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:
| From/To: | 75°F | 80°F | 85°F | 90°F | 95°F |
| 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.
Is the chart different if you live in the desert? I’m in Arizona and it is 106 degrees right now, Our indoor temp was 81 and took almost 3 hours to get to 78. One digit at a time.
Hello Jane, it depends largely on how big an air conditioning system you have. In general, how long should it take to cool a house depends on humidity levels as well; air with a lot of air moisture is harder to cool. Given that you are in the Arizona desert, the air should be dry and therefore the cooling time should be reduced. So, it seems that AC being small or insulation levels are sub-par are two factors that prolong the time it takes to decrease the temperature in the house. Hope this helps.
We live in Eastern NC and are currently experiencing temps of about 100 degrees daily outside. We are moving into a 100 yr old, 3500 Sqft home this weekend that sat with no power for 3 days during said temps above. Currently it has been 6 days since the power was restored and the home has been set to 72 degrees with almost no foot traffic in and out, no open windows, and no cooking or baking. The house is still sitting at roughly 84 degrees on the second floor, 78 on the first floor, and hot hot in the dome above the second floor.
There is a thermostat on both the first and second floor, but we have only seen one unit on the outside of the home, but the owner states it is a larger unit than a usual residential unit would be. The question here is how much longer should I give the system to play catch up before I request to have it looked at? We officially move in in 2 days and don’t want to be sitting in a hot house.
Thank you!
Hi Patience, that 84-degree indoor temperature really has to be dealt with. Now, a 3500 sq ft house that is 100 years old (usually has subpar R-insulation values compared to modern houses) will need a lot of cooling output. If you just go by the 20 BTU per sq ft rule of thumb, you would require about 6 tons of cooling output. That one larger outdoor AC unit might not provide sufficient cooling power.
If you add in the below-average insulation, the high outdoor temperatures can be quite devastating. It might be that the current AC unit is sufficient to handle 90 degree heat; however, it can be overwhelmed at 100 degree days. All things considered, 84 degrees after 6 days of running just means that something is amiss here.
The best advice would be to call a HVAC guy. He or she will be able to tell (using Manual J calculation) if the AC unit is sufficient, especially given the insulation properties of 100 year old houses. If you are new in the area and don’t really have your HVAC guy here, you can use this form to get some contacts from local HVAC companies. It’s best to assess the situation and solve it now than later, I think. Given the 84 temperature, it doesn’t seem that the AC will be capable of pushing that temp below 80 in 2 days. Hope this helps and that you find a good solution here.
Same here, I’m in AZ and came to this site for info but I turned on my AC at 2PM and it was 80 inside the house (2200sq ft) with ambient temps of 110 to 115 today but it’s been running 7 hours straight and it’s only dropped 4 degrees to 76 inside at 9 PM now 104 outside. Brand new build, well insulated. Is the AC underpowered for this home? I’m afraid it’s going to break.
Hi Nate, in these very hot days, it’s really challenging even for an appropriate sized units to lower the temperature adequately. The AC units are well-designed to run at very high capacity for longer periods of time; a new unit should not break even when fighting 115 ambient temperature.
The plan here would be to run the AC unit on Auto (it will prompt the unit to run at near 100% capacity) and hope that will lower temperature to at least 75 degrees. If these high temperature days (110+) continue and you will the AC is not sufficiently sized to deliver enough cooling output to lower the temperature, getting an additional portable or window AC might be a good idea. Hope this helps a bit.
The temperature in my house right now is 89 degrees. The thermostat is set at 68 degrees. How long should it take to cool my house to make it comfortable to stay in the house.
Hi Verna, of course, it depends on quite a lot of factors such as insulation, AC tonnage, outdoor air temperature, windows, and so on. But if we follow the general rule of thumb that it takes on average about 18 minutes for the temperature to lower by 1 degree, you are looking at about 6 hours for the house to cool down to 68 degrees. Some people are already comfortable at about 78 degrees, that can take about 3 hours. Hope this helps.
This is fantastic, thank you so much for this!
Last night at 11:30 outside temperature it was 88 degrees inside was 83 degress 5 degrees diferentes by 5:30 am outside it was 81 and inside was 78 degrees only 3 degrees difernce usually have it automatic till 78 grades never turn off I know it needs to hit till 77 to turn off Didn’t happens during night even that is more fresh it’s not going to happens during day time.Doing the math it take 6 hrs for 10 degress.Do you thing it needs freeze on or werheter the name is .To me is like something is wrong but I rent this house and the owner of the house things it works ok .What you think? Help me with your advise please And I ask for another filter for this year didn’t bring that .It may affect to cool down faster .How often you have to change the filter I thing my is from last year.
