# Four Lessons Sir Isaac Newton’s Third Law Can Teach About Duct Upgrades

Jan. 3, 2023
Duct upgrades often change airflow conditions and interactions in a home.

When many of us hear Sir Isaac Newton’s name, we think of an apple falling near him, leading to his discovery of gravity. Another discovery by Newton is the third law of motion. It states, “For every action, there is an equal and opposite reaction.”

Anytime you modify a duct system, it’s a good idea to consider this law, or it might come back to haunt you. Duct upgrades often change airflow conditions and interactions in a home. For every modification you make, there’s an equal and opposite reaction that can cause unintended consequences.

Successful duct upgrades change airflow and pressures to resolve your customer’s issues. However, unsuccessful upgrades might create additional problems that affect safety, health, comfort, and the entire HVAC system. In other words, you might correct one problem but unintentionally create a larger one. Let’s look at how Sir Isaac Newton’s third law of motion can help you prevent unintended problems when you alter a duct system.

#### Don’t Skip the Test-In

A proper test-in is the single most significant contributor to successful duct upgrades. During this step, you uncover hidden issues that could cause upgrades to miss the mark. Skipping the test-in keeps you in the dark about potential problems and leaves you guessing.

After completing the test-in measurements, you should do the following:
• Compare static pressure readings against static pressure budgets or manufacturer data to identify areas of restriction that need correction
• Determine fan airflow and compare it to required fan airflow to estimate the extent of renovation repairs needed
• Measure system temperatures and compare them to equipment temperatures to identify excessive duct system temperature losses or gains.

Forward-thinking HVAC professionals often offer additional testing before work begins. They offer their customers the choice to invest in this level of expertise that few in our industry have. Some advanced tests include:

• Measuring supply register and return grille airflows and comparing them to design values to identify possible airflow and pressure imbalances.
• Conducting room-by-room heat loss/gain calculations and equipment selection based on real-world conditions.
• Testing building leakage rates with a blower door to estimate the impact on load calculations.

Don’t consider these additional tests unnecessary or a waste of time. They help you discover deeper problems that may prevent your success and your customer’s happiness. If you miss any issues, you own them after you do the work. Remember, you should get paid for this level of expertise. Don’t do them for free and degrade the value of the services you provide.

#### Check Refrigerant Charge

After duct upgrades, airflow across the coil will probably change. The best practice is to check your refrigerant charge before and after upgrades.

You’ll likely end up pulling refrigerant out of the system after you complete your repairs. Excess refrigerant often masks airflow problems and keeps the coil above freezing, so be ready for it. Missing this issue could leave you replacing a compressor because of an overcharge condition.

#### Consider Pressure Zones

Balanced supply and return airflow to individual rooms is one goal of duct upgrades. Improperly designed pressure zones usually lead to airflow imbalances. However, pressure imbalances can also occur because of duct leakage, interior door closure, lack of air balancing, and central returns. Pressure imbalances cause a variety of issues, including:

• Carbon monoxide (CO) poisoning
• Hot and cold rooms
• Large temperature swings
• Poor humidity control
• IAQ issues (dust and allergens).

Consider your upgrades and duct layout carefully with these potential interactions in mind. Don’t be lazy, and recommend door undercuts as a cure to equalize zone pressures. They are ineffective unless the undercut mimics a bathroom stall door. Instead, consider other options like adding additional return ducts and transfer ducts.

#### Air Balance and Test-Out

No system design is bulletproof. As good as your design skills are, you still need to verify your design intentions. Air balancing is a critical step that often gets overlooked once upgrades are complete.

Unless the system receives a proper air balance and test-out, you do not know how much improvement you made. One of the biggest opportunities to thrill your customers is showing them measured and verified improvements to their system. You can also find and quickly correct overlooked issues.

If you compare the readings you measure during test-in to test-out, you can use Newton’s law to your advantage. If you’ve been thorough, you will see an increase in performance.

Without air balance and test-out measurements, you are at the mercy of Newton’s third law. However, if you compare the readings you measure during test-in to test-out, you can use Newton’s law to your advantage. If you’ve been thorough, you will see an increase in performance.

#### Learn from Newton

Cause and effect interactions are a reality of any duct upgrade. Understanding these interactions is the first step to building confidence as you solve your customer’s problems.

Recommend and perform a full test-in to find the hidden defects that cause unsuccessful duct upgrades. Then, design a scope of work that corrects these defects. Once you sell and install the solution, air balance and test-out to verify the system operates as designed.

Newton’s third law of motion states, “For every action, there is an equal and opposite reaction.” There’s a parallel law. “For every duct upgrade you successfully complete, there’s a thrilled customer who will rave about you solving their problems.”

David Richardson serves the HVAC industry as Director of Training for the National Comfort Institute, Inc. (NCI). NCI specializes in training that focuses on improving, measuring, and verifying HVAC and Building Performance.