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HVAC System Repair Study

April 8, 2009
In 2006 the National Comfort Institute (NCI) conducted a study of duct renovation and system performance test results. There’s been a renewed interest in the results of this study as we are receiving an increasing number of requests about common system defects and their effect on system performance. Here is a summary of what we learned.

In 2006 the National Comfort Institute (NCI) conducted a study of duct renovation and system performance test results. There’s been a renewed interest in the results of this study as we are receiving an increasing number of requests about common system defects and their effect on system performance. Here is a summary of what we learned.

Over 60 HVAC contractors participated in the study and provided their results from interviews and live field reports about combustion efficiency and air balancing tests. Consumers were interviewed about their satisfaction with the educational approach to the sales process, the manner in which the work was completed, and their satisfaction with the final results. Test information from over 1000 systems from around the country is included in the survey.

The primary purpose of the study was to identify the current operating efficiency of installed residential systems. We also wanted to identify the approximate percentage of improvement that each group of repairs made to the system efficiency.

All tests were performed using NCI’s Residential Practical Standards written for HVAC contractors. The rating methods used included NCI’s Cooling System Efficiency Rating also known as CSER™ and Heating System Efficiency Rating, also known as HSER™.

Installed Performance Ratings
Real system efficiency compares equipment rated BTUs to the BTUs actually delivered into the home. The survey concluded the typical air conditioning and heating system has an initial average CSER or HSER Rating of 57%. In other words, 43% of the rated BTU didn’t make it from the laboratory, where the equipment was originally rated, into the home.

When the duct renovations were complete, final adjustments including air balance, refrigerant charge, and combustion efficiency adjustments were made. The study showed the final CSER and HSER ratings averaged almost 94%. This is a rating 6% below perfect, but the tests confirm a 36% increase in system performance.

Next, we looked at what changes had been made to get a 36% average performance increase. This was a very difficult task, because almost every change made in a system has a direct effect on the performance of other components in the system. If a coil is cleaned, it changes heat transfer, airflow, and the entrainment of attic air into the home from a poor seal between the register, and the boot increases. The effects of increased airflow also changes the filter pressure drop. In spite of the challenges, we grouped the repairs into the following categories and determined a percent of the total improvement in system performance for each repair.

Duct Renovation
The NCI Duct Renovation Procedures accounted for 32% of the total improvement in system performance. Typical repairs include:
• Repairing or replacing ducts showing inadequate BTU delivery
• Adding new return ducts or increasing existing return duct size
• Adding new supply ducts or increasing existing supply duct size
• Repairing damaged or poorly installed duct joints
• Straightening and extending flexible ducts to increase support and suspension
• Replacing restrictive transitions
• Installing sheet metal wyes, fittings, and balancing dampers
• Renovating combustion venting and combustion area pressure defects

Duct Insulation
We were surprised to find 14% of the improvement came from adding air distribution system insulation. The need for this insulation came from simple duct temperature testing of ducts found in unconditioned space.
These simple repairs include:
• Insulating bare metal or air distribution components
• Installing R-19 duct blanketing per NCI procedures.

System Test and Balance
We were pleasantly encouraged that consumers found test and balancing to be their most visible and appreciated benefit of system renovation. Customers gave it a high rating because it made the system work better by providing equal temperatures throughout the home. A properly balanced system and all the changes made in the system to get it to balance accounted for 13% of the improvement.
This work consisted of:
• Testing and adjusting the system per NCI practical standards
• Rating the installed HVAC system performance per NCI practical standards after all the renovation and adjusting work was completed.

Combustion and Refrigerant Adjustment
A change in airflow requires a system refrigerant charge adjustment. Combustion efficiency testing netted a much larger increase in system efficiency when combined with venting renovations, but these repairs were included in the duct renovation portion of the system renovation. Between heating and cooling season these repairs netted a 12% increase.

Cleaning Coils and heat Exchangers
Since NCI Members constantly measure BTU delivery, they reported that they often found coils that looked clean but were plugged inside the fins. This is one of the reasons the refrigeration charge yielded such a low increase in performance compared to the industry’s previous opinions. Charging a system without testing the pressure drop over dirty coils is a poor practice in our industry.

Several contractors reported new “high efficiency” coils were too restrictive and needed to be replaced for the system to achieve the desired BTU delivery combined with “high efficiency” (undersized) fans.

Clean coils, heat exchangers and heating coils added an average of 10% to system performance.

Restrictive filters were replaced as determined by pressure testing. When high performance filtration was needed for the health of the occupants, return air filter grilles or filter housings had to be added to increase filter surface area and reduce restricted airflow. By increasing heat transfer in the system and decreasing blower motor watts, filters accounted for an average performance increase of 7%.

Fan Repair and Adjustment
Increasing airflow resulted in an average increase of 6% in system performance. Typically fan speed was increased to achieve required airflow. Fans also needed to be cleaned and repairs to increase performance. Many fans and fan motors had to be replaced to meet the minimum acceptable system performance standard of 90% as set by NCI practical standards.

Grilles and Registers
Our last category is grilles and registers. Defects were identified with airflow and temperature testing. Many contractors regularly measure for sloppy connections at boots and grilles using temperature diagnostics. Grille repairs were at the bottom of the list at 5% savings.

Notice the results of this survey exclude the effect of equipment on overall system performance. This varies significantly from how the industry viewed system efficiency. Most in this industry believed that it was the efficiency of the equipment that determined system efficiency.

First, nearly one-third of these system renovations were performed on new construction homes. For these systems, the equipment was existing and new. Over a third of the systems renovated kept the existing equipment. So, the equipment had zero effect on over 60% of the systems included in the survey.

The final third of the systems renovated did receive new equipment as part of the system renovation, but let’s take a look at what equipment efficiency means in light of this test.

Increasing furnace efficiency from 80% to 90% is only a 10% gain, if the equipment works perfectly. Also, considering that most systems only perform at 60% of capacity, at best, new equipment typically yields only a 6% increase in system performance. So do you replace the filters for a 7% gain, or spend an extra thousand dollars, maybe two, on a new furnace?

The answer is to fix the whole system and then install the best furnace your customer can afford. The same principle goes for cooling equipment. We will always sell new equipment and offer the best our customers can afford.

Maybe on the next service or sales call we’ll look at the system and be inspired to do more than replace the system or just get the system running again.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in receiving a free summary of the NCI System performance Study, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, technical articles and downloads.

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 NationalComfortInstitute.com for free information, articles and downloads.