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