Author Topic: a/c sizing  (Read 794 times)

Offline robstar

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a/c sizing
« on: June 11, 2017, 11:36:03 AM »
Hi Guys,

I just wondering what formula you guys using to size an a/c

Offline walker

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Re: a/c sizing
« Reply #1 on: June 13, 2017, 04:02:30 PM »
a great start would be a heat load calculation.

Offline Admin

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Re: a/c sizing
« Reply #2 on: June 13, 2017, 04:28:03 PM »
That's the safe bet for sure.  I follow the HVAC design sizing but to be honest 90% of the time I divide the square feet of the house by 950 and come up with the same size.  The designers tend to undersize now to improve comfort.

Offline Jorgebaloy

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Re: a/c sizing
« Reply #3 on: July 12, 2018, 12:17:30 PM »
Measure the space: Whether you want to cool just one or two rooms or a whole house, you’ll need to figure out how many square feet the area is. First, measure the length and width of the room or rooms to be cooled.

Most of time, you can find the square footage of your entire home in your mortgage paperwork or property tax statements, which will save you from having to re-calculate. If you do need to get out the tape measure, remember not to include your basement, attic space, or closets.

Calculate the square feet: Once you have the length and the width of the area, you’ll need to multiply those two numbers together to come up with the square footage.

Determine the cooling capacity: How fast an A/C can produce cool air is measured in BTUs (British Thermal Units) per hour. You need approximately 30-35 BTUs/hour for each square foot of space if you live in a warm climate and 50-60 if you live in a cooler climate.

Offline tenletters

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Re: a/c sizing
« Reply #4 on: July 12, 2018, 03:49:56 PM »
a great start would be a heat load calculation.


Typical Internet response. I bet my life your company is not doing heat loads on all retros. If you say you are, I call complete BS.

Offline walker

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Re: a/c sizing
« Reply #5 on: July 13, 2018, 05:08:52 PM »
a great start would be a heat load calculation.


Typical Internet response. I bet my life your company is not doing heat loads on all retros. If you say you are, I call complete BS.

thanks for the vote of confidence.  My company who knows, I do only service work for them.  On side work, yes I do them, its looks good to the customer.  but as Admin pointed out 9.5 times out of 10 they come out exactly as you'd think they would using rule of thumb.

Offline tenletters

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Re: a/c sizing
« Reply #6 on: July 13, 2018, 05:53:46 PM »
a great start would be a heat load calculation.


Typical Internet response. I bet my life your company is not doing heat loads on all retros. If you say you are, I call complete BS.

thanks for the vote of confidence.  My company who knows, I do only service work for them.  On side work, yes I do them, its looks good to the customer.  but as Admin pointed out 9.5 times out of 10 they come out exactly as you'd think they would using rule of thumb.

I just disagree is all. Doesn't mean yourself or the company does bad work. I really do not believe you're completing a certified heat load on side jobs, unless it's for new housing that is.

It's possible you have a spreadsheet with fill in the blanks like reliance sales staff, but that's not a heat load and often times tells the installer to put a 120k btu furnace and a 1 1/2 ton AC in an 800 sq ft house.

If you actually are doing a real certified heat load, awesome. It looks really good on you.

Offline walker

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Re: a/c sizing
« Reply #7 on: July 15, 2018, 07:08:01 AM »
a great start would be a heat load calculation.


Typical Internet response. I bet my life your company is not doing heat loads on all retros. If you say you are, I call complete BS.

thanks for the vote of confidence.  My company who knows, I do only service work for them.  On side work, yes I do them, its looks good to the customer.  but as Admin pointed out 9.5 times out of 10 they come out exactly as you'd think they would using rule of thumb.

I just disagree is all. Doesn't mean yourself or the company does bad work. I really do not believe you're completing a certified heat load on side jobs, unless it's for new housing that is.

It's possible you have a spreadsheet with fill in the blanks like reliance sales staff, but that's not a heat load and often times tells the installer to put a 120k btu furnace and a 1 1/2 ton AC in an 800 sq ft house.

If you actually are doing a real certified heat load, awesome. It looks really good on you.

I use coolcalc manual J which is ACCA approved, good enough for me.

Offline tenletters

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Re: a/c sizing
« Reply #8 on: July 15, 2018, 11:14:45 AM »
a great start would be a heat load calculation.


