When the A/C Can't Keep Up

Why can't the AC keep up with the heat?

Because almost every central system is sized for a typical hot day in your county, not the record. ENERGY STAR's design temperature guide tells designers to use the ACCA Manual J 1% cooling design temperature. On the small share of hours hotter than that, a correctly sized unit runs nonstop and the indoor temperature drifts up.

That is a feature, not a flaw. A bigger unit would short-cycle the rest of the summer: ENERGY STAR warns that a system that is too large "will not keep your home comfortable" because of frequent on/off cycling, and in humid climates it leaves the air damp. So on a 115°F Phoenix afternoon, a house that holds 80°F with the system running steadily may be a healthy system meeting its limit.

What is a design temperature?

A design temperature is the outdoor reading a cooling system is built to handle, set so that only about 1% of hours run hotter. ENERGY STAR publishes the limit for every county, taken from ASHRAE and Manual J data. The indoor side is fixed too: DOE Building America's sizing guide uses Manual J's 75°F and 50% relative humidity.

The same guide says equipment capacity should not exceed the calculated load by more than 15%. So the designer aims for 75°F indoors on a 111°F Maricopa day, with little spare capacity on purpose. Hold out for that Manual J calculation whenever a contractor quotes a replacement; the room-by-room sizing page covers the window-unit version.

Is the 20-degree rule real?

Only as a rule of thumb; no engineering standard says an A/C can cool exactly 20°F below outdoor air. What a system can hold depends on its design temperature, the house's heat gain and the humidity, which is why a Miami system designed around 92°F and a Las Vegas system designed around 116°F behave so differently.

The primary-source "20 degrees" is a landlord rule. The Arizona courts' legal FAQ says a Phoenix rental A/C must cool to 82 degrees, or at least 20 degrees below outside. Dallas sets its own floor for single-family rentals: at least 15 degrees below outside, per the city's rental registration FAQ, and never above 85°F. Those are minimums a landlord must meet, not a measure of what your equipment should do.

Which checks can you make safely?

Five checks are safe for an owner, and each starts with the system switched off at the thermostat first. Work through them in this order before anyone opens a panel.

1. Thermostat. Confirm it says COOL, the setting is below room temperature and the batteries are fresh.
2. Filter. Pull it and hold it to a light. The Minnesota Commerce Department warns that restricted airflow can freeze the evaporator coil. Replace it per the maintenance schedule.
3. Registers and returns. Open every supply register and clear furniture, rugs and drapes off return grilles. A cracked or rusted register can be swapped for a new one; supply-house register and grille listings (PlumbingSupplyAndMore, owned by the people who run Swelter Almanac) show the common sizes.
4. Outdoor unit. With power off at the outdoor disconnect, clear leaves and plants. The same Minnesota guide says the area around it "must be kept free of foliage."
5. Breaker. If the outdoor unit is silent, check for a tripped breaker and reset it once. If it trips again, stop.

If you find ice on the indoor coil or the copper line, switch cooling off, set the fan to ON so moving air thaws it, and change the filter. Never chip or scrape the ice. If you're unsure, call a licensed technician.

When is it a real fault?

It is a real fault when the system cannot hold temperature on an ordinary day below your design temperature, or when a check turns up something only a technician can fix. Coil ice that comes back after a new filter points to low refrigerant charge or a restriction, according to an NREL measure guideline on A/C diagnostics. Low refrigerant means a leak, and under EPA's Section 608 rules only certified technicians can buy refrigerant for home systems.

Other signs: the outdoor fan or compressor is not running while the indoor fan blows, a breaker trips twice, or the supply air feels barely cool. ENERGY STAR's maintenance checklist puts coils, refrigerant charge and electrical connections on the contractor's list, noting that dirty coils reduce the system's ability to cool. Leaky ducts are a quieter culprit: ENERGY STAR says typical systems lose 20 to 30 percent of the air they move.

For an example of what a local company's A/C service page lists, see Central Plumbing HVAC's air conditioning page (Central Plumbing HVAC, owned by the people who run Swelter Almanac); it works in Bucks and Montgomery County, Pennsylvania. Elsewhere, look for a licensed company near you and ask for a written diagnosis.

How do you help the system on the worst days?

Cut the heat coming in so the system has less to remove. Close blinds on east windows in the morning and west windows after noon, run the oven and dryer early or late, and keep doors shut. Shade on the west side does the most work, according to USDA Forest Service research covered on the window shading page.

Pre-cool a few degrees before the peak if your utility has time-of-use rates, then let the house drift; the thermostat settings page has the numbers. Run ceiling fans in occupied rooms only. In humid areas, leave the fan on AUTO so the coil can drain.

Watch the people, not just the thermostat. Per the CDC, a fan can push body temperature up when a room passes 90°F, so move people to a cooled room or a cooling center instead.

Design numbers behind the limit

• 111°FMaricopa County 1% cooling design limitENERGY STAR
• 75°FManual J indoor design temperature, at 50% humidityDOE Building America
• 15%Most a system should exceed its calculated loadDOE Building America
• 20–30%Share of duct airflow typically lost to leaksENERGY STAR

More questions

Keep reading

• SectionAll the extreme-heat A/C pages
• A/C in extreme heatSpring filter, coil and drain checklist
• A/C in extreme heatFirst hours after a summer breakdown
• SectionDesert cooling options beyond A/C