Why air conditioner “sizing” can be inaccurate and just plain wrong

Last updated on March 10th, 2024 at 03:15 pm

When you size your room for an air conditioner, or when a professional uses energy calculators to size a room for cooling system installation, there is a hidden variable that everyone misses – even the pros.

Do you know what it is?

The answer is people. People and to a lesser extent, pets.

In other words, the occupants of the house.

I’m not talking about your energy habits, like keeping the thermostat in check and weatherproofing your windows – though that is important.

There is actually a more obvious but “hidden” component we all forget about. But its important.

You see, when you are in a room, you give off body heat. Now one person in a decent sized room doesn’t give off enough heat to matter in terms of keeping the room cool. But take a few people and stick them in a 10 x 10 or 12 x 12 room, and now you’re talking some serious heat generation that the air conditioner has to combat.

If you think about it, this makes sense. Consider that when you lay in your bed at night, when you first slide under the covers you are cool. In a few minutes, you are warm. The covers are insulating you, keeping your body heat close to you. Thick covers do a better job. When you’re in a room, you still give off heat. And you keep giving off heat as long as you stay in the room.

Now as mentioned, usually one person doesn’t make a difference (unless you’re in a closet or something). But multiply your heat times two or three, and the temperature of the room will rise. Humidity will rise too, generally speaking, because we exhale water vapor.

Here’s another example. In the colder months, have you ever attended a party where one room, full of people, is absolutely blistering while the other rooms were just warm? When everyone gathers, their body heat multiplies and adds to the air temperature of the room.

So, how do you account for this when planning for cooling? Well, as mentioned, one or two people in a room isn’t a big deal. More than that and you may have a cooling problem. You can either oversize the a/c unit (not generally recommended), disperse the people throughout the other rooms (doesn’t work as well), or adjust the thermostat temporarily. None of these are good options – most people just have to “live with” the temporary increase in heat and thus an increase in energy bills directly related to cooling.

The best option probably lies somewhere between adjusting the thermostat and dispersing people. People will naturally gravitate to areas that make them comfortable. In colder months, if you are having a lot of people over you should consider moving the thermostat to a lower temperature once the guests start to arrive (no one likes to arrive in a cold house) because their heat will keep them warm and you won’t overheat them. In the summer, it is more difficult. You should probably move the thermostat slightly lower again once guests arrive to combat the extra heat. Yes, this will cost you a little more. If you find guests scattered more, then raise the thermostat to normal. You have to balance your guests comfort with energy saving.

For me, its a no brainer – I want my guests to be comfortable. There’s no sense having people over if they’re miserable. Besides if you follow energy conservation and energy saving tips, you’ll be saving in other areas year round. Plus, in the warmer months, manby people like to go outside and enjoy the weather – fewer stick around in the house to “enjoy” air conditioning.

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