# How much energy (money) do you waste when you use air with the doors open?

I’m trying to figure out approximately how much energy is wasted when an interior space is cooled in the hot summer, with the doors wide open. In other words, how much money would you save if you closed the doors vs. keep them open. Are there any easy ways to estimate this?

Anyone know a good place to start to get an answer to this question? Is there an “ask an engineer” site on the web somewhere?

The quick and correct answer is no, there is no easy way to estimate the heat gain caused by leaving all the doors and windows open. What’s the inside temperature? What’s the outside temperature? The humidity? Is it daytime or nightime, is the wind blowing, are you in a single story or multi-story dwelling, how many square feet of open area are there, how many cubic feet in the building are being cooled, etc.?

It can be done, but not quickly or easily, or even accurately without a lot more information.

Thanks for your answer, yes I know you need all those measurements, I was hoping to get information on what measurements I’d need for this, since I don’t even know where to start. I figured inside and ouside temperature, size of the area. Assuming no wind, how do
the other factors play into it? I guess you’d assume daytime no sun, since there’s no easy way to account for those factors. No wind. I don’t need a precise measurement, just an idea.

Here’s the situation: I’m trying to prove that a public agency is wasting a whole lot of money and energy with this practice. I need some way to quantify it, some method to generate some figures. Any ideas would be greatly appreciated. If you have a large space with lots of air conditioning units cooling the air locally, does it waste huge amounts of money if the giant space is open to the air, or is it negligible?

Your’re still going to have density differences between inside and outside air, which will drive a flow.
Engineering Toolbox has a bunch of stuff that might help. For example, this section on Air Flow and Velocities due to Natural Draft might get you the amount of air moved from inside to outside per hour.
That should let you calculate how much cooling you’d need per hour, und so weiter.

Once you know how much air will infiltrate you’ll then need the specific heat capacity of dry air, which can be found on the same engineering toolbox site, and calculate how much moisture will need to be removed, and figure the change in heat required to condense the water vapor.

I’ve been in open outdoor theaters that are recessed (like a bowl), so the cold air tends to stay put. I would guess that these are not horribly inefficient, even if being completely enclosed would be better.

One way to get an idea of how bad a problem this is, is to compare their energy usage with a similar enclosed space.

Put a power meter on the AC, and see how much power it uses when it’s on. Assume that it would be on 100% of the time, and that it wouldn’t really be cooling much of anything for long.

What’s a reasonable amount of time % wise for the AC not to be drawing power?

That really depends on what you have it set on and what the local climate is. If I had to guess, mine’s on maybe 10 minutes of every hour, which is what, ~16%.

Keep track of how often the AC is on per day and assume it’d be on all the time if your doors are open. This is probably the least accurate method.

Can a large facility tell how much electricity it’s using on AC vs. other electrical needs? Do they keep records of this type of thing?