To figure out the right size a/c unit...try this method

robarSeptember 13, 2012

I saw this on the internet. Take the square footage of your home and multiply it by 25, then divide this amount by 12,000. This will tell you the size (ton) you will need for your central a/c. Example....2000 sq ft. home multiply by 25 equals 50,000 divided by 12,000 equals 4.16. You would need a 4 ton unit.

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I don't think so.

I've got a 2000 square foot home and it has a 3 Ton unit that is sized properly.

    Bookmark   September 13, 2012 at 8:11PM
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Loads are a factor of many many things, and doing the math is the only way to know that the load is correct.

Using square footage doesn't account even for 8 foot or 10 foot ceilings. That alone would alter the volume of air by 20-25%.

Not to mention, R value of insulation, east and west windows. skylights, and building materials.

    Bookmark   September 13, 2012 at 8:25PM
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Robar...that exactly what every installer in my area uses to quickly determine the equipment people need.

This is completely wrong and you end up with the wrong equipment about 80% of the time.

This is the lazy way of doing it to make a quick sale.

Take a look at this free online Manual J calculator to get an idea of all the factors that go into calculating loads:

I'm not affiliated with the site...I just found it and learned to use it. It took me a few days, but it really helps you understand what's the appropriate BTUs for your house for cooling and heating.

    Bookmark   September 14, 2012 at 12:36AM
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This is also called the 500 sq. ft. per ton calculation. This is wrong and leads to oversized equipment.

    Bookmark   September 14, 2012 at 12:55PM
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there is a reason you spend the money for the
software to correctly do a load calc.
rule of thumb sizing causes a LOT of problems.
for homeowners.
hvac companies make more money by selling
larger equipment..with shortened lives due to
excessive stop & starts..utility companies
profit in that it causes more to start up the
unit than to run it for long lengths of time.

its bs OP.

a bit more classy than standing across the
street and holding up fingers to cover house
and using that as guide for sizing..but bs none
the less.

do a search for load calcs, manual j, s, d....

best of luck

    Bookmark   September 14, 2012 at 7:28PM
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+1, the 500sqft per ton is obsolete. Newer homes can do 1,000sqft per ton in southern climates.

    Bookmark   September 18, 2012 at 8:12AM
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If two twin 2000 sqft houses in said southern climate are built next to each other by the same builder using the same materials, but one house was built with no insulation in the walls, no insulation in the attic, and no insulation on the floor and the other had R38 in the attic, R15 in the walls, and R11 in the floor, then their cooling and heating needs would be different.

So, we are trying to raise awareness that using sqft per ton is obsolete regardless if you use 300 sqft/ton or 400 sqft/ton or 1000/sqft per ton. They are all the load calculation.

    Bookmark   September 21, 2012 at 1:21AM
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SO how do you know whats right?? I have 2 estimates for central a/c. Both for RHeem units. I have a 2 floor colonial with 35 windows. 1st fl - 2300 sq ft 2nd fl 1700 sq ft. I have 2 pretty different estimates. #1- says I need a 3 ton and 4 ton. #2- says I need a 4 ton and either another 4 or 5 ton. He is going to do more calc and get back to me. I have oil baseboard heat. no ducts yet.

    Bookmark   October 2, 2012 at 8:11PM
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The load calculation (Manual J) takes into account a lot of factors like insulation, windows on east/west facing walls, sqft, doors, type of floor, climate, volume of house, doors, appliances, people in house, and so on.

So, a Manual J calculation will spit out your cooling and heating needs in BTUs and this calculation is far more useful than the 400 sqft/ton lazy way of estimating. This is how you know you have the right sized equipment.

    Bookmark   October 2, 2012 at 10:51PM
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I'm sorry I don't mean to b a PIA but if I live on Long Island with a family of 5. 15 windows west 20 windows east. Avg insulation. Im worried b/c I know the size of unit is very important and I got 2 different opinions. All bedrooms (4) and 2 baths upstairs. I like to sleep in cold a/c my DH makes fun of me I'm trying to freeze him like Walt Disney. LOL

    Bookmark   October 3, 2012 at 6:10AM
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Both estimates could be wrong. You are getting estimates of 7 - 9 tons for a 4000 sq ft house. This seems oversized unless you have poor insulation and windows.

In general contractors will tend to oversize in order to play it safe when there is no existing AC.

    Bookmark   October 3, 2012 at 8:50AM
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OK I don't have existing a/c so I guess that could be why. I gues I'm worried because everyone on the forum seems to say getting the correct size unit is very important.

    Bookmark   October 4, 2012 at 12:57AM
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Getting the correct size is very important. This is why you need to ask a lot of questions of the contractor.

You have a big old house, so it is easy to make a lot of incorrect assumptions. I suggest you have the contractor show you the load calculation print out. Make sure he has accurately entered the dimensions of all rooms and windows. Ask what assumptions have been used for insulation and air infiltration. Find out what software he is using. If he can't answer these questions or won't show you the results, then a load calculation was not done.

When you have some data start your own thead with an appropriate title.

    Bookmark   October 4, 2012 at 8:51AM
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Thanks for the great input. I will certainly post results.

    Bookmark   October 5, 2012 at 8:11AM
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I keep reading on forums that a load calc is needed. I have talked to 2 HVAC places that said they would not do a load calc. Finally the 3rd place I called the boss told me he would do one for $450. Maybe in middle Tennessee HVAC people
feel like it's a waste of time. I will keep trying but I understand why people don't get a load calc.

    Bookmark   August 20, 2013 at 4:30PM
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Here I had to get a Load calc in order to get the permit. There was no separate charge for it.


This post was edited by saltidawg on Tue, Aug 20, 13 at 17:10

    Bookmark   August 20, 2013 at 4:38PM
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