Do Your Comfort System Designs Work?

June 1, 2005
ACCA Manuals J and D are the undisputed champs when it comes to HVAC system design in residential buildings. They've become the gold standard for HVAC

ACCA Manuals J and D are the undisputed champs when it comes to HVAC system design in residential buildings. They've become the gold standard for HVAC system design. But here's the golden question: Of 100 systems installed in your region of the country today, how many were truly designed usiing Manual J and D calculations?

Here's the next question: Have you ever entered a load calculation, then wrinkled your forehead when the program suggested a 2-ton unit, after you already decided it needed a 3-ton? When that happens, most of us go back and adjust the infiltration rate to get the program to agree with 3-tons.

Design decisions are based on experience with systems in the field, lessons from real-life situations, and the day-to-day pressures from customers.

Here are some ideas to consider when you design your next system.

Design vs. Actual. My dad gave me a duct design chart when I first came into the industry. "A 6-in. duct gets 100 cfm, a 7-in. gets 150 cfm, and an 8-in. gets 220 cfm. Got it, son?" Every one of my designs worked perfectly — until I measured my first one.

Design alone isn't enough. The truth is, until you've consistently measured the performance of the systems you install, there's no proof that your designs are worth the paper they're printed on.

Design is verified when it's compared to measured installed performance.

Design is just the Beginning. The process to build a well-performing system has four basic steps that demand our attention:

  • Proper system design
  • Selecting the right components
  • Top quality installation and startup
  • Test, balance and verify that the installed system actually operates as designed.

An omission or mistake in any one of these steps may result in a poorly performing system.

What Could Go Wrong? Over a year ago, NCI contractors implemented new efficiency measurements that rate the installed efficiency of an operating system in the field. Called CSER? and HSER?, these installed system efficiency ratings are a ratio of a system's measured delivered Btus compared to the systems original design Btus.

Thousands of these tests are now completed and the bottom line is the average residential cooling system only delivers between 55% and 65% of design capacity. The average heating system is delivering 50% to 60% of design capacity.

Component Selection. As manufacturers scramble to produce competitive 13 SEER equipment, some models may struggle in the field. Some equipment efficiency is achieved by smaller blower motors. Fewer watts equal more apparent efficiency. But can smaller blower motors move adequate air? No, that defies the laws of physics. Be very cautious when selecting air handlers. Remember, the equipment isn't the system; only part of it.

Another change: the practice of adding more fins per inch in the coil. The coil may be rated at greater efficiency, but can blower motors rated at .50-in of static pressure or less move required airflow through such a restrictive coil?

Let's talk filters. Filters are the number one source of poor indoor air quality today. Why? Because high efficiency filters usually create high pressure drops. In other words, as they stop more particulates, they can reduce airflow to the point of poor system performance. You can, however, reduce filter pressure drop by increasing filter surface area.

When it comes to selecting blower motors, coils, and filters, look at the fan performance data and make the following calculation:

Fan rated static pressure at 400 cfm/ton - the wet coil pressure drop - the loaded filter pressure drop (rated pressure drop x 1.5) = the static pressure left over for the duct system.

Example: Rated static pressure: .70 in. Less wet coil - .28 in. Less loaded filter - .18 in. Available static pressure .24 in.

A minimum available static pressure needed to allow a generously sized duct system is to function properly is often .20 in. A two-story home or a longer length duct system may need .40-in. available static pressure.

Good quality variable-speed fans have the pressure capacity to allow us to use today's restrictive system components. If you take a close look at fan performance charts, you'll select a variable speed fan for nearly every system.

But remember, as variable-speed fans ramp up, amp draw and energy consumption skyrocket. Many contractors make the mistake of using variable-speed fans to compensate for poorly designed and installed duct systems. Higher pressure means higher pressure drops, more duct leakage, and higher utility costs.

Also, prepare for taller equipment that may make some upflow basement or attic installations impossible. Fans have a problem with a short tee at the equipment discharge. These can reduce airflow up to 40% and are often difficult to identify by static pressure measurement.

Duct Design. While manual D is the duct design standard, how often does the industry use 2%? 5%? A vast majority of our duct design is done with duct calculators.

Let's take a look at two major duct design problems — laboratory engineering data and friction loss per 100 ft. of duct.

When using a duct calculator, line up the friction loss per 100 ft. of duct indicator with the cfm that will be passing through the duct. The rule of thumb is to set the pressure drop at .1 in. However, that may work for ideal rectangular sheet metal duct installations, but not every installation calls for rectangular duct. And that means we must change the friction setting on our duct calculators.

Although this goes against traditional duct design ideas, the argument is over when systems deliver design airflow. We've learned that traditional duct calculator design based on .1 in., or even .08 in. of friction loss per 100 ft. of duct frequently delivers only 70 to 80% of design airflow. Add to that other duct losses and the system is in trouble.

If using a sheet metal duct calculator, we recommend the following pressure drops per 100 ft. of duct. For rectangular sheet metal duct, try .07 in. Round sheet metal pipe and duct board, use .06 in., and for flex duct systems, use .05 in.

Now, you can't really quote such numbers in a magazine article without qualification, so here's the disclaimer: If you think your duct systems work fine, but haven't measured airflow, pressure, and Btus, think again. If you consistently measure airflow, and your systems work just fine, there's no reason to change your design. But if you're not sure, give this design method a try. The argument ends when airflow is consistently delivered and verified. Design it, build it, then test the results yourself.

Installation and Startup. System performance is always subject to the quality of the installation. The most common installation problems include poorly pinched or restricted ducts, flexible duct loosely installed, "beer can cold" refrigerant charge, restricted and undersized returns, poor transitions at the discharge or intake of fans, inadequate venting and combustion air.

Two practices to always avoid are remote plenums (a large duct running to a single box, attempting to distribute air to multiple smaller ducts) and triangular duct board wyes. This list is endless.

Remember, with equipment changeouts, because these installation defects are found in nearly every system, you should rarely install new equipment on an existing duct system. First, you have little chance of delivering the efficiency you promised on your proposal. Second, you'll miss the most profitable part of the changeout, the duct renovation.

Test, Balance and Verification. During your first air balance job, you'll immediately discover that each 6-in. duct delivers a different volume of airflow. That will change your design beliefs instantly. One might deliver only 45 cfm, while another may get 110. To achieve design, all systems must be tested and balanced.

Verification of system performance is the next step beyond current commercialtype balancing. Once airflow is adequate and properly distributed throughout the building, the system pressures and temperatures should be taken. The refrigerant side cannot be properly charged until adequate airflow is verified.

Motor amperage and rpm should be documented. A good air balance report displays all the related system values. Experienced balancing technicians review all the numbers to be sure they agree and verify the system is operating as designed.

The final step in true system design verification is using the air balance numbers to calculate delivered Btus. If the field Btus match the design Btus then the design, component selection, installation, and balancing are delivering the capacity and quality you intended to build.

The definition of a working system is changing. Before, if you started the system and the fan and compressor kicked on, the old tech would exclaim, "she's a workin'." But true performancebased contractors surpass their competition as they self verify system performance and design by air balancing and Btu measurement.

Rob Falke is president of the National Comfort Institute. He can be reached at 800/633-7058, e-mail robf@ If you're interested in receiving a nocost (that means free) NCI Duct Sizing Chart, drop Doc a line at the email address above.

About the Author

Rob 'Doc' Falke | President

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC-based training company and membership organization. If you're an HVAC contractor or technician  interested in a building pressure measurement procedure, contact Doc at [email protected]  or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.