Air conditioning maintenance, task by task, before the first hot week

It comes out and gets held up to a light, and the rack gets as much attention as the filter. One that has collapsed in a loose track lets air slide around the edge of the media, which is how a coil behind a clean-looking filter ends up filthy.

The residential trap is the rack itself. A one inch filter in a hallway grille holds very little media, so the most restrictive one on the shelf trades away air the system needs, while a four inch cabinet at the air handler can filter hard without strangling the blower. This is also the one task on the list that belongs to you between visits.

Power off at the disconnect, top grille off, fan lifted clear, and whatever has collected in the base of the cabinet taken out by hand before any water touches it. Then the coil is washed from the inside out: the spray goes from within the cabinet outward, against the direction air normally travels through the fins, so a year of debris leaves the way it came in. Wash it from the outside and you drive that debris deeper into the fin pack where nothing will reach it again.

Low pressure, a cleaner meant for aluminum, and the fins combed straight only once the wash is finished, because a pressure washer flattens fins and a flattened fin blocks air as thoroughly as dirt does.

The coil is where refrigerant gives its heat up to outdoor air, and only if air passes through the fins. Blanket it and the refrigerant will not condense until it is pushed to a higher pressure, so head pressure climbs, the compressor draws more current, and the system delivers less cooling for more electricity on exactly the afternoon it was bought for.

This is the coil nobody sees, and on a house system it is awkward: sealed inside a sheet metal case on top of the furnace, or inside an air handler in an attic or closet, and on a great many installations no access panel was ever cut into it. It also runs wet all summer by design, and wet metal holds whatever gets past the filter instead of letting it blow through. A loaded evaporator restricts air and insulates its own fins at once, so the house reads warm while the machine sounds healthy.

On a routine visit it is inspected as far as it can be seen and read from the numbers around it. One found genuinely loaded needs a chemical clean with the runoff caught, which is a job of its own and gets quoted as one.

The blower is a caged wheel and each blade is a small scoop. Dust builds on the concave face until the scoop is filled level, and a wheel in that state turns at exactly the same speed while moving noticeably less air. Nothing sounds wrong, which is why almost nobody suspects it; it reaches you as an upstairs that never keeps up.

With the door off, the wheel gets looked at. Light dust comes off in place; a wheel with a real coating has to come out of its housing, which is separate work with its own price. On a ducted system this is the furnace's own blower, so a loaded wheel costs you in both seasons.

The indoor coil pulls water out of the air all summer, and a humid Long Island August produces a great deal of it. It runs off into a pan, through a trap and down a line that spends its life dark and wet, which is precisely what biofilm wants. A slime blockage there is the most common reason a working air conditioner puts water on a floor.

The drain is cleared and then proven: water poured in at the top and watched arriving at the far end, because looking at a pipe tells you nothing. The trap is primed, the pan checked for slope and rust, the secondary pan under an attic unit checked too. Then the float switch, which is what stands between a blocked drain and a ceiling. Testing it means lifting the float by hand and confirming the system genuinely stops. An untested float switch is a smoke alarm with the battery out.

With the power isolated, connections at the disconnect, the contactor and the terminal board are checked for tightness, because a loose connection heats and discolors long before it fails. Contacts are looked at for pitting, and for insects, which nest under the armature more often than a homeowner would expect.

The run capacitor is measured against the rating printed on its own can, and this is the reading that earns a spring visit more than any other, because nothing about a weakened capacitor shows from the outside and the meter is what finds it before the season does. Then current, every motor read against its data plate, months before any of it becomes a phone call.

What those readings mean once they are out of range belongs to air conditioning repair.

A sealed system does not consume refrigerant. If the charge was correct last year it should still be correct, so it is checked by measurement and then left alone, and only after the coils are clean and the airflow is right, because both move every number you would read.

If the readings say the charge is down, the finding is not low refrigerant. The finding is a hole. Adding gas on principle every spring quietly funds a leak for a decade while the compressor runs short of the oil that travels with the charge. Locating and repairing that leak is separate work with its own invoice, and you should hear so on the day.

A thermostat is compared against a reference thermometer where it actually hangs, because one sitting in afternoon sun or above a lamp reports a room no one in the house is living in. Setpoints, the schedule against how the house is really used, the deadband that stops heating and cooling arguing with each other, the low voltage wiring at both ends and the changeover on a heat pump all get checked. Safety controls get tested, not assumed. A safety that has failed in the closed position would never have stopped anything, and it looks exactly like one that works.

The last task is the record: return and supply air temperatures, the capacitance figures, the amp draws, and the outdoor conditions on the day, because a reading without the weather around it cannot be compared with anything. One year of readings is a snapshot. Two years is a trend, and a trend is what tells you a capacitor is drifting or a motor is working harder than it was. Readings taken on our visits are recorded, and you can ask for them.