Thank you coxman,
That kind of misinformation drives me up the wall. I was about to respond and saw you beat me too it.
Most of what has been said here is correct. However a few adders.
First, the design temp for the midlands SC is 96/76. That's not 96 outside 76 inside. Thats 96 outside dry-bulb, 76 outside wet-bulb. (Wet bulb is a measure of the humidity level spoken in terms of degrees instead of %. Normally the temp difference a properly sized unit can hold inside to outside is around 24 degrees. basically what was stated here.
the 3 degrees per airchange is not a value I've heard but it seems possibly within reason.
I do know that when air pulls through the unit it can usually drop the air temp around 23-24 degrees. So, say your stat i set to 75, and the unit is on so the room is not currently satisfied, so lets say its 76. Well, air is being returned to the unit, maybe from a high place in the room, at 77 degrees. It gains heat as it moves down the duct towards the unit, so it reaches the unit at 78-79 degrees. That air should discharge at between 53-56 degrees. Hence the 23-25 degree drop. If the unit is producing 53-56 degree air right at the registers then the unit is doing all it can do.
If it is doing that and the room is running 80, then the house is leaking in too much heat, or the unit is undersized.
If the air at the register discharges are at 60+ then the unit is not performing well.
(Disclaimer, if you just returned from vacation and your house is 85 inside, and its producing 60 degree discharge air, that is GOOD, cause again DX equipment can only drop he air about 23-25 degrees. I mean, if its mid 70s inside and you're seeing 60+ at the supply register, that could be a problem.)
If you're seeing 58-ish air, well that's borderline and could mean the unit isn't quite charged right, or it could mean a dirty coil either inside or outside, OR it could mean your supply ducts are seeing notable heat gain between where the air leaves the unit and when it reaches the supply register.
Note, if your discharge air is too warm, then its not removing enough moisture from the air, and that'll cause your room to feel warmer at 75F then if the air was drier at 77 degrees.
Everything said about sizing too big a unit is true. It will cause the compressor to short cycle, the refrigerant can slug and shorten equipment life.
ALSO, While I'm standing on the soap box, NEVER use those stupid expensive filtrete filters. I HATE those things and the BS marketing behind them. Those things will kill an AC unit. Look up the pressure drops of your filters online.
The average ENTIRE residential AC unit can handle only about 1.5" of total static pressure drop from the fan motor. The Coil and box of the unit account for around 0.6". Your entire supply duct system accounts for about 0.5", and your return duct system accounts for about 0.2". That leaves just 0.2" for your air filter and ultimately your dirty air filter.
A basic clean blue mesh air filter has about 0.05" drop when its clean. When dirty, about 0.2". So, when that filter is dirty, it is MAXING out your fan.
Those expensive filtretes are about 0.25" CLEAN, and about 0.5" dirty. So, the moment you install it your unit fan is working too hard and burning the motor. As it gets dirty it gets worse. It also slows down the amount of air crossing the coil, which causes the refrigerant thats moving back to the compressor to be "too cold" compared to how it should be. This means some of it is still in a liquid state vs a gas state. This causes compressing slugging. (like slamming your fist into water, (it hurts) and will eat up your compressor.
DON'T do it, you've been warned!
Lastly, for anyone interested, you can measure wet bulb temp by taking a papertowel, soaking it in water, wring it till just drier than dripping, wrap it around the mercury end of a thermometer, and see what the temp reads. As the moisture evaporates, it'll hold the heat temp down on the mercury and give you an accurate wet bulb temp. Note: Charleston has a lower drybulb design temp at 94. but a higher wet bulb design temp at like 78-79, because of the higher humidity on the coast, than here in Columbia.
Oh, ONE MORE thought. Be careful misting your outdoor condensers. Those things were not designed to run in a wet environment. Even your city water supply has chlorine and lime stone and other corrosives that you're just spraying right on your coil fins and they will corrode it faster. No question a unit installed near a water body (coastal town) will rust before a unit in a dry area, and thats the reason. Everything is corrosive and rusts to some degree but moisture is a solvent and accelerates that process.
Furthermore, again by dropping cold water right on a coil, it again permits the possibility to slug the compressor which is unhealthy as stated above. A big tree above an outdoor unit will allow it to run in share so it takes on less heat, but yet allows it to vent its heat away fairly easily. Never build a structure close to or above the unit because it needs to be able to blow the heat away from it so more "cooler" air can come in to take more heat from those coils.
FINALLY LAST, YES A heatpump running in the summer months is EXACTLY the same function process and equipment as an airconditioner that has a gas furnace for heat. The only difference is a heatpump has a reversing valve that switches modes for winter and instead of pumping heat out of your house, it tries to pump heat into your house from the cold winter air outside. DONE!!