Hi Elizabeth, what is your thermostat setting? If you have it set at 78 degrees, the air conditioner is working fine. If not, there might be something wrong with the AC. You can check here how often should you change air filters in AC; it’s anywhere between 1 and 6 months. So, twice a year seems quite reasonable. Hope this helps.
Hi
Out side air temperature is 45 degree and humidity is 50 %. How long will it take for a 2 ton ac to tredude the inside temperature from 31 degrees to 25 degrees?
Hi there, 45 degrees Celsius? That’s extremely hot. It depends on the square footage and insulation R-values, and so on. If you take the statistical average 18 minutes per 1 degree, you are looking at 6-degree difference or about 2 hours of cooling time.
Hi I live in a 2 bedroom 960 sq ft apartment. Was 81 in apartment. 89 outside. Took it 6 hours to get to 75. We are second floor. No one above us and no one on one side. Is this ok???
Hi Eva, if the AC can manage to adequately lower the temp, that’s all you actually need. Given that it took longer than 18 min per degree, you might have below-average insulation R-values, a lot of windows, or a bit undersized AC, or a combination of these factors. The important part is that you got to 75 degrees.
Hi I have 3.5 tone ac with 14 seer for my 2 storey 2700 square feet house I Canada ( now the outside temp is 30 degree cent grade). How long it take to cool the house to get 17 degrees .
Thanks
Hi Paul, alright, so the key part of the estimate here is that if you have an adequately sized AC unit, you should expect a 1-degree indoor temperature decrease every 18 minutes. That’s the average and there are many factors that can affect it (weather, outdoor temperature, insulation, humidity levels, etc.).
Now, for a 2,700 sq ft house in Canada, a 3.5 ton AC might be a little undersized. You are basically delivering 15 BTU of cooling output per sq ft; the rule of thumb is that you should have about 20 BTU per sq ft. A big 2 story house will also need a bit more time than average because it presents a large distribution space.
So, in short, you can expect the indoor temperature to fall by 1-degree every 30-minutes and up to 1 hour. This is a rough estimate, hope it helps.
I have a dual zone unit HVAC with only 1 condenser (huge grey box) outside and 2 controlled thermostats. My house is 4,200
Sq ft and has 3 levels with a finished basement. One thermostat controls the basement and main floor. The other thermostat controls the upstairs (top level). It was 85 degrees in my house and outside was around 87 degrees. The HVAC tech fixed my unit as it stop running completely. Before he left, I told him that I believe my AC capacitor might need replacing as the another HVAC maintenance tech who came months ago explained that we might need to replace it soon. This HVAC tech said everything seems to be working right and he can’t say for sure if it needs to be replaced as it’s operating right now. Now it’s been 8 hours running nonstop…It’s still 85-87 degrees…upstairs and downstairs and there’s little air coming out the vents. So I don’t know how long it takes for my house to get cooled from 85 degrees to 75. I don’t want to be wasting time and $ for this HVAC that’s been running for many hours nonstop only to end up having a huge electric bill. I have left several messages for tech but no return calls yet. If I need to replace my capacitor would be helpful as to why my house is still warm with no changes in 85 degrees. How long does it take to cool down 85 degrees to 75 degrees in a 4,200 sq ft home with 3 levels and a dual zone HVAC on a 87 degree weather outside day? Please help!
Hi there, thanks for describing the situation in detail. Now, the thing about capacitor is that A) it works, B) it doesn’t work. Very rarely you will see ‘just works sometimes’. Here’s the HVAC techs mentality: If it works, don’t change it, if it doesn’t work, change it. So, the tech made the right call here. If the AC has been working 8 hours nonstop, then the capacitor should be OK.
Alright, but you still have 85 degrees, not 75 degrees. This could be explained by undersized AC unit; if this is a new unit, it just might be that it’s too small. For 4,200 sq ft, you would need (applying the basic 20 BTU per sq ft rule of thumb here) about 84,000 BTU or 7 tons of cooling output. Given that you have 2 air handlers, each of them should deliver 3.5 tons (42,000 BTU), and that’s just not feasible. Air handlers are usually 9,000 BTU, 12,000 BTU, 18,000 BTU. So, the most likely problem here is that you have a way undersized AC unit. If that’s the case, you can run it 24/7, and you still won’t be able to cool your 4,200 sq ft home to 75 degrees.
