Many needed repairs that improve the performance of almost every air conditioning and heating system are overlooked during routine service calls. Let’s take a look at the most common repairs needed, as recently discovered by evaluating test data from 400 poorly performing systems.

Why Are These Repairs Overlooked?
Most routine maintenance checklist come from tasks originally devised to keep equipment running through its warranty period. We now realize that just because equipment is running doesn’t mean it’s performing well. This understanding is pumping new life into maintenance, service, and system upgrades.

Most of these issues are hidden and cannot be identified through a checklist alone. Some level of testing and a few basic calculations are needed to detect and diagnose these invisible problems.

This level of diagnostics and repairs can easily be learned and included in your daily service, installation, or sales routine.

Adjusting Fan Speed
Air moves heating and cooling through the equipment. Too much or too little air and equipment efficiency takes a dive.

It’s estimated that the amount of equipment airflow actually measured at startup on new or replacement equipment is less than 15%. If this is true, 85% of your customers depend on your luck, not your skill, when it comes to comfort and efficiency. In which category do you classify your jobs?
The easiest way to determine fan airflow is to plot it. This takes a few minutes. It requires you to verify fan speed and measure total external static pressure. These two values are then plotted on the fan table to reveal the amount of airflow the fan is moving. You can then adjust fan speed up or down to the required amount.

However, if system static pressure exceeds fan rated pressure, repairs to the filter, coil, or duct system may be needed to allow the fan to operate within specifications.

Filter and Coil Restrictions
Filters and coils are becoming increasingly restrictive to airflow with each passing season. Overly restrictive coils and filters have been documented to reduce installed equipment efficiency up to 30%,

A discussion about filter and coil efficiency is incomplete without considering the effect the component has on the performance of the system. The restriction caused by a coil or filter is easily measured by testing the static pressure before and after it.

Subtract the pressure measured before and after the filter or coil to find its pressure drop. In a gas furnace system with an external coil, the filter and coil pressure drop can be compared to a percent of the rated fan pressure to accurately evaluate its performance.

Air filters should typically be 20% or less of fan rated pressure. External coils should be 40% or less of fan rated pressure. Email Doc at the address at the end of this article to receive a procedure for these pressure measurements.

Undersized Duct Systems
Data evaluated from National Comfort Institute’s ComfortMaxx software confirms the typical duct system’s resistance to airflow is approximately double what it should be.

This fact establishes the need for increased duct sizing and better installation practices than currently being used. It questions our current design and installation methods and calls for verification of duct sizing by measuring duct system performance and then comparing test results to design.

I was completely convinced I was an amazing duct designer, until the first day I measured the airflow of my duct systems. It was a hard day to be me, but my ability to design ducts rapidly improved shortly thereafter. I invite you to subject yourself to verifying your duct system designs by measuring the airflow delivered by your systems.

I hope your test results turn out better than mine did.

Further data shows the typical residential HVAC system requires the addition of one 14” (or equivalent size) return duct and the upsizing of approximately three supply ducts to allow the fan to operate within manufacturer specifications.

Nearly every duct system can and should be improved. The trick is to use testing to discover how poorly each system is performing through measurement and then help your customer see and pay for the needed system improvements.

Refrigerant Charge Adjustment
The change you make in a system when adjusting refrigerant charge is also invisible. The refrigerant remains out of site as it passes from the jug into the system.

The biggest problem with refrigerant charge adjustments is that most of the time, the necessary testing that has to accompany it is ignored. Usually the airflow through the equipment is assumed to meet the required system specification.

Since the average cooling airflow in existing and many new systems is less than 300 cfm per ton, the carefully calculated refrigerant charge completely misses the mark and fails to improve system performance as intended.

However, this repair is often needed and could be effective, if only fan airflow would be measured and compensated for when charging the system. Use the fan adjustment technique mentioned earlier in this article to make this needed repair before adjusting refrigerant charge every time.

Your Customers Need Your Expertise
Since each of these repairs are invisible to you, your customers will certainly be unaware they have these issues with their systems.

Because your profession is highly technical, customers are often cannot describe problems they are experiencing because they don’t have the knowledge to assess their situation. They rely on your knowledge and skills to make them comfortable and reduce their cost of owning and operating their systems.

Test more, diagnose better, and pull yourself and your customers to a place of higher performance.


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 free step-by-step test procedures mentioned in this article, 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.