest rooms. Even a small room with a normal-sized window might be too much for one of these units simply because of the heat that the UV rays contribute.
I recommend going with a 9000 or 12000 BTU unit. Ideally, I'd say 12000 BTU, but they can get pricey. Usually, 9000 BTU units offer the best compromise of price and cooling ability. Remember that since it's a portable AC, even if you're not planning on moving it much, situations might come up where you move it, so the unit's ability to cool rooms of different sizes is important. Plus, you want your room to cool down as fast as possible, correct?
Finally, down to power. You may believe that the less the BTUs, the less power consumed. Generally, this logic may be correct, but remember, the 6.2 L V8-powered Corvette can get up to 30 miles per gallon on the highway, the same as little compacts with four-cylinder engines. The point here is efficiency. Try to buy from a manufacturer that shares either the power consumption, the energy efficiency ratio, or both. Usually, if they're not sharing this information, it's not worthy of being shared. In my reviews, I have come across units that are rated at 12000 BTUs, yet consume less power than many 7000 BTU designs.
The best thing is to do look at what's available to you, then look at all the reviews and information available, so that you can make an informed decision.
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