Written One potential problem with portable air conditioners is the condensation back flow which should be exhausted outside through a hose - dirt and debris can accumulate if not properly cleaned; depending on the design. A unit used for one season shouldn't be a problem, though, and they generally last quite a long time - essentially indefinitely if it's well designed and constructed, and properly maintained. Buying used is always a crapshoot so it's really a value-judgement decision: is saving $100 amortized over a few years worth the risk of potentially buying a lemon without a warranty. I'd feel comfortable enough to buy used because I feel sufficiently knowledgeable to know what to look for and fix it if it breaks - a risk I'm willing to take.    EASY ANSWER:Plug it in, and make sure it is working!NOT SO SHORT ANSWER:Bring a thermometer to take the temperature of the air. You want to measure the Outside Air temperature, the Condesner Air temperature, the Supply (cold) Air temperature, and the Inside Air temperatre. 

You want to measure that heat is being transfer across both coils, and that the unit is doing work.Example:80F Outside Air95F Cond. Air Temp= 15 degree difference70F Indoor Air Temp55F SUpply Air Temp= 15 degrees differenceYou are rejecting heat from inside to outside and doing work.  air conditioning repair in salt lake cityExample of no work being done, but "feeling cold air"60F Outside Air70F Cond Air Temp=10 degrees80F Indoor Temp70F Supply Air Temp= 10 degreesYou seem to be doing a 10F split across both coils, so WOHOO, BUT WAIT!!!!air conditioning units for the garageIf we think about this more clearly we can see the outside temperature is colder than the indoor temperature and we are unable to cool the indoors, AND have a paltry 10F  split.  heat pump hvac definition

So what is likely happening is that your AC system is acting more like a dry cooler, meaning that you are cycling the refrigerant outside, rejecting some heat and bringing it back in, but your compressor is acting more like a pump and this only seems to be working because the outside temperature is so low you are naturally going to have a heat transfer toward the outside.  The Indoor Temp should be 70 and the Supply Temp should be 60 or lower.  Other factors could be present and things could get more complicated as far as heat loads etc.  But the key thing to be mindful of is that the compressor is in the system to force some work to be done, raising the pressure on the high side, so that we can capitalize on this through the evaporator.  So regardless of heat loads and more complicated aspects of HVAC, you should be able to feel that some SIGNIFICANT work is being done and you should bring a thermometer because it is easy to feel the blowing and think it feels colder when in fact you are doing a mere 5 degree split or maaaybe a paltry 10 degrees, but likely from the fan more so than the unit working well.

I live in a flat in an older house and I have the disadvantage of living next to an outer wall. In autums/springs there is a period when the central heating isn't turned on yet, but it's already getting cold outside. During this time the wall (and generally air inside) gets quite moist and mold starts forming. Also the moist air makes it feel much colder than it really is (the thermometer shows 20C, but it feels like 15C). Thus I'm looking at dehumidifiers and air conditioners. The first are cheaper by half, the latter are more powerful (according to spec) and can also be used for cooling the air in the summer. But I really wonder how much I can trust the spec and how much it would really help. Does anyone have any experience with this? I would consider a dehumidifier which will pull moisture out of the air. I'd also check into why the wall is getting moist. Are you sure there is enough insulation in the wall, because you definitely should not have moisture condensing on or in the walls.

An air conditioner will make the room colder which you probably don't want if the weather is already turning cold. The dehumidifier will pull the moisture out of the air without cooling the air the way an air conditioner would. For the time being the dehumidifier should pull moisture out of the room for you. The specs for the appliances (dehumidifier or air conditioner) are usually fairly accurate, most notably on larger name brands. I've found knock-off and smaller brands to be a bit sketchy when it comes to performance, so I've learned my lesson and usually go for the larger more established brand names when it comes to things like this. Every air conditioner I've ever seen works not only to cool the air but also to pull moisture out of it. That's often why you get dripped on when you walk underneath a window unit. There might be condensation on the cooling coils, but more often than not the "conditioning" of the air includes bringing it down to a comfortable humidity: literally pulling moisture out of the air and draining through a pipe -- often that part of the a/c's job is more important than altering the temperature in terms of comfort.

So yes, an a/c will be more expensive than a dehumidifier, but that's because it does both jobs: cooling and reducing moisture in the air. I'd go the double-duty air conditioner route, but then again, I live in Florida and we really can't get enough a/c down here! Also, I've seen dehumidifiers in action and I wasn't impressed. You usually have to manually empty the water collection area, and even then it really doesn't do all that great of a job lowering the humidity. I use my Sharp portable AC to dehumidify my man-cave/office fairly often, it works really well - and doesn't require any condensate drains or tank emptying nonsense. Really all the machine is doing is running the AC at a minimum setting to remove the moisture from the air, but this is the most effective method of dehumidifying a room - short of purchasing a dedicated dehumidifier which realigns the airflow path across the condenser and evaporator to give you warm/dry air. Or as a second alternative - buy a pallet of desiccant packs and spread it around the room :)

In all seriousness - I actually do something like that in my motorhome (I bought a big desiccant holding "bucket" from West Marine and it does a decent job keeping the dampness out, but you have to replace the pellets and drain the water periodically, so it's kind of messy). Just for a more concrete example, we have a portable air conditioner in a 15'x15' finished room (in a fairly new house) that's currently filling a 5-gallon bucket every 12 hours (currently investigating draining it to the outside), so using them as a dehumidifier will certainly work. Ours also has a mode to dehumidify without cooling which seems like what you were interested in. Dehumidifiers tend to produce a fair amount of heat, which is fine if you're only worried about cool weather. If you think you might also use it in the summer then you might want to go for the A/C unit. Even if you weren't going to use the cooling feature, at least you can exhaust the heat it creates to the outside. (I haven't seen any non-A/C dehumidifiers that have exhaust connections.)