Free Air Conditioning Repair Airflow and Duct Systems Questions and Answers — Questions and Answers
Question 1: A technician measures the Total External Static Pressure (TESP) on a residential furnace and gets a reading of 0.9 inches of water column (IWC). The manufacturer's data plate specifies a maximum TESP of 0.5 IWC. What is the most likely consequence of this reading?
- The blower motor will consume less amperage due to a reduced workload.
- System efficiency will increase because the air is moving more slowly.
- Reduced airflow may cause the evaporator coil to freeze or the heat exchanger to trip on high limit. (Correct answer)
- The system will provide better dehumidification in cooling mode.
Correct answer: Reduced airflow may cause the evaporator coil to freeze or the heat exchanger to trip on high limit.
A TESP reading higher than the manufacturer's maximum rating indicates excessive resistance in the duct system. This high static pressure forces the blower motor to work harder while moving less air, leading to reduced airflow across the indoor coil. Insufficient airflow can cause the evaporator coil's temperature to drop below freezing in cooling mode or cause the heat exchanger to overheat in heating mode, tripping the high-limit safety switch.
Question 2: In a rectangular sheet metal duct system, what is the primary purpose of installing turning vanes inside a sharp 90-degree elbow?
- To increase the velocity of the air around the bend.
- To filter large debris from the airstream before a transition.
- To reduce air turbulence and minimize static pressure loss. (Correct answer)
- To intentionally create back-pressure for air balancing.
Correct answer: To reduce air turbulence and minimize static pressure loss.
Air has momentum and does not change direction easily. In a sharp elbow without turning vanes, air crashes into the outer wall, creating significant turbulence and a large drop in static pressure. Turning vanes are curved blades that guide the air smoothly through the turn, minimizing turbulence and the associated pressure loss, which improves overall system airflow and efficiency.
Question 3: A technician confirms proper blower motor operation but measures significantly low airflow moving through the residential air handler. Which of the following is the most common and likely cause for this issue?
- A dirty or overly restrictive air filter. (Correct answer)
- An oversized return air plenum.
- Leaking supply ducts in the conditioned space.
- A thermostat that is improperly calibrated.
Correct answer: A dirty or overly restrictive air filter.
The air filter is one of the most common points of restriction in an HVAC system. A filter that is clogged with dirt and debris, or one that has a MERV rating too high for the system's design, will severely restrict the amount of air the blower can pull into the air handler. This is often the first item a technician should check when diagnosing a low airflow complaint.
Question 4: A customer complains of a loud "banging" or "popping" noise from their rectangular metal ductwork shortly after the blower motor starts and stops. What is the most probable cause of this noise?
- The use of flexible duct connectors at the plenums.
- Debris such as a loose screw rattling inside the duct.
- Undersized or inadequately supported ductwork experiencing "oil canning." (Correct answer)
- Excessive air velocity causing the registers to vibrate.
Correct answer: Undersized or inadequately supported ductwork experiencing "oil canning."
This phenomenon, known as "oil canning," occurs when large, flat sections of metal ductwork flex due to changes in air pressure. When the blower starts, the duct pressurizes and bows outward; when it stops, it relaxes inward, creating a loud bang or pop. This is often caused by undersized ducts creating high static pressure or duct walls that are too thin or lack proper cross-bracing for support.
Question 5: To get an accurate measurement of a system's Total External Static Pressure (TESP), where should the two test probes of a manometer be placed?
- In the return grille and in the furthest supply register.
- Immediately before the blower wheel and immediately after the blower wheel.
- In the supply plenum after the indoor coil and in the return plenum before the filter.
- In the supply plenum after the indoor coil and in the return plenum after the filter but before the blower. (Correct answer)
Correct answer: In the supply plenum after the indoor coil and in the return plenum after the filter but before the blower.
Total External Static Pressure (TESP) measures the total resistance the blower motor must work against that is *external* to the air handler or furnace cabinet. This includes the supply and return ductwork. The correct measurement points are on the supply side after all internal components (like the coil or heat exchanger) and on the return side before the blower, but after the filter (as the filter is considered part of the external system resistance).
Question 6: When comparing two duct runs of the same diameter and length, which of the following will result in the highest friction loss and the greatest reduction in airflow?
- A straight run of smooth, rigid metal duct.
- An undersized, flexible duct with several sharp bends and sagging sections. (Correct answer)
- A properly sized, rigid metal duct with two 45-degree elbows.
- A properly stretched, flexible duct with one gradual 90-degree bend.
Correct answer: An undersized, flexible duct with several sharp bends and sagging sections.
Flexible ductwork has a corrugated inner liner that creates significantly more friction than smooth rigid duct. When it is also undersized for the required airflow, the velocity increases, which raises the friction loss exponentially. Adding sharp bends and allowing the duct to sag further increases turbulence and restriction, creating the worst-case scenario for airflow.
Free Air Conditioning Repair Airflow and Duct Systems Questions and Answers
A technician measures the Total External Static Pressure (TESP) on a residential furnace and gets a reading of 0.9 inches of water column (IWC).
The manufacturer's data plate specifies a maximum TESP of 0.5 IWC.
What is the most likely consequence of this reading?