What Exactly is Manual S in HVAC Design and Why Is It Important?

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This is a guest post by David Butler of Optimal Building Systems in Arizona and a frequent commenter here in our blog. I've known David for a couple of years now, and I can attest to his expertise in the field of HVAC. When an HVAC question stumps Chris and me, we call on David because he'll most likely know the answer and will explain it in great detail.  If you're a home energy pro, make sure you download and read his paper, The Elephant in the Room.

I wonder how many energy raters and auditors are familiar with Manual S, which is an important part of the full HVAC design process? For those who don't know, the S stands for selection, as in equipment selection. Why should you care? Well, if you don't at least understand the basics of Manual S, you're in no position to judge if an air conditioner is sized properly, as required by the ENERGY STAR new homes program. This is one reason why objected" target="_blank">I objected to this well-intended policy when it took effect in 2006.

The new HVAC contractor checklist being introduced with ENERGY STAR Version 3 includes several fill-ins intended to demonstrate Manual S compliance. The problem is that most HVAC contractors currently sidestep the Manual S procedure, relying instead on AHRI or nameplate capacity for equipment selection. I'm sure many of you have heard this criticism before (I've said it often enough), but you may not know why sizing based on AHRI or nameplate capacity it's a bad thing. Here's the rest of the story...

As it turns out, the capacity of an air conditioner is a moving target. It depends on three environmental variables - outdoor temperature, indoor temperature, and indoor humidity - and one system variable - airflow at the evaporator coil. Further complicating matters, an air conditioner's total capacity is split between sensible capacity (satisfies the thermostat) and latent capacity (removes moisture from the air). As it turns out, the sensible-total ratio, also known as sensible heat ratio (SHR), is also a moving target. It's important to note that the SHR is not a design condition but rather a result of the design conditions. Change any one condition, and you change an air conditioner's sensible and latent capacity.

An air conditioner's nameplate capacity is based on a specific set of operating conditions - 95° F outdoor dry bulb, 80° F indoor dry bulb, and 67° F indoor wet bulb. (Wet bulb is a way of expressing the amount of moisture in the air). These conditions, established by AHRI Standard 210, provide a convenient reference for comparing equipment but not much else. Not surprisingly, equipment manufacturers optimize their designs so that performance peaks at AHRI conditions. After all, nameplate capacity is all that matters in the marketing world.

In the real world, a given air conditioner will perform very differently in Phoenix than it will in Savannah. In fact, AHRI operating conditions have little relevance in any climate. For example, no one designs to 80° F indoor dry bulb. The ENERGY STAR homes program requires 75° F and some contractors go even lower. And if we reduce indoor dry bulb to 75° F, then we'll get into trouble if we leave indoor wet bulb at 67° F. A psychrometric calculator reveals that this combination converts to a relative humidity of 67%! Working backwards, if we design to a relative humidity of 50% at 75° F dry bulb, then the corresponding wet bulb is 63° F. This is the most common assumption for the indoor conditions.

Furthermore, nameplate capacity tells us nothing about an air conditioner's sensible-latent split. Neither does its AHRI rating. Unfortunately, many Manual J trainers teach students to use a generic sensible heat ratio, thus perpetuating a short-cut that bypasses Manual S. To make matters worse, this bogus equipment selection method is institutionalized by popular Manual J software tools. Right-J by Wrightsoft and RHVAC by Elite provide an equipment selection search engine based on AHRI ratings and a user selectable sensible ratio. It's no wonder so many HVAC contractors never bother to learn or follow the Manual S procedure.

Flagrant oversizing may make this a moot point in practice, but as I've written so many times before, oversizing has a big impact on comfort, efficiency, and moisture removal. In this ill-conceived ploy to avoid call-backs, HVAC contractors are throwing out the baby with the bathwater.

Manual S picks up where Manual J leaves off. It lays out a fairly simple (albeit non-intuitive) procedure for selecting equipment based on the design loads. In the case of a furnace, the procedure is trivial: output capacity should be between 100% and 140% of the design heat load. But for an air conditioner it's not so simple. You need to know the system's sensible and latent capacity at the design conditions specific to the project. This information can be found only in the manufacturer's expanded performance tables. These tables are at the core of the Manual S procedure for air conditioners and heat pumps.

Some manufacturers have developed proprietary equipment selection tools that utilize expanded performance data. However, these tools are limited to the company's own product line and are available only to dealers. Unfortunately, since there's no central database of expanded performance data, independent software companies like Wrightsoft and Elite cannot offer a Manual S module. It's up to the practitioner to obtain performance data from the manufacturer for each model of interest. Lennox and Carrier's engineering books are available online in PDF format if you know where to look. In other cases, you have to contact the distributor. Moreover, manually looking up performance data for several prospective equipment combinations can be time consuming.

In an ideal world, AHRI would maintain a database of expanded performance data that could be accessed through third-party software tools. But don't hold your breath. Manufacturers see zero benefit in having us look over the shoulder of their dealers, many of whom don't follow the rules.

For those who want to learn more about the Manual S procedure, here's an excellent article by ACCA Technical Services Director Wes Davis. The one shortcoming is an overly simplistic explanation of choosing the cooling design airflow (Figure 5), although understandable considering the venue. System designers are advised to follow the complete procedure outlined in the book, available from ACCA for $58.


