# 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

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

That’s the rule-of-thumb we used for the Texas TREC exam. Of course, Texas is a bit warmer than some climates.

I passed the National Exam about a month and a half ago, I used 500 sq feet/ton.

1 Like

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.

Doesn’t negate the fact that it’s on the NHIE…:neutral:

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.

What Ronald said!

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.

Thanks Mike! That’s my point exactly…