David Richardson

In many areas of the country, duct sealing and tightness testing is now mandated at local and state levels. This has brought about renewed interest in sealing existing duct systems among HVAC professionals as awareness increases.

While a properly sealed duct system can lead to improved IAQ, comfort, and possibly a reduced risk of carbon monoxide poisoning, is it possible that too much is being assumed for tight ducts to really be right ducts? Is there a possibility that duct sealing could negatively affect an HVAC system?

Cause and Effect

When a duct system is sealed, static pressure increases as you are sealing existing relief openings. Depending on the severity of leakage in the duct system, this static pressure increase could be substantial. When this increase in pressure is combined with a duct system that is already undersized or restricted, this is a recipe for disaster.

Many of you reading this may have just slammed on the brakes and said, “Duct sealing works great on a properly sized duct system.” You are 100 percent correct; it does. There is one issue with this though: How many properly sized duct systems do you encounter in the field?

The majority of existing duct systems were sized using rules of thumb. These duct systems have also never been tested to verify performance, which is something to consider as we look at some of the effects caused by sealing an undersized duct system.

Constant-speed Fans

Airflow is the medium used to transfer Btu in an HVAC system. In order for the equipment to have a long, trouble-free life, the fan airflow has to be within acceptable levels for the heating and cooling system requirements. When fan airflow drops below acceptable levels, equipment life is drastically reduced and problems begin.

When a restricted or undersized duct system is sealed, the amount of airflow a constant-speed fan is moving decreases. In many instances, the leaks that existed in the undersized duct system were the only thing keeping the fan moving the minimal amount of airflow needed for borderline equipment operation. Once these leaks were sealed, the fan airflow dropped below acceptable values due to an increase in total external static pressure.

In the cooling mode of operation, you now begin to see increased compressor failures due to refrigerant flood back. Evaporator coils begin to intermittently freeze-up during periods of extended run time due to lack of airflow. Often, the technicians who get these freeze-up complaints will overcharge the system in an attempt to keep it from freezing up again. The vicious cycle continues.

In the heating mode of operation, you now have heat exchangers in gas furnaces that are cracking due to overheating from a lack of airflow. Heat pumps lock out on high head-pressure controls and a portion of the refrigerant charge gets removed as an assumed remedy. This leads to longer equipment run times and secondary electric heat strips coming on to satisfy the building load — all of this because of a decrease in fan airflow. The cycle continues through another season.

Variable-speed Fans

The amount of airflow a variable-speed fan is moving will typically stay the same when a restricted or undersized duct system is sealed. This comes at a high price, though, as there could be negative impacts on the equipment and its operation.

The variable-speed fan will consume more energy to move the programmed amount of airflow in this scenario. Any energy savings that may have been assumed from a variable-speed fan will be lost. This is contrary to the claims made by some that the fan will only consume a certain amount of watts per day.

Condensate blow-off from the evaporator coil is another issue that appears when an undersized duct system is sealed and a variable-speed fan is present. This can lead to IAQ issues from condensate building up in the air-handling equipment or being forced into the supply duct system.

Air-handler drains that once drained properly now might not drain at all due to excessive negative pressure in the air-handler cabinet. This is due to the fan ramping up against the higher static pressures created by sealing an undersized duct system.

Two Missing Ingredients

There are two missing ingredients needed to correct the duct-sealing recipe for disaster. They are static pressure and fan airflow. Oddly enough, these two ingredients are typically ignored when it comes to duct tightness and duct leakage testing.

A simple static pressure test as a pre-qualifier before duct sealing is performed is a great start. The equipment’s fan-rated pressure, which is found in the manufacturer’s engineering data, should be compared against the actual measured static pressure as an initial step. This would ensure the ducts have enough capacity to be sealed, otherwise further duct system repair is needed.

The next step would be to determine fan airflow. If the fan airflow falls within an acceptable range, then duct sealing can be looked at as an option. If the fan airflow is already at a borderline level, then other options would need to
be considered.

Above all else, make sure you aren’t creating more problems than you are correcting. The above side effects are in direct opposition to the good intentions of duct sealing. Duct sealing is an important aspect of proper duct design, but it has to take place on a properly performing duct system. Make sure you aren’t unintentionally signing the death sentence on the HVAC equipment when you seal ducts that might be undersized or restricted.

Publication date: 7/7/2014

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