Related articles:

HVAC Design Done Right - Manual J, S, T, & D

Got Manual J? Don't Assume It's Correct.

ENERGY STAR Version 3 Train-the-Trainer Class - Day 2

The Magic of Cold, Part 2 - Intermediate Air Conditioning Principles


Green Curmudgeon

Nice piece, lots of good, solid, (if slightly impenetrable information - particularly the linked article by Wes Davis.) It does, however, raise an important question - how can we get quality (and code compliant) HVAC designs when most people don't know how to do them, accurate data for Manual S is not readily available, and few, if any people are willing to pay anything extra to build or renovation a building in these challenging financial times? I always enjoy learning more about building performance, and this article filled a gap in by knowledge, but it also serves to point out how far we have to go and makes me wonder if, as an industry, we will ever get to where we need to be.

M. Johnson

It should be worth a mention that Trane for example, publishes a grid of sensible and latent capacities for their air conditioners. It would seem that this sidesteps the Manual S guidelines, and might even be more accurate. 
But whenever such info is not available, Manual S is something that should be understood by every AC contractor. Far better than oversizing as a CYA method.

Christopher Cadwell

With a perfect install in a dry climate, I have proven time and time again that Man S is too conservative and still over-sizes equipment. Let us not forget the 20% fudge factor Man J, S have inherent to them.  
So if Man J over-sizes by 20% and then Man S does that too, then what are we left with? 
To me it comes back to "sizing is secondary to getting air flows correct."  
We concentrate too much on tonnage, and ignore the air movement. Just my opinion. By the way how many AC contractors have the tools to measure total air flows and air balance? Probably about as many that actually own the manuals J and S. 
Ohh yeah, air flows are much easier when the units are sized correctly - as in small, small. 
Next up Man D and T. 
Good work as always David.

David Butler

M.Johnson, the "grid" of sensible and latent capacities published by Trane is exactly what I'm talking about. This data does not sidestep Manual S. Rather, it's at the core of the procedure. 
BTW, when contacting a manufacturer, it helps to use the correct name when requesting the expanded data. Here are some examples: 
"Expanded Rating Tables" (Lennox)  
"Product Data" 
(Carrier & related brands),  
"Product & Performance Data" 
(Trane & related brands) 
"Technical Information Manual" 

David Butler

Chris, not sure why you would suggest that S leads to additional oversizing. The procedure instructs you to add back half of the excess latent capacity to the sensible capacity, so in a dry climate, this will allow you to choose a smaller system than for a house back east with same sensible load. The idea behind this is that excess latent capacity will act to reduce RH below design. This 'rule of thumb' is decidedly unscientific, and in fact, may be a bit ambitious in a dry climate where monsoon doesn't coincide with peak sensible loads. 
I totally agree with your comments regarding overemphasis on 'tonnage', and lack of emphasis on airflow.

Sam Young

The most important part of the procedure is to find and use the de-rating factors. Oddly enough, you can make the outdoor conditions as hot as you want without much impact. However, moving the design cooling temperature from 80 to 75 degrees can affect the size of the system by more than 0.5 tons. 
The adjustment factors are also critical for sizing equipment in dry environments. 
When you learn how to do Manual S, the HVAC contractor calls you to decide whether equipment fits in their own home!

Christopher Cadwell

"Chris, not sure why you would suggest that S leads to additional oversizing." 
Sorry. To be specific it is the manufacturer charts that I think do this. It is my belief that those charts have fudge factor in the figures. Unproven, except by myself on my own post tests.  
I guess I should not generalize on that.  
On the post tests on my own installs when I verify airflow with a flow hood and calculate in temperature split and altitude, I get 25-20% more Btuh output than the charts say I should.  
Sorry for being skeptical. Technically it is not Man S that would have the fudge factor in that case.

Chris Cadwell

To correct I get 15%-20% more Btuh than the charts say I should.

Joel Burdette

Reading this post and David Burler's article about Manual S, I realize that the informal, unmentioned and unpublished "Manual SWAG" might still be applied in occasionally. 
Thanks for the education.

Allison Bailes

Great comments, everyone! This is a tough subject, and David has expertly pointed out the difficulties with doing this job correctly and why we need to get over our capacity fixation. (When is a 2 ton air conditioner not a 2 ton air conditioner? Most of the time!) 
Carl: Your question kind of sums it all up: 
How can we get quality (and code compliant) HVAC designs when most people don't know how to do them, accurate data for Manual S is not readily available, and few, if any people are willing to pay anything extra to build or renovate a building in these challenging financial times?" 
As I wrote a few months ago (Why Won't the HVAC Industry Do Things Right?), this is part of the central problem we face with high performance homes.


On Old Houses, contractors make many guesses as to ACH, Duct Loss/Gain/Leakage, etc. These variables can make dramatic changes in the loads, but you don't know what these values are till you test. Most manual J's on older homes are dog and pony shows where the contractor uses values that will give him the numbers he expects. Then he looks smart. How is this science? What values are correct for ACH and duct losses on old houses? 

Allison Bailes

AC BAD DOG: Yes, doing load calculations on older houses can be tricky. The best way to do it is with actual data for infiltration rate and duct losses. Otherwise, it's just guesswork -- educated guesswork, better than using rules of thumb, but still guesswork.