My landlord installed this brand new out of the box. Our apt is 900 sq ft tops. This unit can't even cool down our living room and kitchen. If the whole 2 bedroom apt is 900 sq ft, you can imagine how small our living room is. I'm so irritated. I have a 10 week old baby that is super cranky because of the heat. We have it on HIGH, set at 66, for 2 days and I mine as well have the windows open!!

First of all you need to check if your power supply to unit is correct (120v or 220v).

that is a 10000 btu unit and a good rule of thumb to go by when 'sizing' a unit is 12000 btu for every 600 sq ft of cooling space (if the place is well insulated) or 400 sq ft (if not well insulated).
So, you have a 10000 btu unit which is 5/6th of 12000 btu - so - can reasonably expect that unit to cool approximately 500 sq feet.
In other words the room it's in now could be only - say - 300 sq feet (and well insulated) - in which case the AC unit is cooling it - well.
However, if you try to cool a 800 sq ft room - it will not work as well.
Hope this has helped.

Assuming the unit is 'cooling' but not cooling it 'well,' I would think the unit is too small.
NOTE: 2000 Btu for every 100 sq ft of cooling space if the room is well insulated. If not - then 2000 btu for every 66 sq ft.
For example - a 12000 btu AC unit should cool 600 sq feet of cooling space if well insulated.
or
400 sq feet if not well insulated.
hope this has helped.

There are a few variables that determine the size needed to cool your home - the size of your home, type insulation, are just two. I have a 2600 sq ft home - about 1300 sq ft per floor. I choose to cool the entire second floor (bedrooms and baths) and two of the largest rooms on the first floor (kitchen & living room) via flexible ducts from an air handler installed in my attic. My house was built in 1960, and at the time had electric heat. This means it is fully insulated. I replaced all the windows with energy efficient types, and vinyl sided. I installed soffit, ridge and gable vents to keep my attic well ventilated. I can cool my house in Boston, MA to 70 degrees (when it is 85 degrees inside) with a 4 ton unit in a little over an hour with no problem (one ton of cooling is equal to about 12,000 BTUs). Your condenser should not run non-stop. If it is not cooling then it is not a thermostat problem, but could be a gas charge problem. If you haven't paid the contractor in full yet - that may be the reason why. The contractor should know how much cooling you need for the space you have and installed a properly sized unit. Make sure your air filters on the return are clean and replaced regularly. Call the contractor and explain the problem - he should be able to solve it for you very quickly.

Unfortunately, 7000 Btu's will not cool 1180 sq ft. At best, it's rated for 400 sq ft, as it's only rated as a "Room Air Conditioner". To cool 1180 square feet, you'll need at least 30,000 Btu's (like in Central Air Conditioning) or 4 more window units spread out throughout the condo. It takes more Btu's to cool, than it does to heat.

I really sorry. I wish I had better news for you. Thanks for choosing FixYa.

If you have the model number correct, that is a two ton unit, generally depending on what part of the country and how tight the house is, here in Oklahoma that wouldn't cool more than 1000 sq ft.

That size of unit it should do a good job, but not if its not running.
All AC units get dirty, both coils probably need cleaned, its the first thing that should be done every other year or so, more in dirty locations and less in clean areas.
To clean slide the unit out of the case and take it outside and take a garden hose with a nozzle to it and a solid stream, hose out the coils keep water away from the rest of the unit, and keep a eye on that you do not bend the fins with to much pressure or back off from the coil. Hose the coil start at the top and work across and work your way down. If you get things wet that should not get wet let it dry out.
If it is not making water do not worry the outdoor fan has a slinger that throws any water into the outdoor coil and between the water and any dust that makes a mud on the coil you will see it when you clean the coil. Good luck

First off - the 'rule of thumb' is 600' per ton of Air Conditioning. In other words your old unit is a 2 ton unit. So - 2 tons x 600' = 1200'. As you can see if you install the 2.5 ton unit - you will be installing a AC that 'could' cool a 1500 sq ft house (2.5 x 600' =1500 sq ft.). Slightly more than what you need; and the 3.5 ton unit is 'way to big,' (3.5 x 600' = 2100 sq ft.).

Note: fyi - many in the AC business will sometimes refer to tonnage in btu's, i.e. 1 ton = 12000 btu - hence a '2 ton unit' can also be referred to as a 24000 btu unit and vice versa.

So... from the above - you can easily see that "2 tons" of Air conditioning is what is required to cool the 'average' home of 1100 sq ft. "roughly speaking."

Note: it is always best to have a professional 'size' your cooling/heating needs.

One of your questions was could you 'mix tonnage?'

The answer is 'usually you don't mix the tonnage of your outside/inside units.' However, professionals sometimes do (mix the tonnage) in certain situations, and installing a 2.5 ton outside unit with an existing 2 ton inside unit is often done, however, there are some 'tech issues' here and - I would "again" recommend that you call a Service Tech to help you with the sizing/mixing of your cooling/heating needs.

hope this has helped

There are more factors than sq ft. You have to factor in window surface and R factor of insulation.

Mark is right 5 tons is way too big and you are probably icing up. Even here in Az we size units to no less than 500 sq. ft. per ton and more like 700 sq. ft. per ton. What is the make and model number of your unit? I'm a Goodman Authorized Contractor so I should have most info on hand.