What temperature should my AC be blowing in my house?
It’s August. It’s HOT outside. We want to be COOL inside. But just how cold should the air temperatures be coming from the supply vents? While most people just assume that “cold” is the correct answer, there is a range of acceptable temperatures. I’ll get into that in just a minute, but first I feel it’s important to understand (just a little) of how our HVAC system works.
When the air conditioning system is on, and working correctly, the process that is occurring is that warm air is being pulled from the home (let’s say 75 degrees) through the return vent and moving through the A/C units evaporator coil. This is where the magic happens!
Science of evaporative cooling
Through the science of evaporative cooling, when this warm air passes through the evaporator coil, a volume of heat is removed from that warm air and we get cold air coming from the air ducts! It is this process that cools your home! Through this process, humidity is also removed, which helps make us feel more comfortable. Now, from the example above, with 75 degree air being pulled in at the air filter, what temperature should we expect to be blowing out? Well, the general consensus is somewhere between 14 to 20 degrees colder than the intake temperature. From my example, 55 to 61 degrees would be an acceptable range.
An air conditioner cools a room by blowing cold air. Exactly how cold should an air conditioner blow? What temperature should come out of the AC vent?
These are useful answers to know if you suspect that something is wrong with your AC. One thing is for sure: If your AC is blowing warm air, something is definitely wrong. You can read a bit more about what causes the air conditioner to blow warm air here.
HVAC experts are less sure about exactly what temp should air be coming out of the vents. You see numbers anywhere from 10 degrees below the room temperature to up to 22 degrees below room temperature being thrown about. Without a clear-cut exact number, here is how we can categorize how cold air should be coming out of AC vents:
- Starting AC: Delta T between 16°F and 22°F. When you start AC, the difference between the current temperature in the room and the air coming out of the AC vents (in HVAC, we refer to this difference as ‘Delta T’) should be the highest.
- Running AC: Delta T between 14°F and 18°F. When the AC is running for some time, you should see the temperature coming out of the AC vents lower by about 1°F-4°F.
- AC Close To Set Temperature: Delta T between 10°F and 14°F. When the temperature in the room is close to the temperature you set on the AC thermostat, you should see that the air coming out of the AC is not as cold as it was below (Delta T reduction).
In any case, the air temperature coming out of the vents should be significantly lower than the room temperature. It is important, however, to have a nuanced approach here (it matters if the AC is just starting, running, or is close to the set temperature).
Example: Let’s say that your start AC when the indoor temperature is 80°F and want to lower it to 72°F. At first, the AC should blow 58°F-64°F temperature air (Delta T = 16°F – 22°F). After some time, you should see the temperature lower to let’s say 77°F and the air coming from AC should have a temperature of 59°F-63°F (Delta T = 14°F – 18°F). When the room temperature gets close to the set 72°F temperature, the AC should be blowing 58°F-62°F air (Delta T = 10°F – 14°F).
Here we notice a very important detail. Namely, the temperature of the air coming from the AC is pretty much constant (between 58°F-64°F in all stages). The room temperature, however, is constantly decreasing; that means that Delta T is decreasing as well.
We will explain how the temperature of the air coming from AC vents is measured, introduce concepts like Delta T, and use the difference between supply air vents and return air vents to diagnose if the air coming out of your AC is cold enough.
To help everybody out with how cold should an AC blow, we have prepared a full table of temperatures that you should come out of AC vents, based on the above categorization and taking into account the 10°F to 22°F lower-than-current-temperature range.
Let’s start by looking at how to adequately estimate how cold air is coming out of your vents (and if there is something wrong with your AC):How To Measure Air Temp Coming Out Of AC Vents (Delta T Calculation)
We usually notice that the air coming out of AC vents is not sufficiently cold. If there are concerns about something being wrong with the AC, a HVAC professional will measure Delta T to confirm the suspicion.
Here is how to measure Delta T:
- Measure the temperature of the air in the supply vents.
- Measure the temperature of the air in the return vents.
- Delta T is the difference between these two temperatures, calculated using this equation:
Delta T = Tsupply – Treturn
You can measure that yourself. Usually, you won’t measure the temperature in supply and return vents; you measure the current room temperature and the temperature of air coming from the air conditioner:
- In the case of central AC, you measure the temperature of air coming out of central AC vents.
- In the case of mini splits, you measure the temperature of air under the air handler.
This is not exactly Delta T as a HVAC professional would measure it (Tsupply – Treturn) but it’s pretty close. In worst cases, you are making a 10-20% measurement error.
Example: You measure the current temperature in the room; let’s say it’s 78°F (this is your Tsupply). Now you measure the temperature of air coming from AC vents or indoor air handler; let’s say it’s 63°F (this is your Treturn). Here is how you calculate Delta T:
Delta T = 78°F – 63°F = 15°F
This means that the air coming out of the AC vents is 15°F lower than the current room temperature.
This is just 1 case. How cold should the air from the air conditioner be? Here is a full chart with expected AC vent temperatures at:
- Different room temperatures (70°F to 90°F).
- Different AC stages (starting AC stage, running AC stage, close to set temperature stage)
These are benchmark estimates of how cold should your air conditioner blow. If you measure Delta T below these numbers, this is a sign that something might be wrong with your AC.
If there is hot air coming through AC vents ac, you do have a problem. The case for warm air coming from AC vents is usually low refrigerant levels, but it can also be loss of AC compression, clogged air filters, clogged expansion valve, power issue like blow fuse or tripped amp breaker, or a thermostat issue. You can read more about air conditioners blowing hot air here.
Hopefully, this illustrates a bit how to think about what temperature you should expect from your AC vents or air handlers.