When ACHR News reported on the 2001 to 2009 growth in residential air conditioning, the headline figure was households. The summary describes "an increase of 16 million in the number of households with a/c." That count tells you how many homes gained cooling. It says nothing about whether the ductwork in those homes was ever sized to carry it.
Many of those installs, in older houses especially, added a condenser and a coil to a duct run that was originally built for heat, or for a smaller cooling load. The ducts did not change when the equipment did. If you own a house like that, the most useful test you can run on a cooling system is not the amp draw on the compressor. It is the resistance the air handler has to push against.
Why the added system strains the old duct
An air handler moves a fixed volume of air against whatever resistance the system presents. That resistance, called external static pressure, comes from the return grille, the filter, the coil, and the supply branches. A coil added to an existing air handler narrows the path. A dirty or undersized filter narrows it more. A return with one grille for a whole floor can choke the blower even when every supply register looks open.
When the total resistance rises above what the blower and the manufacturer's data expect, airflow drops across the coil. Low airflow across the evaporator can lead to a frozen coil, reduced capacity, and longer run times. The compressor is usually the last part to fail in this sequence, which is why the symptoms show up as comfort complaints first.
The readings to take
You need a manometer or a digital differential pressure gauge with two hoses. Many homeowners who own one use it to check a furnace filter. Use the same tool here.
Start by finding the rated external static pressure for your air handler. It is listed in the installation data, the unit literature, or the model documentation that the data plate points you to. Write down the model and serial numbers before you start, so the number you compare against belongs to the equipment actually installed.
Measure the static pressure with the system running on cooling, with the filter in place. Take one reading on the supply side downstream of the coil and one on the return side upstream of the blower. Subtract the return reading from the supply reading to get the total external static. Then measure across the filter and across the coil, individually, with the blower on. Those two numbers show you which component is taking the most of the budget.
If the total external static is near or above the rated figure, look at the cheapest fixes first. Replace a filter that is rated tighter than the system was designed for. Check whether a return grille is blocked by furniture, a rug, or a closed door that cuts off the return path. Confirm the blower is set to the speed the installer chose for the coil and the duct run. Those are all changes a homeowner can make without opening the refrigerant circuit.
Keep the work where it belongs
Gauge readings are diagnostic information, not a repair. A reading that is out of range tells you where to look. Changing a blower tap, adding a return, or reworking a branch is a job for a licensed installer who will document the change and confirm the airflow afterward. Ask for the airflow and static readings in writing, along with the model numbers they were taken against.
The attic is where many of these problems hide. Look for crushed flex duct, joints that have separated, and branches that run through unconditioned space without insulation. Check that the duct is supported rather than sagging across the joists. If you find a damp stain on the decking above a duct run, or evidence of water coming in around a vent penetration, stop the duct inspection and address the source. A roofing contractor such as Keys Roofing can look at the roof penetration while you handle the duct side, so the two repairs do not get done against each other.
What the numbers do not tell you
A high static reading does not prove the coil is at fault, and a normal reading does not prove the duct is sound. A duct that leaks into the attic can pull in hot, humid air and still show acceptable static pressure at the blower. For that reason, pair the gauge readings with a temperature split across the coil and a look at the supply temperatures at the far rooms. The gauge tells you about resistance. The rooms tell you whether the air is arriving where it is needed.
Growth in air conditioning ownership is a fact about households. Your duct system is a fact about your house. Take the readings, write them down, and let the measurements decide the next step.