Getting the air conditioner through a heat wave

A central air conditioner is sized for the design temperature of its region, the number that summer rarely exceeds, and it is expected to hold the house about twenty degrees cooler than that. On the days when the outside temperature goes past the design number, the system runs continuously and the house creeps up a few degrees, and that is normal. It is not broken. It is at its limit, and a few habits keep it from going past it.

What helps

• Set the thermostat and leave it. Every time it is turned down after the house has warmed up, the system runs a long hard cycle to catch up. A steady 76 costs less than a house swinging between 72 and 80.
• Shade the windows on the sun side. Closed blinds and curtains on the west windows in the afternoon cut the load more than anything else you can do for free.
• Run the ceiling fans, counterclockwise, in the rooms you are in, and turn them off when you leave. A fan cools people, not rooms.
• Use the oven, the dryer and the dishwasher in the evening. Each one adds heat and moisture the system then has to remove.
• Keep the filter clean and the outdoor unit clear. Shade the outdoor unit if you can do it without blocking the airflow; a unit in full afternoon sun works measurably harder.
• Close the fireplace damper. It is a chimney for cooled air.

What does not help

• Turning the thermostat to 65. The system produces cold air at one rate; a lower setting only makes it run longer, and if it runs long enough the coil ices over and the airflow stops.
• Closing registers in unused rooms. It raises the duct pressure, and the air finds its way out through the leaks.
• Fan set to "on" instead of "auto" in humid climates. The blower running between cycles re-evaporates the water on the coil and puts it back into the house.

The signs it is in trouble

• Air from the registers is cool but not cold, or the airflow has dropped. Check the filter first. If the filter is clean, look at the indoor coil or the refrigerant lines at the outdoor unit for frost. Ice means the system has to be shut off, fan only, for a few hours to thaw, and then a Pro needs to find out why: low airflow or low refrigerant.
• Water on the floor by the air handler or a stain on the ceiling below it. The condensate drain is clogged. Turn the system off and clear the line or call; the pan overflows every hour it runs.
• The outdoor unit hums but the fan does not spin, or the fan spins but the compressor does not start. Usually a failed capacitor, a common, inexpensive repair, and one of the most common heat-wave calls.
• The breaker trips. Once may be nothing. Twice is a call.

When it quits on the hottest day

Every HVAC Pro in the county is booked and running behind, and the honest ones say so. Ask for the wait time, ask whether they charge extra for after hours, and meanwhile close the house up, shade the windows, run the fans, and move the family to the coolest room, which is usually the basement or the north side downstairs. Anyone elderly, very young or unwell should be somewhere with air conditioning, a relative's house or a cooling center, not waiting it out. Heat kills more people in the country than any other kind of weather.

And in September, when the Pro finally has time, ask what a tune-up would have caught.