Combination heating and cooling units are by far the most common found in residential homes today. The reason for this is cost. The duct distribution system is used for both heating and cooling.
This is the only advantage this system has over two separate systems. The disadvantages are location of registers, ducts not insulated, system design, units longevity and cost of operation. The location of registers plays a major factor on how well this system works as far as your comfort and fuel bills are concerned. Heating registers are located near the floor and cooling near the ceiling.
The reasons for this are hot air rises and cool air drops. In most cases with combination units the registers are located at the floor level. The reason for this is heating is considered more important than cooling. What usually occurs here is the heating of the home is fine, but the cooling is not.
The problem is the return are registers located at the floor. In the cooling mode the return is taking the coolest air in the home, which you want to stay in the home, and brings it back into the system. The warmest air in the home is at the ceiling and it just stays there. The most cost-effective way to alleviate this is to the create a single large return at the highest point in the home. You must be able to control this return along with all the others in the home. The high return must be closed during the winter because you want the warmest air in the home to stay and the low returns must be open because you want the coolest air in the home brought into the system and be heated.
The reverse is true for cooling. Most combination units ducts are located in the basement and they are rarely insulated. In the cooling mode you will see these ducts sweat. The reason for this is the same reason a soda can sweats when you take it out of the refrigerator during the summer. The soda in the can is so cool that it forces the air outside the can to condense on the can. The more the soda can sweats, the warmer the soda gets. The same principal applies to your ducts in the cooling mode. The more they sweat, the warmer the air inside the ducts get, the more it cost you to cool your home. Have ever seen those foam rubber cups they slide soda cans in? What occurs is the air cannot touch the can, the result is the soda stays cooler longer.
The same principal applies to your ducts in the basement. If you wrap insulation around the ducts, the air cannot touch the ducts, the result is the air inside the ducts stays cooler and it cost you less to cool your home. The ducts that are enclosed in ceilings and walls are already separated from the air. By the way you will save on you fuel bill with heating too, but not as much as cooling. The system design is flawed and there is very little you can do about it, but you do need to understand it.
Contractors use what is known as Manual "D" to determine the distribution system, meaning the length and size of the ducts. If you know anything about this Manual, you will see the heating and cooling duct design is a compromise. The specifications for heating only and cooling only are much different than heating and cooling.
Another problem is the fan is located before the heat exchanger and the cooling coil is after the exchanger. In the heat mode the cooling coil will have very little affect on the air flow, because warm air move fairly easy. In the cooling mode the heat exchanger, because the new energy efficient system design is usually increased surface area and cool air is heavier and denser, this will impede the air flow to the cooling coil. For this reason, it is almost mandatory that a two stage fan be installed in combination units.
The second stage of the fan increases velocity to move the air pass the exchanger to cooling coil and through the ducts. Whenever you heat a material, the heating deteriorates the material. Imagine a portion of the year you extremely heat this material and the other portion of the year you extremely cool it. The longevity of a combination unit is clearly less than that of a heating unit only. The extremes in a combination unit accelerate its deterioration.
The cost of operation on a combination unit will be more than a heating or cooling system only. For all the reasons stated above.