This is the third in a series of HotMail articles by ‘Doc’ Falke focusing on specific techniques designed to test, diagnose and improve the performance and efficiency of residential HVAC systems.

The quest for higher efficiency cooling equipment has hit a barrier. Unsuspecting installing contractors are being handed hidden liability that’s difficult to identify and resolve. Certain cooling coils are being built for the highest possible SEER rating, but when installed in live field conditions fail to perform as designed. This may leave you with an unhappy, uncomfortable customer and an expensive problem to deal with. Let’s take a look at how you can quickly identify and solve this recurring defect you may be facing today.

The Cooling Coil Issue

One method used to manufacturer higher efficiency cooling coils is to build a greater number of rows of fins per inch in the coil. Lower efficiency coils may contain 8 to 10 rows of fins per inch. Some push the limit and use 14 to 16, but in recent years some manufacturers are moving to 24 or more rows of fins per inch.

The result of this practice is cooling coils that are so restrictive to airflow that when selected and installed in an equipment replacement application, the operating total eternal static pressure of the system is often double the fan rating. When this happens, system airflow may be less than half of what’s specified. When system performance is measured and diagnosed, it’s not uncommon to discover system delivered BTUs are less than half of what the equipment was originally rated to deliver in the laboratory.

In addition to poor efficiency performance, many coils are built in a manner that condensate water is often blown off the coil and into the ductwork when installation and pressure conditions are not ideal. This results in potential moisture and mold issues that may pose a serious health risk to building occupants, not to mention a significant financial liability to the installing contractor.

Avoid the Problem in the Design Phase

When selecting cooling coils, good designers will check two of the specifications of each cooling coil. The first is the rated pressure drop of the wet coil at the maximum system design airflow, and the second is the coil’s ability to manage condensate water. This information is published with each coil by the coil manufacturer.

According to a national NCI study of well performing HVAC systems, the coil pressure drop budget should be less than 40% of the rated fan total external static pressure.So, if a fan is rated at .50-in. w.c., a remote cooling coil pressure drop would be 40% of .50 or .20-in. w.c. Few cooling coils today are manufactured at this low of a pressure drop. If an .80-in. w.c. rated fan is selected at .32-in., coil pressure drop would meet budget. .32-in. w.c. is a reasonable coil pressure drop.

So, how much fan pressure would be required to overcome a .50-in. w.c. coil pressure drop? A 1.25-in. w.c. rated fan could handle that coil. Try to find one of those for a residential system. Are you getting the picture? Choose wisely.

The other specification to check is condensate blow-off potential. The best suggestion I can offer is that if a coil specification includes a condensate blow-off chart, consider it a red flag and choose a less efficient, better performing coil and save yourself the headache.

If you missed the excessive coil pressure drop in the design phase, go easy on yourself. How many distributors knew enough to match the fan and the coil pressure drop when they bundled and priced the equipment? Few understand the relationship between fan capacity and coil pressure drop. We’re all learning together here.

Identify High Coil Pressure Drop in an Existing System

The problem of high coil pressure drop is invisible when inspecting a system. Excessive coil pressure drop can only be identified through measurement.

This is done by measuring pressure on both sides of a cooling coil and subtracting the lower pressure from the higher pressure. This pressure difference is the coil pressure drop. If the coil, pressure drop is higher than 40% of the fan rated pressure, you may have a problem.

To effectively measure coil pressure drop, please follow a few rules:

1. Use a manometer with two pressure ports. If analog, the range of 0-in. to 1-in. w.c. is ideal. If digital, a range of 0-in. to 5-in. is ideal. If the range is too great, the accuracy of the reading will be questionable.

2. An accessory for the manometer is a pressure hose that fits snugly on the meter and on the static pressure tip that’s inserted into the air stream. You’ll also need a ¼-in. or 3/8-in. drill bit, a sheath to limit the distance the drill bit penetrates the air stream, and hole plugs to seal the air leakage of the test holes.

3. Before drilling near the coil, remove the screws and inspect to be sure the area drilled contains a vacant space behind it. This assures you will not hear those nasty hissing or sparking noises when the drill bit penetrates unintended areas of the equipment.

4. For a procedure on how to measure coil pressure drop, see the contact information at the end of the article.

What to Do When Coil Pressure Drop is Excessive

When the coil pressure drop exceeds 40% of the fan rated budget, changes in the system normally are possible to compensate for the problem.

Compare the coil pressure drop to the original manufacturer’s engineering data at the current operating airflow. If you’re lucky, a higher pressure drop will indicate the coil is dirty. So cleaning the coil may solve the problem.

First, measure the system’s total external static pressure and compare it to the rated pressure. If you’re lucky, the total external static pressure may be less than the fan rated pressure and the system may still perform well even though the coil pressure drop is excessive.

You can reduce the filter pressure drop by installing a less restrictive filter. This may free up some static pressure and allow the system total external static pressure to fall within the manufacturer’s specifications. Adjust the fan airflow to the amount required and verify system operating pressure.

You may also add additional supply or return duct capacity to offset the excessive static pressure of the coil. A single 14-in. return duct can offset the excessive coil pressure drop in a hurry.

You might have to replace the fan with a higher pressure capacity variable speed fan. Watch for excessive watt draw.

The last resort is to remove the overly restrictive coil and replace it with a less restrictive, less efficient coil. This is a bold move, but if you measure the operating efficiency of the system after the coil change, you’ll find a significant increase in system operating efficiency.

Hopefully you’ll be more aware of the hidden defect of excessive coil pressure drop. Now you should be able to measure and identify excessive coil pressure drop and eliminate this defect from the systems your company designs, installs and services.

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 free Coil Pressure Drop Test Procedure, contact Doc at robf@ncihvac.com or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles and downloads.