InterNACHI Inspection Forum Rule of Thumb A/C sizing question from Practice Exam
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 Inspecting HVAC Systems Topics include heating, venting, and air conditioning inspections.

#1
5/23/06, 11:04 AM
 Gregory A. Liebig InterNACHI Member Join Date: May 2006 Location: Sheboygan, WI Posts: 787
Rule of Thumb A/C sizing question from Practice Exam

I have a question regarding a question on the practice test. On page 11 there is a question that seems to conflict with the information I was looking up. The correct answer is 700 sq ft but some of the research I did supported 500 sq ft per ton. What is the correct answer? Wisconsin requires passing the National Exam and I’m doing everything I can to get ready for it within 2 weeks.

Greg Liebig
NACHI06051095

A rule of thumb for air conditioning is a ton of air conditioning is needed for:
• 700 square ft
• 500 square ft
• A one story house
• A hot day

References:
http://www.healthgoods.com/education/healthy_home_information/Space_Heating_and_Cooling/sizing_heat_and_ac.htm

The contractor asks you how many square feet of living space there are in your house. He (or she) then tells you what size unit you need. This is called "sizing by square footage" and is the most commonly used inaccurate method of sizing. A typical value used for air conditioners is one ton (12,000 Btu/hour) per 500 square feet (46 m2). This does not take into account differences among houses in design, construction, or energy efficiency.

Table 1: Capacity rules of thumb for room air conditioners

Rules of thumb for estimating air conditioner size vary by manufacturer. This example, from Carrier, includes the following suggestions for adjustments: If a room is heavily shaded, reduce capacity by 10 percent; if the room is very sunny, increase by 10 percent; and if using the unit in a kitchen, increase capacity by 4,000 Btu per hour.

Room area Capacity
(ft²) (Btu/h)
100 to 150 5,000
150 to 250 6,000
250 to 300 7,000
300 to 350 8,000
350 to 400 9,000
400 to 450 10,000
450 to 550 12,000
550 to 700 14,000
700 to 1,000 18,000

Source: Platts
#2
5/23/06, 12:17 PM
 rdawes Active Poster Join Date: Nov 2004 Location: Plano, TX Posts: 425
Please Note: rdawes is a non-member guest and is in no way affiliated with InterNACHI or its members.
Re: Rule of Thumb A/C sizing question from Practice Exam

That's the rule-of-thumb we used for the Texas TREC exam. Of course, Texas is a bit warmer than some climates.
#3
5/23/06, 10:37 PM
 jwilliams7 InterNACHI Member Join Date: Apr 2006 Location: Springdale, AR Posts: 36
Re: Rule of Thumb A/C sizing question from Practice Exam

I passed the National Exam about a month and a half ago, I used 500 sq feet/ton.
#4
5/23/06, 10:48 PM
 Gary Reecher Active Poster Join Date: Aug 2004 Posts: 273
Please Note: Gary Reecher is a non-member guest and is in no way affiliated with InterNACHI or its members.
Re: Rule of Thumb A/C sizing question from Practice Exam

Rules of thumb are dumb.

Using Air Conditioning Contractors of America (ACCA) Manual J heat loss/gain calculation method is more precise with less chance of oversizing equipment and reducing comfort.
#5
5/24/06, 12:14 AM
 Joseph Burkeson, CMI InterNACHI Member Join Date: May 2003 Location: Greater Tampa Bay Posts: 22,004
Re: Rule of Thumb A/C sizing question from Practice Exam

Quote:
 Originally Posted by Gary Reecher Rules of thumb are dumb. Using Air Conditioning Contractors of America (ACCA) Manual J heat loss/gain calculation method is more precise with less chance of oversizing equipment and reducing comfort.
You Betcha!

IRC - M1401.3 Sizing.
Heating and cooling equipment shall be sized based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.

Yes, I'd personally choose a CMI every time over every other inspector. ~ Nick Gromicko, InterNACHI Founder

Certified Master Inspector - 2010
Florida Licensed Home Inspector - HI176
Florida Licensed Mold Assessor - MRSA208

Square-One Inspection "Assurance begins here"

#6
5/24/06, 8:10 AM
 Michael W. Gault InterNACHI Member Join Date: Apr 2005 Location: Mount Pleasant, SC Posts: 1,093
Please Note: mgault is a non-member guest and is in no way affiliated with InterNACHI or its members.
Re: Rule of Thumb A/C sizing question from Practice Exam

Quote:
 Originally Posted by Gary Reecher Rules of thumb are dumb. Using Air Conditioning Contractors of America (ACCA) Manual J heat loss/gain calculation method is more precise with less chance of oversizing equipment and reducing comfort.
Doesn't negate the fact that it's on the NHIE...
#7
5/24/06, 8:33 PM
 rdawes Active Poster Join Date: Nov 2004 Location: Plano, TX Posts: 425
Please Note: rdawes is a non-member guest and is in no way affiliated with InterNACHI or its members.
Re: Rule of Thumb A/C sizing question from Practice Exam

Rules of thumb may be dumb but if I inspect a house that's a ton undersized based on that rule and we are here in hot Texas then I will call for a HVAC guy to be consulted so he can do those calcs and tell the buyer if the AC is properly sized. I won't say it is not properly sized but I will say that it should be looked into.
#8
5/24/06, 10:56 PM
 Michael W. Gault InterNACHI Member Join Date: Apr 2005 Location: Mount Pleasant, SC Posts: 1,093
Please Note: mgault is a non-member guest and is in no way affiliated with InterNACHI or its members.
Re: Rule of Thumb A/C sizing question from Practice Exam

What Ronald said!
#9
5/25/06, 2:36 AM
 Gregory A. Liebig InterNACHI Member Join Date: May 2006 Location: Sheboygan, WI Posts: 787
Re: Rule of Thumb A/C sizing question from Practice Exam

I also agree that one should defer if they cannot give the client the best decsion. That still doesn't take away the facts of the question on the exam.
#10
5/25/06, 2:38 AM
 Gregory A. Liebig InterNACHI Member Join Date: May 2006 Location: Sheboygan, WI Posts: 787
Re: Rule of Thumb A/C sizing question from Practice Exam

Thanks Mike! That's my point exactly...