Originally Posted by Lynkage
You can also use an ERV to bring in fresh air.
I will be doing some kind of air exchange in my new theater as well. It will be completely sealed so it will need some fresh air.
Kudos to you for two things. First trying to learn more before buying a unit, secondly for not trying to cobble up the system for the rest of your house just to heat and cool your HT.
Not knowing how big your basement or HT is this. is mostly speculation, but a 2 ton might be too big, 3 ton absurd, 4 tons totally insane. In air conditioning bigger is not better, in fact it's the worst thing you can do. An AC will dehumidify but only while it's running. Oversized units cost more initially for the larger equipment, you will use more energy running larger motors, and they will likely short cycle (<20 min run times) which will not remove much humidity.
The only way to properly size a unit is by using ACCA Manual J. There are some sites on the web that you can use for free or low cost for 1 calculation, or you will probably have to pay a contractor to do one. If a contractor has the (expensive) software and knowledge to use it, he won't show his calculations until you sign a contract with him. This is to keep you from bid shopping another contractor that won't do the calculation. Basement loads are lower for heating and much lower for AC. There are about 20 or so factors that go into a manual J, sq footage, cubic footage, insulation above, below, and around the area, sq footage of exterior glass (doors and windows), your local climate information, area under ground / above ground, etc.
There are a number of really good minis on the market, and some are really cheap ones too. Midea, a Chinese company makes a lot of these units marketed under a lot of labels (including Carrier) and most of the ones you see in the DIY area of big box stores. Mitsubishi, LG, Fujitsu, and Panasonic all make really good units. They all offer variable speed units which I strongly recommend, they will adjust compressor and blower speeds to match the load.You can get units to support up to 4 indoor cassettes. They are very energy efficient partially because of the variable speed, and they use plastic fans as opposed to metal on traditional systems which require less torque and horsepower to run.
The folks that talked about fresh air intake are, I believe, on a good track. Mitsubishi, Fujitsu, and Panasonic I know of, maybe more have a fresh air option if you use a ceiling mount cassette. The ceiling mount cassette looks like one you would see in an RV only much quieter since the compressor will be remotely mounted, only the blower will be running which will be very quiet. The ceiling mounts are less obtrusive than wall mounts in my opinion. The controls on these units are very unique to the brand name, but if you can control the fresh air separate from their thermostat there are a couple of good options. Both Honeywell and Dwyer Instruments make a carbon dioxide control, make sure you get one that has relay outputs. The optimal range is between 400-700 PPM, the lower the better. One of these could be used to control a damper on the fresh air intake.
Something else you might want to look at is a dehumidifier. There are a number of central ducted units on the market that will be totally automatic. You can buy a very low cost temp/humidity meter from Amazon or Walmart and monitor for a while. One of the best times to test is coming pretty soon, what's called "shoulder season", typically in spring or fall depending on climate when your home needs little if any heating or cooling and rainy or humid outdoor conditions. The ideal indoor humidity range is between 45-55%. Correct humidity levels are comfortable and healthy. It helps prevent mold growth, dust mites, and damage to wood floors and furniture.
Home Theaters are likely to be the best air sealed and insulated areas of the home. Depending on the size of the room and number of occupants that might be there for an extended time will quickly increase both humidity and carbon dioxide levels by just exhaling their breath.
Again, kudos for doing your research. Keep researching what will work best for you before you purchase. It's an expensive but long term investment.