On a hot day, few things feel as refreshing as standing in front of an air conditioning vent and basking in the cool breeze. How cold that breeze should be is a surprisingly involved question. Your air conditioner must obviously put out enough cold air to lower the ambient temperature to a comfortable degree, but how cold should air conditioning be to do that? Does the outside temperature matter? Is all that cold air energy-efficient? Where should the air output be measured? Is it a bad sign if the air from the vent does not feel appreciably colder than the surrounding air?
A fundamental principle of physics governs how air conditioners work: Heat always flows from a hotter object or system to a cooler one. Cold air from the vent isn't really cold; it's just less hot. The heat has to go somewhere, and an air conditioner in good working order sends as much of it as possible out of your home.
As hot ambient air moves over evaporator coils, it sheds its heat into the refrigerant in the coils. The unit then circulates the warmed refrigerant through a closed system, compressing the gas into liquid form. It is then sent through the evaporator coils as a low-pressure gas again so it can absorb more heat. Meanwhile, exhaust air blows across the condenser coils, picking up the heat waste they produce and returning it to the outside environment. It works on the same principle as your refrigerator, and that is why the back of the refrigerator feels hot.
Supply and Return: Where Does Cool Air Come From?
To understand how cold an air conditioner's output should be, it's important to understand the difference between supply air and return air.
In HVAC terms, supply air is the hot air outside your home. Some window units are designed to take supply air directly from outside; central units may take supply air from outside or use return air, recirculated air that has already been conditioned. Because return air already has less heat to exchange with the evaporator coils in your air conditioner, it is more efficient to cool.
If you have ever turned your air conditioner off while on a lengthy vacation, you know that cooling your home again takes some time; that's due to the lack of cooler indoor return air to mix with hot supply air. You will also notice this phenomenon when you get into a car that has been parked in the sun on a hot day. It takes some time to dissipate the heat and start to feel comfortable. Return air also makes your air conditioner more energy-efficient because it takes less work to maintain a comfortable ambient temperature.
Measuring Air Temperature from Your Air Conditioner
Your air conditioner can provide about 20 degrees of cooling from the supply air and return air that passes over the evaporator coils. In other words, on a 100-degree day, even the most efficient air conditioner could only achieve an 80-degree inside temperature. Thanks to recirculating return air, though, the air from your vent may be in the 50’s even on that 100-degree day. That 20-degree heat loss from hot outside supply air is still warm enough to be uncomfortable, especially in humid climates. However, when it mixes with 70-degree indoor air, you feel the difference at the vent.
If your air conditioning unit has been running steadily, the temperature at the vent should be between 55 and 60 degrees Fahrenheit. If you have only recently started the unit after some time without air conditioning, it may take a day or longer to reach these temperatures as the return air slowly sheds its extra heat.
In general, your air conditioner should show a 20-degree split between the air temperature measured at the vent and the temperature of the air nearest your return air intake just in front of the filter grille. Note that this relationship should be fairly consistent regardless of outside temperatures or personal comfort preferences.
Cooling Problems with Your Air Conditioner
If you notice a temperature disparity greater than 20 degrees, your fan may not be blowing quickly enough. Over time, this problem could lead to freezing condensation on the evaporator coils that could diminish your system's efficiency. If you notice a small temperature gradient and your air conditioner has been functional for at least two days, the condition could have a number of possible causes:
- Loss of refrigerant
- Dirty or damaged evaporator coils
- Damaged or malfunctioning exhaust fan
- Damaged, blocked or inefficient ducts
- Clogged air filters blocking return air
- Dysfunctional compressor
A trained and qualified HVAC specialist can discern why your unit isn't putting out the cold air it should provide. Regular maintenance can prevent many problems before they start, so arrange a maintenance check in the spring to prepare for summer's heat just as you get an annual check for your heating system in the fall to ready your home for winter.
While comfort is vital when deciding where to set your thermostat, energy efficiency also matters. The higher the temperature at which you set your thermostat, the less work your air conditioner has to do. However, turning the AC unit off entirely overnight or for short periods may not be a money-saving measure because the unit will have to work so hard to overcome the heat load in the return air. Talk to a certified heating and air conditioning specialist to find out which settings are best for your system.
How cold should air conditioning be for comfort is largely a personal choice. Most HVAC specialists recommend leaving the thermostat at 78 degrees or higher for the best intersection of comfort and efficiency. In humid climates, though, turning down the thermostat another degree or two could make the difference between a comfortable night's sleep and a fruitless search for the cool side of the pillow. Newer units are more energy efficient than older air conditioners, so another solution may be replacement. A newer air conditioner can be as economical at a lower temperature as an older one with a higher thermostat setting.