Typical Internet response. I bet my life your company is not doing heat loads on all retros. If you say you are, I call complete BS.

thanks for the vote of confidence.  My company who knows, I do only service work for them.  On side work, yes I do them, its looks good to the customer.  but as Admin pointed out 9.5 times out of 10 they come out exactly as you'd think they would using rule of thumb.

I just disagree is all. Doesn't mean yourself or the company does bad work. I really do not believe you're completing a certified heat load on side jobs, unless it's for new housing that is.

It's possible you have a spreadsheet with fill in the blanks like reliance sales staff, but that's not a heat load and often times tells the installer to put a 120k btu furnace and a 1 1/2 ton AC in an 800 sq ft house.

If you actually are doing a real certified heat load, awesome. It looks really good on you.

I use coolcalc manual J which is ACCA approved, good enough for me.

That looks pretty decent actually. I just ran through one quickly. Came pretty close on my house. Good stuff man.

Offline Jorgebaloy

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Re: a/c sizing
« Reply #9 on: July 17, 2018, 02:17:25 PM »
Hi Guys,

I just wondering what formula you guys using to size an a/c

Determining Your BTUs

Air conditioners are sized by the number of BTUs they put out. BTUs stand for British Thermal Units and refer to the amount of energy your air conditioner is using per hour. To figure out what size air conditioner you require, you first need to determine the amount of BTUs you require. This is done by calculating your square footage. Break up each room into as many squares and rectangles as you need to be able to measure the entire space accurately. This means separating out closets, bump outs, and hallways each as their own separate area.

Variations and Variables
While this chart is used as a general rule of thumb for selecting air conditioner size, variations can occur depending on several variables you need to consider.

Shade and Sun
If the room or house you are cooling is heavily shaded, you may need 10 percent fewer BTUs to cool it. If the house or room is very sunny, however, add 10 percent more BTUs.

Room Occupancy
The more people that use the area you are cooling, the higher the BTUs need to be to compete with body heat. As a general rule of thumb, for every person over an initial 2 people, add 600 BTUs.

Hot Appliances
If the room you are planning on installing an air conditioner in is the kitchen, you need to factor in additional BTUs for the stove, oven, and microwave. Add about 4,000 BTUs to the unit size to cover this.

Get a Professional Opinion
If you are unsure about the size of unit you need, or you have trouble calculating the size of area you plan to cool, get a professional opinion. Getting the right-sized air conditioning unit is too important to leave to chance.

Offline rmuntz

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Re: a/c sizing
« Reply #10 on: July 20, 2018, 10:25:00 AM »

Calculate the square feet: Once you have the length and the width of the area, you’ll need to multiply those two numbers together to come up with the square footage.

Determine the cooling capacity: How fast an A/C can produce cool air is measured in BTUs (British Thermal Units) per hour. You need approximately 30-35 BTUs/hour for each square foot of space if you live in a warm climate and 50-60 if you live in a cooler climate.

Thanks for clearing up how to calculate area !

Why would you need more BTU's in a cooler climate ??

If you care about the longevity of your business, do the HL/HG calculation properly so you have a hard copy of why you did what you did. The name of the game, today, is liability.

Offline Jorgebaloy

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Re: a/c sizing
« Reply #11 on: July 25, 2018, 07:29:37 AM »

Calculate the square feet: Once you have the length and the width of the area, you’ll need to multiply those two numbers together to come up with the square footage.

Determine the cooling capacity: How fast an A/C can produce cool air is measured in BTUs (British Thermal Units) per hour. You need approximately 30-35 BTUs/hour for each square foot of space if you live in a warm climate and 50-60 if you live in a cooler climate.

Thanks for clearing up how to calculate area !

Why would you need more BTU's in a cooler climate ??

If you care about the longevity of your business, do the HL/HG calculation properly so you have a hard copy of why you did what you did. The name of the game, today, is liability.

If you live in an area that experiences frigid winters, you will definitely want to consider a heat pump mini split system that will provide you with heat even as the temperature dips below zero.

Look for the HSPF (Heating Seasonal Performance Factor) rating for each brand below to get an idea of the heating capabilities - the higher the HSPF, the better it will perform with greater efficiency.

Offline Admin

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Re: a/c sizing
« Reply #12 on: July 26, 2018, 08:47:04 AM »
I think you mean 800 to 1200 sq ft per ton.