I know this is not a good information, but based on the data, it is the most likely case. Hope you find the least expensive solution for this, best of luck!
What could be causing a greatly reduced airflow after replacing my 4 ton A/C condensor & coil. My Coleman A/C quit 6 days ago after 27 ¹/² years. The new unit is a different brand. The outdoor temperature reached 100 today. The indoor temperature reached 91. How long should it take to cool my 2100 square foot home that has a new metal roof installed on top of previous existing one, all blinds closed, all duct work intact, and ceiling fans on? I would like the temperature to be 72.
Hi Yevette, thank you for the question. Namely, the 1st order of business is to check if the new outdoor unit is correctly installed (superheat/subcooling, airflow measurements). Now, when the outdoor temperatures are at 100+ degrees, the AC unit will works slower, but the 91 degree indoor temperature is significantly over what would be considered normal indoor temperature.
4 ton unit is sufficiently sized for a 2,100 sq ft home. If it doesn’t bring the temperature below 75 degrees (or at least 80 degrees), something is probably wrong with the unit itself. Hope this helps a bit.
I have been attempting to educate myself about HVAC units the last several weeks and I’ve read a lot of conflicting data online making it a frustrating effort. This site has been a breath of fresh air as it has a wealth of information, and I can’t thank you enough.
I have a few questions regarding a new HVAC system that was installed as part of a new-build condominium. The property is east/west facing, 2400sq.ft. single top floor with 10’ tall ceiling throughout the entire unit (24000 ft³). There are 27 windows + one slider and located in Southern CA. All things considered, I learned that such a property would likely require a 5-ton unit. However, the developer installed a 3-ton.
My first question; am I right on my calculations of a 5-ton? If yes, would there be any time a 3-ton would suffice using the same configuration?
Secondly, a heat pump was placed outside alongside the house on a narrow walkway that also has a short 3.5’ wall on two sides of the unit, the condo on the third side and the walkway leading to it on the 4th side, which is unobstructed. Clearance on the discharge side is 20.5”, 6” on the back side and 7” on the coil side against the house. When the hot air is discharged it seems to be bouncing off the short wall and as a result it’s circulated back to the coil side of the compressor.
My second question; Isn’t the clearance around the unit less than optimum and will it cause problems?
For now, the system does cool, but so very slow. It takes 3+ hours to get down 5°. To me that doesn’t seen normal and doesn’t correspond with other properties we have that can double that in 3 hours but are smaller in size.
Thanks in advance for any assistance.
Hi David, thank you for your kind words. We try really hard to bring the no nonsense data to help everybody.
Alright, 1st question: If we just use the basic 20 BTU per sq ft 8 ft ceiling height premise, this would means that 2,400 sq ft 10 ft ceiling height would require 2,400 sq ft × (20 BTU / 8 ft) × 10 ft = 60,000 BTU of cooling output. That’s exactly 5 tons, as you have correctly calculated.
Can you use 3 ton unit instead of 5 ton unit? If these condos are in Canada, Illinois, Minnesota, and you have world-class insulation, that might do. Might. But I’ve never seen such a configuration to be conditioned with a 3 ton unit. An on-site HVAC specialist will have to use a detailed Manual J calculation but it’s almost impossible to get to 3 tons. Very likely, you need a larger unit. 5 ton would be the first rough choice, obviously.
When you have 100+ degree days, it’s doubtful the 3 ton unit will suffice. 3+ hours for 5-degree difference is a good practical example that this unit is undersized.
2nd question: You want the outdoor unit to be surrounded by cold air (for cooling). Shade would be great, adequate clearance is good, but the hot air circulating back to the compressor is pretty much the worst idea ever. It’s like your burning a fire near the compressor; it will have a harder time to expel the excess heat.
Now, I get that realistically you can’t have an ideal location for the outdoor unit, given how the place is structured. But hot air bouncing back to the outdoor unit is just wrong.
Honestly, both of these things seems to be off. Calling a HVAC guy to recheck everything is a good idea here; he or she will likely have much the same thoughts that you already have. Hope this helps a bit, and good luck with quite an unpleasant situation.