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Surviving Houston's August heat waves: HVAC strategy when it's 100+ outside

When the heat index hits 110 deg F and the grid wobbles, your AC has the hardest job of the year. Here is how to ride out a heat wave without breaking equipment - or your bills.

  • By Avatex Service Company
  • Reviewed by Jonathan Avalos , Owner
  • Last reviewed June 13, 2026
  • Reading time 5 min

The short version

  • On a 100 deg F day, your AC may not be able to maintain 70 deg F indoors - and that's not a failure. Most residential systems are sized to maintain 75-78 deg F at design temperature.
  • Setting the thermostat colder than usual does NOT make a struggling AC catch up faster. It just runs longer at the limit.
  • Pre-cool overnight and during morning hours when the system has spare capacity. Hold the line during peak afternoon (3-7 PM).
  • If the system runs continuously and indoor temperature climbs anyway, the issue is usually a dirty condenser coil, low refrigerant, or a tripped capacitor.
  • ERCOT grid stress events typically peak 3-7 PM on extreme days. Reducing AC load during those hours helps everyone - and a well-tuned smart thermostat handles it automatically.

What 'design temperature' actually means

Residential AC systems in Houston are sized for an outdoor design temperature of approximately 96 deg F (ASHRAE 1% design dry bulb for Houston). At that outdoor temperature, the system is engineered to maintain about 75-78 deg F indoors at 50 percent humidity. When outdoor temperatures exceed 96 deg F (the famous 100+ heat waves we see in late July and August), the system is operating beyond its design point. Indoor temperatures will drift up by 1-3 deg F.

This is normal physics. Your system is not broken. It is at the edge of what it was sized to do. Setting the thermostat to 68 deg F will not change this - the system can only deliver so much cooling capacity at a given outdoor condition.

Pre-cool, don't catch up

The smart move is to pre-cool when the system has spare capacity, not to fight peak heat with maximum thermostat setting. Practical schedule for a Houston heat-wave day:

  • Overnight (10 PM - 6 AM): set thermostat to 72-73 deg F. The system will run efficiently and cool down the home's thermal mass (drywall, furniture, contents).
  • Morning (6 AM - 11 AM): set 73-74 deg F. Maintain.
  • Midday (11 AM - 3 PM): set 75 deg F. The system can still hold this comfortably.
  • Peak (3 PM - 7 PM): set 76-77 deg F. Let the home drift slightly. The system will not be running flat-out and will use less power during grid peak hours.
  • Evening (7 PM - 10 PM): set back to 74-75 deg F as outdoor temps drop.

When the system genuinely cannot keep up

If your AC runs continuously through the afternoon AND the indoor temperature climbs more than 3-4 deg F above setpoint, something is mechanically off. The most common causes during heat waves:

  1. Dirty outdoor condenser coil. Cottonwood seeds (June), grass clippings, leaves all accumulate. The coil cannot reject heat, head pressure rises, capacity falls. A simple gentle hose-down (after turning off the breaker) can recover 10-20 percent of capacity in 10 minutes.
  2. Low refrigerant from a slow leak. Symptoms: longer run times, ice on the suction line, warmer-than-expected supply air. Refrigerant top-off without finding the leak is a temporary fix, not a repair.
  3. Failing capacitor. Symptoms: outdoor unit hesitates to start, hums for 5-10 seconds before kicking in, sometimes shuts down on overload. Capacitors are cheap to replace but hard on the compressor when they are weak.
  4. Tripped float switch on the condensate drain. Pan filled with water, safety switch shut down the AC. The drain line needs clearing.

What ERCOT grid stress looks like for homeowners

ERCOT (the Texas grid operator) issues 'Conservation Appeals' and 'Energy Emergency Alerts' during extreme heat, typically peaking 3-7 PM on the worst days. Voltage may sag slightly during these hours. Here is what that means for your AC:

  • Slight voltage sag does NOT damage modern HVAC equipment. Compressors are designed to tolerate brief voltage variation.
  • Brief outages (lights flicker for 1-2 seconds) are fine. The system will restart automatically with a 5-minute compressor delay built into modern thermostats.
  • Extended outages (more than 5 minutes) followed by restart on partially-restored grid CAN damage equipment. If the grid is in trouble and you have time, turn the AC OFF at the thermostat manually until you see the lights are stable.
  • If you see lights brightening AND dimming randomly (a 'voltage swing'), turn the AC off immediately and call your utility. That is unsafe operating voltage.

Stay-cool tactics that don't depend on the AC

  • Close blinds and curtains on west-facing and south-facing windows from 11 AM onward. Solar gain through unshaded windows is enormous.
  • Run ceiling fans in occupied rooms. Fans do not cool air, they cool people - moving air increases evaporation from skin, which feels 3-4 deg F cooler at the same air temperature.
  • Limit cooking that uses the oven during peak hours. Microwave, slow cooker, or grill outside instead.
  • Run major heat-producing appliances (dishwasher, dryer, oven) overnight when the AC has capacity to spare.

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