Simplified sizing of air conditioners
June 9, 2005 8:38 AM   Subscribe

BTUFilter: I'm looking for a plain and simple calculation to size an air conditioner based on cubic feet.

From searching the web, I found some references to "Manual J", which contains what looks to me like rocket science for sizing air conditioners (pretty much over my head). I also found some web based calculators that usually ask for length and width of a room (square feet), but I want to figure this out by cubic feet. Doesn't the ceiling height matter? Where I work, we have a 12 foot ceiling, and I'm guessing that the calculators assume an 8 ft ceiling. I have also come across a rule of thumb that says "1 ton per 500 sq feet". (1 ton = 12,000 BTUs.) Could I just use that rule of thumb and multiply by 1.5 (since 12 is 1.5 times 8)?

I realize there are a lot more variables, such as builing material, number of windows, which way the windows face, etc, but I'm just trying to get in the ballpark.

So, how many BTUs per cubic foot?
posted by jclovebrew to Home & Garden (4 answers total)
I realize there are a lot more variables, such as builing material, number of windows, which way the windows face, etc, but I'm just trying to get in the ballpark.

The problem is, there's more than just space and shade -- there's also the consideration of the number of human occupants in the room, the amount of heat generated by the amount of work equipment in the room, and the type and amount of lighting used, just to begin with. If this is for work, those considerations can be extremely significant, given constant lighting, staff, and equipment -- and unless the ceiling height is abnormal, those factors will almost certainly outweigh any difference between square and cubed feet.

You're going to have someone install/augment the HVAC system, right? The person you'll have do the installation can give you a much better estimate than a web site or someone on AskMe without information on the space.
posted by eschatfische at 8:57 AM on June 9, 2005

Heat gain from windows is a huge factor in sizing airconditioning and can't be disregarded. Unless you just looking at buying a window shaker to cool a room all those calculations should be done. If you just trying to cool a room with a standalone unit then you can use a rule of thumb and multiply by .5 to account for your higher ceilings. 1 ton per 500 sq ft sounds a bit small unless your micro-climate has relatively low summer temps (below about 32) and normal humidity. If you regularily see better than 37 and high humidity you'll need more. For example a lot of the motels you see with thru the wall units are running 220V units rated for 1.5 tons because the units are turned on in the afternoon when it is already hot and they need to cool the room quickly.
posted by Mitheral at 9:38 AM on June 9, 2005

We probably *should* have had an HVAC expert come and set up a solution, but we waited until it actually got very hot, so it became an emergency/rush job. We went to a local electronics store and bought the largest window unit we could get for our 220v/20a circuit, which ended up being 25,000 BTU's.

The space is roughty 1500 sq feet, with 12 ft ceiling. It's also roughly rectangular. While I'm describing it, I might as well mention that we have 3 huge windows in the front, facing north. There is plenty of light, but it's basically indirect light. There are other offices above, below, and next to us.

Based on various back-of-the-envelope calculations we did, we came up with anything from 36,000 to 50,000+ BTUs required. So I fear we undersized it. But we were sweaty and desperate.

Sitting here right now it's actually quite tolerable, but I don't envision the temperature in here ever actually getting to 74 where we set the thermostat, nor do I imagine the compressor shutting off any time before September... I'm thinking that we might augment this with a second window unit, maybe even a smaller one that runs on 110v. But I would like to have some confidence about the calculation. Maybe there's still room to call an expert in. Unless somebody here can lend some good advice.

We're in NYC by the way, where it's currently about 88F and pretty humid. The summers here tend to be very hot and humid. We certainly haven't seen the worst of it yet.
posted by jclovebrew at 10:15 AM on June 9, 2005

When I was doing this kind of work I could do the calcs and bid a job in an hour tops. So if you've got the price of a service call to spend it wouldn't hurt to schedule a call whenever they can get to it (about september probably :( ). FYI A/C can be installed in the winter and you'll get a much better price if there is no demand. A/C contractors like people who schedule installs in the off season 'cause it keeps their guys busy. So if your planning to go central and can sweat it out it'll be worth it.

Also set your thermostat and then don't touch it. You'll get the best system performance that way. If it feels weird that the A/C is coming on in the morning you've got your set point too low turn it up a bit then leave it there.
posted by Mitheral at 11:53 AM on June 9, 2005

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