Question for any HVAC experts
 

Our house (about 2000sf) has a whole house air conditioner that is only about 6 years old. It struggles to cool the house down, but it is comfortable most of the time. It can run all day long and seems never to go down one degree on hot days. Question: can I replace the outside unit for a bigger one without changing out the furnace too? The furnace is fine in winter. I was told you had to change both. What say you experts. SmileWavy

Is it functioning well? Could it be remarkably dirty/not able to pull air through it for heat exchange and/or low on refrigerant? I'd want to be sure it's functioning well before replacing it.

If you haven't had your evaporator pulled & cleaned it's probably about time...
Also probably a good time to have your ducts sealed....

+1 to ensuring the system is functioning at peak efficiency.

Assuming you are talking about a A/C-furnace combination as opposed to a heat pump, you can upsize the A/C portion, but it depends on the size of the furnace. The furnace blower will generally have 3 speeds. If your A/C is using highest blower speed, you cannot upsize the evaporator and condenser. If your A/C is using a lower speed, upgrade away.
jurgen

The problem with up sizing the condensing unit is that the evap coil cannot absorb enough heat or return a large enough volume of refrigerant to get the back (suction) pressure up above the freezing point, resulting in frozen coils.
Now most systems are matched for efficiency and should be installed as a set.
If you live in an area with low summer humidity, having an evaporator that is over sized by a half ton isn't a bad thing, you will never freeze it up but it will run longer.

Best bet is to get the number off of the indoor coil and furnace. The system that is currently in my house has a 3.5 ton condensing unit but the coil inside is a 5 ton coil. This is by design from the factory.
Most coils, especially cooling only coils are sized within a range.
As the others said make sure everything is within spec and clean. I have climbed in attics to find the small door missing where the filter is changed. Owner said oh is a small opening 1" X 20" looks small. But, would you run it with a 4 X 5 hole cut in the side of your return?
Sorry to ramble. Make sure its clean and charged and check the delta T at the supply and return...