The second floor of my house is also 1500 sq. feet. The cooling load calculation for my area in NJ is about 2.4 tons. The summer climate is similar to Ohio so I would it expect a 2.5 ton AC should be adequate.

The big difference is all the duct work in my house in inside conditioned space and in the a cool basement.. Your system has much more duct loss than mine. You could put in a 3 ton AC, but your duct work may not be able to handle the increased air flow. A better option is to minimize the duct losses. Go in the attic and inspect the duct work for leaks and insulation value. You should have R6 or R8 rated duct work. If not, the spend money replacing it.

Check also the insulation value of the attic. You want a minimum of R30 insulation value on attic floor. If there is blown in insulation perhaps you can add more and cover any duct work laying on the floor.

I would also check on the attic ventilation. Lowering the temperature will also help.

10 years is considered old equipment. It's not uncommon for problems of this nature to occur with an 'older' system. In fact it's quite common for systems in the range of 5-8 years old to suffer these problems.

Typically they are related to 'capacity' issues.

What can effect capacity of an air conditioner?

Dirt. (uncleanliness)

Restricted filtration. (dirty filters)

Improper refrigerant charge. (refrigerant leaks)

Mechanical failure. (compressor runs, but the valves are damaged- rare but does happen.)

Improper repairs. System over charged with refrigerant, fan motor replaced with improper motor among many other possibilities.

Thermal Envelope breaks. The house is getting older and in many cases insulation batts may have fallen that were just laid over openings in the attic that provide entry into the structure like: fire place flue, chases used to run ducts, etc.

It is never a good idea to over size an air conditioner.

I am not having any problems during the winter keeping my home heated. If it is a problem with the ducts and/or insulation, why would it only affect the air conditioning? -thanks-

" If it is a problem with the ducts and/or insulation, why would it only affect the air conditioning? -thanks-"

Because it's not 120*-150* in your attic in the winter time.

However it is down to 0-degrees in the attic which isn't causing me any problems in keeping my home heated to 72*. My furnace can keep up during the winter but my a/c can't keep up during the summer.

Jim, if that's the case it's most likely to be one of these issues or combination of them. I've pasted these again from my previous post and omitted the thermal break as a possibility.

Typically they are related to 'capacity' issues.

What can effect capacity of an air conditioner?

Dirt. (uncleanliness)

Restricted filtration. (dirty filters)

Improper refrigerant charge. (refrigerant leaks)

Mechanical failure. (compressor runs, but the valves are damaged- rare but does happen.)

Improper repairs. System over charged with refrigerant, fan motor replaced with improper motor among many other possibilities.

Thanks for your comments. However the situation is still baffling. My neighbor across the hall in an identical unit installed the same size a/c the year before me (different manufacturer). We both have annual maintenance performed on our air conditioners by two different hvac companies. Nonetheless we both have the same capacity issues with our air conditioners but not with our furnaces.

I would estimate in the winter the temperature in your attic is 20 - 30 degrees warmer than the outside air temperature. This is due to the heat rising up from the house and the solar gain from the sun during the day.

Buy yourself an Accurite temperature and humidity guage and leave it in the attic. Make note of the temperatures in the summer and winter.

What size furnace do you have? It is possible to pair a 100K BTU furnace with a 2.5 ton AC.

Sounds like your AC isn't cooling properly. Have the service company check the temperature drop across the coil. If its 15-20 degrees at the correctly specified air flow, then the unit appears to be undersized (for your condo). This could also be due to the ducts being in an uninsulated attic if they aren't insulated (but I suspect they are). If the temperature drop is less than 15-20 degrees then it's not operating at the efficiency it should be and needs to be serviced, repaired or replaced.

The apparent contradiction of cooling inadequacy vs. heating sufficiency is not a contradiction at all. It is basic physics and it can not be messed with. If you have the heat transfer rates and capacities/contributions of both heating and cooling appliances arithmetic will make sense of it. I specify contribution because you need to know if they are operating up to their rated capacity. Condition of the ducts also contributes, to both heating and cooling as you pointed out. Of course, and the calculations involving existing construction vs. new construction is sometimes challenging due to unknowns in the old stuff.

Rather than estimates of heat transfer using assumptions about the construction, you can do better with fuel use rates. You should be able to get real heat contribution from the furnace and fuel use records (and heating degree days) from the coldest weeks of the year when space heating makes hot water and cooking insignificant. You also need to know the efficiency of the furnace. An energy rater or engineer should be able to use that data to calculate the heat transfer rates of the structure. (BTU in must equal BTU out.) Those rates can also be used for heat transfer in the other direction in the summer. Those calculations can be done today. You have fuel use records and you can get degree days from a nearby weather station.

It is somewhat less common to use energy use of the cooling appliance for estimates, but it should work just as well. You need to separate electricity use by installing a separate meter and the contribution of humidity removal will have to be factored in. That means that you'd have to record humidity inside and outside and include that in the calculation.

It is likely that you have sufficient capacity for you furnace or it is oversized whereas the cooling capacity is not adequate if your cooling unit is functioning properly. Again, calculations will tell you. How do you fix it? Better insulation on the ducts, structure envelope and better sealing of both will always cost less in the long run than installing a larger appliance. Bringing the ducts indoors is also a good idea as pointed out by mike_home.

Don't think of active fans if you consider better attic ventilation. It needs to be passive. it is difficult, however, to draw enough air through an attic to make much of a difference in its temperature. Most of the heat transfer into the living space is due to re-radiation of heat from the underside of the roof deck. That is why radiation barriers were so "hot" for a time. The trouble with them is that they get dirty and lose effectiveness pretty quickly. It was a nice idea, but did not work as expected.