1) I think this would be his primary point: 1000’s of satisfied customers and a successful business would disagree that 500 sf/ton is a bad idea. I don’t remember the last customer that complained that their new system left their house uncomfortable. (We are in Northern California, btw). To be clear, i am not claiming that 500 sf/ft ends up with the same result as a proper load calc would, i am claiming saying it doesn’t lead to an uncomfortable house around here.

2) I think every article i have read on the topic of the problems with oversizing gives an example in the southeast US where humidity is a huge factor, which makes me think that it’s just not a big deal around here.

3)If you install a smaller system than what the customer had and it has any issues on hot days (days above design temp or just doesn’t cool as quick as the customer feels like it should) even though we say it is working properly, you will have angry customer. If you install what they had or bigger, you can just blame it on abnormally hot days and the customer will be fine with it.

4) I suppose the solution to this is to properly “sell” the system by explaining the concepts in articles like this one; but i think that is exactly what he and many, many small hvac owners are trying to avoid. They don’t want to be salesmen. When you have enough business and can pick between customer who use your time by asking about every little thing (that they could easily find on google) and you know will be picky or customers who just want their a/c to cool off the house, you will go with the latter.

I guess that turned into me thinking out loud. To be clear, I want it to be necessary to take the more advanced steps like this. If you just just grab anyone off the street and teach them how to design and install a proper system in a day, then how can i ask for any more money than a handy man or a new employee? Thanks for any help on this topic!

If the calc without leakage ranges between 600 SF/ton and 2000 SF/ton, what are you going to lean towards?

Our process is delivered by HVAC contractors and offers a blower door based load calc.

1000 SF/ton not so frightening when you narrow in on load with measured leakage.

Benjamin, there’s nothing

Benjamin, there’s nothing inherently wrong with low velocity in supply ducts.  Yes, it’s not a good idea for ducts that run through an unconditioned attic because of the temperature rise in summer and temperature drop in winter of the supply air coming out of the vents.  But actually, larger ducts are great for air flow because the resistance is a lot lower.  And fittings, which dominate the total effective length, are rated for a velocity of 900 feet per minute.  When you cut the velocity in half, you reduce the equivalent length of fittings to a quarter of what’s published in Manual D.

And if you’ve never seen 1,000 sf/ton work, you must be working on uninsulated, super leaky houses.  We often design for houses that come in at 2,000 sf or more per ton.

You can’t put any AC system
You can’t put any AC system to any duct work. If you have grossly oversized supply duct for the CFM you’re trying to push you lose your velocity and you won’t have very poor airflow. The returns of course the bigger the better you cannot oversized the return and the bigger the return the quieter the system will be. I’ve never seen in 1000 square feet per ton work I’d have to see it to believe it it would be a hell of an r factor in that house. In a lot of homes you are not going to be able to change out ductwork because it’s all buried you have to cut the house to pieces to do it

I’ve seen it not work on
I’ve seen it not work on double pane windows a good r factor. You’re only having five Outlets for 1000 square feet.

Ben, when you started in HVAC

Ben, when you started in HVAC 33 years ago, single-pane windows were the norm.  R-11 walls and R-19 ceilings were standard in many places.  Brand new houses that leaked at the rate of 10 to 20 air changes per hour at 50 Pascals (ACH50) were everywhere. 

I don’t know where you’re located, but it doesn’t matter so much.  Building codes, materials, and techniques have changed all over the US.  500 square feet per ton is a rule of thumb that never really worked anyway.  It doesn’t take into account the orientation, the airtightness, or pretty much anything else that matters.

Lack of capacity happens occasionally but far less frequently than you think.  The duct system is most often to blame (assuming the refrigerant is properly charged and the equipment is working properly). 

I’ve never seen 1 ton per
I’ve never seen 1 ton per 1000 sq ft work anywhere. I’ve seen 500 square feet per ton not work on 100 degree days. I would have to see it to believe it on a hundred degree day 2000 square foot home working on 2 tons of air. You better have really great ductwork I’d have to see it. I’m in doing hbac for 33 years and when homes are not cooling it’s because lack of capacity

Your comment implies that sizing rules of thumb are appropriate within a geographical area. Nothing could be further from the truth.

I just finished the design on a new home in Savannah. The design cooling load came out to about 1.6 tons for 2,000 ft2, or 1250 ft2 per ton. That’s not that unusual for a verified code-built home.

Contractors who rely on what’s “worked for decades” are the ones doing stupid things now, as today’s code-built homes require far less capacity than homes build decades, or even a single decade ago.

That’s pretty awesome! I owe
That’s pretty awesome! I owe you one! Matt

Charlie, Yes, there’s a Hobo
Charlie, Yes, there’s a Hobo datalogger that measures runtime. See this article I wrote two years ago for details:

My Big Fat Oversized Air Conditioner

http://energyvanguard.com/blog-building-science-HERS-BPI/bid/76640/My-Big-Fat-Oversized-Air-Conditioner