Why Your AC Can't Keep Up With the Heat

What the makers and the Department of Energy guide say

Carrier and Bryant make air conditioners and send readers to their dealers. Bryant's pages describe it as a Carrier company. Each article's author line names a marketing manager at the maker. Its reviewer line names a product manager there. Our reading: these are a seller's account of its own equipment. We cite them for what they say, not as independent testing.

Carrier says an undersized unit will struggle to keep up as outdoor temperatures rise. It adds that a system that runs constantly while the house stays warm may be too small for the square footage or insulation. Bryant says extreme outdoor heat, poor insulation or an older, undersized unit can make it hard to reach the desired temperature.

The Building America guide is posted on a Department of Energy website, and a firm that does right-sizing and field-testing work wrote it. It stresses a different point: comfort problems are seldom a result of undersized equipment. Most, it says, have causes unrelated to capacity. Our reading: the makers and the guide weigh size differently. None of them can say which situation applies to your house, because only measurements taken there can.

What a design temperature is

The Building America guide describes a design condition as a dry-bulb temperature tied to how often it occurs over a year. A dry-bulb temperature is the reading of an ordinary air thermometer. It says the value is exceeded, on average, for the stated percentage of the 8,760 hours in a year. By our arithmetic, a 1 percent design temperature is exceeded for about 88 hours a year. A 0.4 percent design temperature is exceeded for about 35 hours.

The guide says programs such as ENERGY STAR Qualified Homes require the 1.0 percent design temperature for load calculations. It says its own authors typically use the 0.4 percent value. It says HVAC contractors typically use the mean extreme conditions. The guide describes the 0.4 percent value as lower than the mean extreme condition. A design temperature is a chosen share of past hours, not the hottest day on record, so hours above it are part of the definition. Which percentage a designer chose can change the size a calculation gives, so it is worth asking for. The tonnage and sizing page covers what a properly sized system may hold during those hours.

What the published Tampa Bay numbers look like

ENERGY STAR publishes a Design Temperature Limit Reference Guide for homes certified under its Certified Homes program. For cooling, designers are to use a temperature at or below the county's 1 percent cooling temperature. Its 2019 edition lists 95°F for Hillsborough, Pasco and Pinellas counties. The 2015 edition listed 92°F for Hillsborough, 91°F for Pasco and 92°F for Pinellas. The 2019 edition draws on the 2017 ASHRAE handbook, and the 2015 edition drew on the 2013 handbook. Our reading: the editions used different handbook years, so the change does not by itself show the weather changed. The guide is a program rule for certified homes, so it is a county reference here and not a rule for replacing equipment.

The National Weather Service's August climate sheet for Tampa lists a normal daily high of 91.2°F, using 1991 to 2020 normals. Its records began on April 1, 1890, and were updated through December 2023. They show August daily highs as hot as 98°F. Our reading: the design table is a planning percentile for a county. The weather service sheet gives daily values for a station, so the two measure different things.

What counts as keeping up

The Building America guide also reports a comfort standard called Manual RS, from ACCA (Air Conditioning Contractors of America). For the cooling season, it lists a thermostat reading of the setpoint plus or minus 3°F. The guide was last updated in August 2010.

The maker Bryant gives a rule of thumb: an air conditioner can typically cool about 20°F below the outdoor temperature. By our arithmetic, that rule of thumb puts a 95°F afternoon at about 75°F inside. The same page gives a different 20-degree rule. It compares the air at the vents with the air returning to the unit. Our reading: the two rules measure different things, and neither is a specification for your model. The maker's performance tables and a technician's readings should decide.

Capacity falls as the outdoor air gets hotter

A cooling system does not have one fixed capacity. A 1995 report from the Texas A&M University Energy Systems Laboratory measured the change. The report, written for the Electric Power Research Institute, tested ten residential air conditioners in laboratory rooms. Between 82°F and 120°F, split-system units dropped in capacity by an average of 0.46 percent for each degree Fahrenheit. A split system has separate indoor and outdoor units. By our arithmetic, the split-system average adds up to a drop of about 4.6 percent between 95°F and 105°F outdoors. Our reading: these are averages across test units from the 1990s. They show the direction of the effect, not what your system loses.

