Hotter Summers and Cooling Load: What Gulf Coast Heat Does to HVAC Systems

If your cooling bills rose last summer, two very different explanations are possible: the weather got hotter, or your equipment got worse. Usually it is some of both, and the two interact. A system that has quietly lost capacity has no reserve left when a heat wave arrives. This guide explains how meteorologists and energy engineers measure "how hot" in a way that relates to cooling load, what the official data shows for Houston and the Texas Gulf Coast, and what sustained heat does to HVAC equipment.

Cooling degree days in one paragraph

A degree day compares the daily average outdoor temperature with a baseline of 65°F, the temperature at which a typical building needs neither heating nor cooling (this is the baseline the U.S. EPA uses in its climate indicators). If a day averages 85°F, it contributes 20 cooling degree days (CDD). Add up the days and you get a monthly or annual CDD total that tracks closely with the energy buildings use for cooling. Heating degree days (HDD) work the same way below 65°F.

CDD is not a perfect measure. It ignores humidity, solar gain and the fact that large commercial buildings with heavy internal loads often need cooling well below 65°F outdoors. But it is consistent, publicly reported and good enough to separate weather from equipment performance, which is exactly what facility managers need.

What NOAA data shows for Houston

NOAA's National Centers for Environmental Information publishes an annual Local Climatological Data summary for Houston's Bush Intercontinental Airport (KIAH). The 2023 edition includes both that year's observations and the official 1991-2020 normals. The contrast is instructive.

Two things stand out. First, Houston's cooling load dwarfs its heating load: normal CDD are about 2.6 times normal HDD, which EPA also highlights as typical of the South. Second, 2023's annual average of 72.5°F was the warmest in the 30-year table NOAA prints in that summary, and the 109°F reached in August 2023 matched the highest temperature in the station's period of record.

Month by month: where the load sits

Even the "mild" months carry meaningful load: Houston's normal January still has 17 CDD and its normal March has 85. From May through September, more than 2,600 CDD accumulate in a normal year. August 2023 averaged 91.0°F for the whole month, day and night, and every day reached 90°F.

Humidity: the load CDD does not show

The same NOAA summary lists mean dew points in the mid-70s°F for July and August. Air that humid carries a large latent load: before a cooling coil can cool outdoor air that enters through ventilation or infiltration, it has to condense water out of it. Latent load does not appear in CDD at all, which is one reason Gulf Coast buildings often run their cooling equipment harder than the degree-day count alone suggests. It is also why a loss of coil performance in Houston tends to show up first as humidity complaints, not just warm rooms. See signs your HVAC is losing capacity.

Is it getting hotter? What the trend data says

One hot year is weather, not a trend. The longer-term data:

• Nationally, EPA's climate indicator (data through 2023) finds that cooling degree days in the contiguous U.S. have increased over the past 100 years, with the increase most noticeable over the past few decades, while heating degree days have declined. EPA also notes that the trend varies by region, so national numbers should not be applied to a single city without checking local data.
• In Texas, NOAA's 2022 State Climate Summary reports that temperatures have risen almost 1.5°F since the beginning of the 20th century, that very warm nights (lows of 75°F or more) were especially frequent in the 2010s, and that one emissions pathway projects 20 to 30 more days above 95°F by mid-century.
• On the grid, ERCOT's records page (last updated August 18, 2026) shows a new all-time peak demand of 91,089 MW on July 22, 2026, unofficial until final settlement, ahead of the previous record of 85,508 MW set on August 10, 2023.

Warm nights matter more than they sound. When overnight lows stay in the upper 70s, buildings never shed the day's heat, equipment never gets a rest period, and the next morning's pull-down starts from a higher point.

How heat raises load and lowers capacity at the same time

Hot weather hurts an air-cooled system twice:

1. The load goes up. More heat conducts through roofs and walls, ventilation air arrives hotter and wetter, and solar gain peaks in the same months.
2. The equipment gets weaker. An air-cooled condenser rejects heat to outdoor air. The hotter that air, the higher the refrigerant's condensing temperature and pressure must climb, which means the compressor works against a bigger pressure lift for every ton it moves. Capacity falls and power rises.

Southern California Edison's laboratory testing of five-ton rooftop units, published at the 2004 ACEEE Summer Study, measured this directly at condenser inlet temperatures from 85°F to 130°F. Every unit's cooling capacity deteriorated as ambient temperature increased, and one high-efficiency model's compressor drew up to 74% more additional power per degree Fahrenheit than its standard counterpart. The authors also noted that temperatures above 115°F are typically beyond the conditions at which manufacturers test their products.

This is why DOE uses a 95°F rating point for EER and why EER is a better predictor than SEER of how equipment behaves on a Houston August afternoon. Our guide to kW/ton, EER, SEER2 and COP explains the rating conditions.

Run hours, wear and degradation

CDD translate almost directly into compressor run hours. A season 18% hotter than normal means roughly that much more operating time on compressors, fans and bearings, at the most stressful operating conditions of the year. That has three consequences:

• Existing problems cost more. A fouled coil, a low refrigerant charge or a drifting economizer wastes a percentage of energy every hour the unit runs. More hours, more waste.
• Margins disappear. Equipment is sized for a design day. A unit that has lost part of its capacity can still keep up on a mild day, but it falls behind on the hot ones, runs continuously and still misses set point.
• Wear accelerates. High condensing pressures raise discharge temperatures and oil temperatures inside the compressor. See compressor wear and lubrication for why heat is hard on compressor oil.

Using CDD to separate weather from degradation

The practical value of degree days is normalization. Here is a simplified, illustrative example. Suppose a building's June through September electricity use was 1,100,000 kWh in 2023 and 1,000,000 kWh in a year with normal weather. Using the NOAA figures above, June through September 2023 accumulated 2,752 CDD against a normal of 2,253, about 22% more.

A 10% increase in use during a season with 22% more cooling degree days does not, by itself, suggest a degrading system. Only part of a building's load is weather-sensitive, so a proper analysis regresses use against CDD to separate the fixed base load from the cooling-driven part. Our guide to baselines and weather normalization walks through that method. The reverse case matters too: if use rises faster than CDD year after year, something in the system is likely getting worse. The companion article why cooling costs keep rising covers rates, demand charges and other non-weather causes.

Preparing equipment for a hotter season

• Schedule spring maintenance before May, when Houston's normal CDD more than double from April.
• Verify refrigerant charge and airflow under load, not just on a cool spring day.
• Clean condenser coils and check cooling tower water treatment before peak season.
• Log kW/ton, supply air temperatures or temperature split weekly through summer so problems show up as trends.
• Review demand charges and utility incentive programs; see Texas utility commercial incentives.
• Build the work into a documented program; see commercial HVAC maintenance for efficiency.

Where CryogenX4 fits

Weather is outside anyone's control; the condition of heat-exchange surfaces is not. CryogenX4, based in Houston, describes its product as a one-time treatment, installed while the system runs, that is designed to remove insulating compressor oil film from internal coil surfaces and to improve the lubricity of the existing oil. The company reports energy savings of up to 30% and typical payback of 12 to 36 months, with results varying by equipment condition. In a climate with more than 3,000 cooling degree days, any measure that improves heat transfer is used for many hours a year, which is why weather-normalized measurement before and after matters.

Next step

Download your last three summers of monthly kWh and pair them with monthly CDD for your nearest NOAA station. If your kWh per CDD is climbing year over year, start with a structured diagnosis of the equipment rather than blaming the weather.