How fast does an AC cool a house? The key question we have to answer is how long does it take to cool a house by 1 degree. Based on that, we can calculate how long should it take to cool a house from 80°F to 72°F or by 2 degrees, for example. As you might imagine, how long does it take to cool down a house depends on several factors.
Theoretically speaking, we can presume that the heat capacity of air at 72°F is 1.006 kJ/kg*K and its density at 72°F and 14.7 psia is 1.195 kg/m3.
If we use ’20 BTU per sq ft with 8 ft ceilings’ the DOE rule of thumb for sizing AC units, we know that we will need about 1-ton of AC (12,000 BTU/h or 12,661 kJ) to adequately cool a 600 sq ft space (20×30 room, for example). In order to calculate the cooling time, we use these two equations:
Q = m×C×ΔT and m = V×ρ
Don’t worry about how complex this sounds. We are going to simplify it quite a bit further on; but if we want exact figures, we need to make precise calculations. On top of that, you will be able to use the Cooling Time Calculator yourself and check out the Cooling Time Chart as well.
Before we check all the factors that affect the speed of temperature decrease when running an air conditioner, let’s summarize rough estimates, based on these calculations with added losses adjustments:
- It should take a standard AC 18 minutes to cool down a house by 1 degree.
- It should take a standard AC 36 minutes to cool down a house by 2 degrees.
- It should take a standard AC 54 minutes to cool down a house by 3 degrees.
- It should take a standard AC 1 hour and 12 minutes to cool down a house by 4 degrees.
- It should take a standard AC 1 hour and 30 minutes to cool down a house by 5 degrees.
- On average, it should take about 3 hours (180 minutes) to cool down a house by 10 degrees.
Inversely, we can also answer how many degrees should ac cool per hour. Within 1 hour, the AC should decrease the temperature by about 3.3 degrees.
By knowing how long does it take to cool a house down by 1 degree, we can adequately estimate how long should it take to cool a house from 80 degrees to 72 degrees, right?
That’s an 8-degree difference. Here is how we can calculate how long it takes to get from 80°F to 72°F:
Cooling Time (From 80°F To 72°F) = 8 × 18 Minutes = 144 Minutes = 2.4 Hours
In general, it takes 2 hours and 24 minutes to cool a house from 80°F to 72°F. However – this is very important – you can just blindly take this number and assume that your house will be cooled to 72°F in 2.4 hours.
What you vitally need to include in your calculation are the contributing factors. Some factors like good insulation and a big AC unit can decrease the time it takes for the AC to cool your house down. Others like very hot outdoor temperatures (100°F or more) and the size of your home will increase this time.
To get a good feeling of what is a sensible time for your house to cool down to 72°F, you will have to account for these factors. Further on, we will should you how to do just that.
There is more:
Not everybody starts at 80°F. Some houses are at 85°F and you want to cool them to 72°F, others are at 87°F, 90°F, and so on. For everybody to be able to get an estimate of how long should it take for a house to cool to 72°F, or even to 75°F or 78°F, we have prepared a Cooling Time Calculator.
This calculator will estimate (based on the time it takes for a 1-degree drop in temperature) how long should it take to cool a house from:
- Any start temperature. Example: 80°F, 82°F, 85°F, 88°F, 90°F, and so on.
- To any end temperature. Example: 75°F, 72°F, 70°F, 68°F, and so on.
Below the Cooling Time Calculator, you will also find a calculated Cooling Time Chart of how many hours it takes to cool a house from any start temperature to 72°F.
Before we check out the calculator and the calculated table, let’s look at what factors we should take into account when determining how long it takes to cool a house:
Factors That Affect The Speed Of Cooling
On average, it takes about 18 minutes to cool down a house by 1 degree. Of course, in some homes, we will see that a 1-degree drop happens in 12 minutes, and in others, it will take longer; let’s say 25 minutes.
Why is that?
Well, every home is different. The 18-minute mark is just a rough average. Factors that can increase or decrease the time it takes for cooling a home include:
- Size of your air conditioning systems. This is a relative size; if you have an oversized unit, you will likely see that a 1-degree temperature drop happens in less than 18 minutes. If, on the other hand, you have an undersized unit for your home, the 1-degree temperature drop will take more than 18 minutes. You can check what size air conditioner you need in terms of BTUs (adequate sizing) here.
- Insulation. Insulation plays a key role in heat exchange between your home and the surroundings. With better insulation (high insulation R-values), you will be able to cool your house quicker than with subpar insulation.
- Size of your home. The 18-minute drop is calculated for an average 2,200 sq ft house. With bigger houses, the cooling will usually take longer to drop the indoor temperature from 80 degrees to 72 degrees. In smaller houses, you can see a faster change in temperature.
- Outdoor temperature. The higher the outdoor temperature, the slower the process of cooling. For example, if the outdoor temperature is 100°F you will be able to cool your house down slower than if it would be 90°F. This is because no insulation is perfect; outdoor heat will inevitably sieve into the house and the air conditioner has to fight this heat transfer as well.
With all these factors in mind, we can look at the Cooling Time Calculator which uses an average house size, adequately-sized air conditioner, average insulation, and 95°F outdoor temperature conditions to calculate how quickly you can cool a house:
Cooling Time Calculator
Here’s how you can use this calculator:
Let’s say you want to cool your house from 80°F to 72°F. You slide the first input to ’80’, the second input to ’72’, and you will get the result in minutes.
Namely, you can see that, on average, it takes 144 minutes (or 2 hours and 24 minutes) to cool down a house from 80°F to 72°F.
You can also consult this cooling time chart, based on the same presumptions about the AC, house sizing, and insulation R-values:
Cooling Time Chart
This chart tells you how long should it take to cool down a house from different initial temperatures to the desired temperature:
|65°F||180 Minutes||240 Minutes||360 Minutes||450 Minutes||540 Minutes|
|68°F||126 Minutes||216 Minutes||306 Minutes||396 Minutes||486 Minutes|
|70°F||90 Minutes||180 Minutes||270 Minutes||360 Minutes||450 Minutes|
|72°F||52 Minutes||144 Minutes||234 Minutes||324 Minutes||414 Minutes|
|75°F||0 Minutes||90 Minutes||180 Minutes||270 Minutes||360 Minutes|
|78°F||N/A||36 Minutes||126 Minutes||216 Minutes||306 Minutes|
For example, let’s say you want to cool your house by 2-degrees from 80°F to 78°F. This will take, on average, about 36 minutes.
If, however, you want to go to extremes, you can take the cooling from 95°F to 65°F as an example. This is a massive 30°F drop in indoor temperature; it will take about 540 minutes (9 hours) of the air conditioner running at 100% output to achieve that.
Hopefully, now you have a good understanding of how quickly an air conditioner can cool a house. You can use both the calculator and this table to get an idea of how long should it take to cool down a house.
Of course, be mindful about including the specified factors above in the calculation; with some houses, it may take longer to cool down, and with others, it will take less time.