On the same afternoon, the heat coming into the house rises while the cooling the unit can deliver falls. So the gap between them is widest at the hottest hour. The maker's published performance data for your model, at your design temperature, is the place to see the effect. The sizing page linked above explains how to ask for it.

The house may now gain more heat than the one that was sized

UF/IFAS Extension describes a cooling load calculation as a measure of what system size suits a structure, given its square footage, ductwork, insulation and windows. According to the Building America guide, the HVAC design needs to be modified as well if the home is significantly leakier. The same applies, it says, if building specifications change drastically from the design. A calculation describes the house as it was on the day it was done. An addition, new windows or changes to insulation can move the load.

UF/IFAS lists ways to cut the load. They include shading east and west windows and delaying heat-generating activities, such as dishwashing, until the evening. The Department of Energy's cooling guide adds window coverings that prevent solar heat gain, and insulation in the attic and walls. Humid air adds a moisture load on top of the heat, so a house can feel hotter than its thermometer says. The humidity page goes deeper. If the extra heat is mostly upstairs, two-story uneven cooling covers that case.

Delivered versus rated: airflow, charge and ducts

The Building America guide lists causes unrelated to size. One is a system that is not properly charged with refrigerant, which it says results in reduced capacity. It says dirty filters or coils raise the system's external static pressure, which reduces airflow. Static pressure describes how hard the blower must push to move air through the equipment and ducts. It says duct leakage from poor installation sends heating and cooling into unconditioned spaces instead of the rooms. The Department of Energy's cooling guide says a clogged filter limits airflow through the unit and reduces its ability to cool the air effectively. Each of those has its own page: the evaporator coil, air filters and refrigerant leak repair.

A Florida Solar Energy Center paper reported duct tests in central Florida homes. The study began in the spring of 1989 and was due to finish in the fall of 1990. It tested 91 homes and found return leaks that averaged 10.0 percent of the air handler's total flow. A return leak is a gap on the intake side of the air handler. It pulls surrounding air into the system, and in a hot attic that air is attic air. Based on theoretical calculations, the paper says a 15 percent return leak from the attic can cut a system's effective capacity by half. It assumes the summer peak at about 5 PM, when the attic is likely to be 120°F. It says a 30 percent return leak can completely overwhelm the system's capacity and cause the temperature in the house to rise. Our reading: that is a theoretical calculation from a 1990 paper, not a measurement of a current house. It shows why leaks matter most at the hottest hour. For leak sizes and testing, see the Florida ductwork page. For the repair choice, see the sealing-versus-repair page.

UF/IFAS says an outdoor unit needs the maker's clearance distances so that air flow from the unit is not blocked. The maker Carrier says that if you suspect a refrigerant leak, you should not attempt to fix it yourself. Refrigerant, wiring, capacitor and coil work needs a licensed technician, so no steps for it appear on this page.

What you can do safely on a hot day

UF/IFAS advises a monthly filter check during peak season, or on the maker's schedule. For most of the cooling season, it says to keep the house closed tight during the day. That keeps unwanted heat and humidity out. The Department of Energy's cooling guide says ceiling fans let you raise the thermostat setting about 4 degrees without losing comfort. These steps lower the load or protect airflow, and none of them involves refrigerant, wiring or opening the cabinet. If the house still cannot hold its setting after the load is reduced, take measurements instead of guessing at a cause.

What to ask a contractor, in writing

These questions work for a repair visit or a proposal. They differ from the sizing questions on the Florida tonnage page.

1. What were the indoor and outdoor temperatures and the run time when you took your readings, and what were the readings?
2. Which situation applies here: heat beyond the design temperature, less delivered capacity or a changed load? What measurement shows it?
3. What airflow did you measure across the coil, and was any duct leakage test done?
4. How do you verify that the refrigerant level is correct, and may I get the test values in writing?
5. Was a load calculation done when the system was installed? Which design temperature did it use, and has the house changed since?
6. What could reduce the heat load first, such as shading, attic insulation or duct sealing? What would each change, in writing?

Keep every answer on paper, since a remembered conversation is hard to check later. UF/IFAS tells owners to ask to see a valid contractor's license. It says Florida owners can check it on the state's website.