How Cool Should My House Be If It’s 100F Outside (Or More)?

In the summer heatwave, temperatures easily pass 100°F. To fight this heat, we use air conditioners. The key question here is this:

How cool should my house be if it’s 100°F outside?

In most cases, it is recommended that you set your thermostat at 72°F. However, with extremely high temperatures, you might want to set your thermostat higher than 72°F.

air conditioner setting for when its 100 degrees outside
At 100°F or more outdoor temperatures, it makes more sense to set your AC setting to 74°F than to 72°F.

Setting the thermostat above 72°F makes sense because:

  • You don’t want to overwork your air conditioner. If the AC unit is running close to 100% output all the time, the additional wear-and-tear will reduce the lifespan of your unit. You can read more about how long do different air conditioner types last here.
  • You don’t want to pay higher-than-needed cooling costs. When running an AC unit hard, the energy costs will be higher. Example: Running a 5-ton AC unit at 100% output requires about 5,000W of electric input. That means that every hour, such an AC will spend 5 kWh worth of electricity (that’s about $0.66/hour running cost). You can read more about air conditioner running costs here.

The best temperature to set your air conditioner at 100°F or more is a compromise between:

  • Your comfortability levels. Most people prefer 72°F indoor temperature. But with extremely high 100°F+ outdoor temperatures, you will have to see how comfortable you feel if the indoor temperature is 75°F or even 78°F. Example: Some people feel comfortable at 80°F. Others will sweat profusely at the same temperature.
  • Overworking AC + high cooling costs.

Here’s the general view of this compromise:

The higher the indoor temperature you set, the better for the air conditioner lifespan and your wallet.

Note: You can also use tricks to reduce the apparent temperature (the temperature we experience) as we describe further on (it involves a smart use of ceiling fans and humidity levels).

Now, the exact temperature setting for a 100°F or higher outdoor temperature depends on each person. Nonetheless, let’s look at the general recommendation of how cool should your house be in 100°F or higher outdoor temperatures:

General Recommendation For Air Conditioner Temperature Setting

At normal summer temperatures, a 72°F air conditioner setting is the standard. However, what is the outdoor temperatures spike above 100°F?

We can turn to the Department of Energy (DOE) recommendation for striking a balance between comfortability and cooling power consumption. Namely, DOE states that 78°F is an ideal indoor temperature if you want to keep the cooling costs low and still be as comfortable as possible.

We can use this recommendation as to the recommendation of what temperature to set an air conditioner if it’s 100°F degrees outside or more.

That doesn’t mean, however, that we can’t set the temperature to 75°F or 80°F when the thermometer hits 100°F.

The DOE’s recommendation is primarily based on energy savings and secondarily on our comfort levels.

Depending on how well we can handle the heat, we can set the home thermostat to these different temperatures:

How Cool Should A House Be If It’s 100 Degrees Outside?

Here is the recommendation based on our own comfortability levels:

Type Of Person: Temperature Setting:
Doesn’t Handle Heat Well 75°F
Handles Heat Normally 78°F
Handles Heat Well 80°F

These are the temperature settings that look at your comfortability levels first, and the cooling power expenditure second.

The same settings apply if you are wondering how cool should a house be if it’s 95, 105, 110, or 120 degrees outside.

On top of that, we can also use some tricks to make a higher indoor temperature feel cooler. Here are 2 very useful tricks:

  1. Get a ceiling fan. According to the DOE “If you use air conditioning to cool your home, a ceiling fan will allow you to raise the thermostat setting about 4°F with no reduction in comfort”. That means that you can set your AC unit to 80°F when it’s 100 degrees or more outside, and the indoor temperature will feel like 76°F. You have to know if the ceiling fan should rotate clockwise or counterclockwise, of course. To get one, you can check which are the best ceiling fans here.
    how cool should a house be with a ceiling fan
    In the summer, the ceiling fan should be spinning counterclockwise.
  2. Lower humidity levels (with AC or/and dehumidifier). We perceive dry heat to be less hot. Similarly, we perceive wet heat to be hotter. This relationship is described by the Human Heat Index below. The best way to reduce humidity levels in the summer is to run an air conditioner and dehumidifier simultaneously. Some people are not aware that air conditioners reduce humidity levels as well. how to feel cooler when its 100 degrees outside

This index clearly indicates that, for example, if we have an indoor temperature of 80°F, it will feel like:

  • 75°F at 10% relative humidity levels.
  • 77°F at 20% relative humidity levels.
  • 78°F at 30% relative humidity levels.
  • 79°F at 40% relative humidity levels.
  • 81°F at 50% relative humidity levels.
  • 82°F at 60% relative humidity levels.
  • 85°F at 70% relative humidity levels.
  • 86°F at 80% relative humidity levels.
  • 88°F at 90% relative humidity levels.
  • 91°F at 100% relative humidity levels.

That means that, if you have lower humidity levels, you can set the AC unit to higher temperatures (80°F or more) even when the outside temperature is in access to 100°F, and still feel a bit cooler. Obviously, you do want to avoid high humidity levels since this will make the indoor temperature feel hotter than it actually is.

We hope that this indicates clearly how the compromise between comfortability levels and AC energy expenditure (and lifespan) should be taken into account at very high outdoor temperatures.

2 thoughts on “How Cool Should My House Be If It’s 100F Outside (Or More)?”

  1. I wish the cool my bedroom at night down to low 60″ with heavy covers, otherwise I don’t sleep well. I only have 210 sq ft w 10′ ceilings (approximately 250 sq ft equivalent). This will allow me to turn off my central AC at night and not cool my whole home. I would like to install the DIY Mr Cool 12k unit. Would this be overkill? I assume it would just run less frequently with no detrimental affect. Your thoughts.

    • Hello Bud, as have written in our Mr Cool DIY series review here, this is one of the best mini split brands to choose. For a 250 sq ft (effectively), a 12k unit might be a bit oversized. With oversized units, you have to worry about AC short cycling. Here’s why these problems will most likely not occur in your situation:
      1. The low setting of 60″ will actually need a bit oversized unit to achieve. The 12k unit would be the perfect fit.
      2. You only get short cycling (main problem with oversized units) if an AC has to cool several rooms. Since you are only looking to cool down a bedroom (one room), you won’t have problems here.

      In short, for your situation, the DIY Mr Cool 12k unit is adequately sized and you just can get a better DIY mini split than that (here is a spec-by-spec analysis of the best DIY mini splits supporting that).


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