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Why can't your air conditioner maintain 72 degrees in late afternoon to early evening?

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Answered 2014-01-04 14:05:28


Increased insulation and air sealing is the first thing to address. An attic easily hits 130 degrees on 90 degree days. If you only have a few inches of insulation, or even 8-12, your air conditioner has to work awfully hard to keep up a 70 degree temperature.

A few more issues are in play here as well.

1. Lots of heat - in the afternoon the sun has been beating on your house for hours, warming up all of the building materials - the studs, siding, drywall, etc. Your house is much hotter as a whole than at 6 AM when the sun comes up.

2. Temperature Stratification - heat rises (in general) so the upstairs is hotter than the downstairs. With too little insulation there could be a 10-15 degree difference. It should be more like 2-3 degrees.

3. Reverse Stack Effect - In the winter heat rises through your house. In the summer the denser, colder air falls through the house, into the basement, and out the top of the walls of the basement (the band joists). Air comes in to replace it from the attic. This is an example of heat goes to cold or the Second Law of Thermodynamics.

4. Poor Air Sealing - Light fixtures, attic hatches, chimneys, Plumbing stacks, and much more are leaking hot air into the house which your AC then has to cool.

Basically, get more insulation in your attic and be sure it is air sealed (caulk and foam those holes in the ceiling mentioned). Look for a "Home Performance" contractor, not just an insulation contractor.


There could be reduced efficiency due to dirty coils, filter, or ducts. If the filter is clean and the coils, both inside and outside are clean and the duct work is not stopped up or leaking, check to see if the condenser is blocked by plant growth. Check to see if the freon charge is to low.

72° is a bit cool. The recommended temperature is 78°. If your system was sized to efficiently maintain 78° it will be to small to maintain 72° in the heat of the day.


Many homes have load balancing from their electric company, which allows the utility to remotely shut off your outdoor unit during periods of high demand. This is usually in the afternoon when the heat load is the highest on your home, which in turns makes your home get hotter.


According to the International Energy Conservation Code (IECC) section of the International Building Code, the size (capacity) of a residential HVAC system cannot be designed to maintain an indoor air temperature of less than 75°. The processes that must be followed to determine the final system design are ACCA's Manual J (heat load calculation), Manual S (equipment sizing calculation) and Manual D (duct design procedures).

HVAC Equipment Evaluation - A trained service technician should perform a complete evaluation of the system to confirm that it is operating to the manufacturer's specifications. It is not possible to determine if the system is producing the correct amount of cooling by reading the operating pressures, by measuring the temperature difference between the air entering the indoor unit and the air leaving the indoor unit or by doing both. In order to determine the system's total total output capacity, "wet bulb" temperature and and total air flow measurements must also be taken and then the total capacity can be determined using a psychometric chart. If it is not known if the system is operating at its full design capacity, then replacing the system with a larger unit could replace the existing issues with a long list of new issues, not to mention higher utility bills for the life of that system.

The reason your A/C fails to keep the temperature you set late in the day is simply because it is sized wrong. A properly sized and properly installed heating or A/C system will maintain the desired temperature 24 hours a day, 365 days a year.

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