Free Air Conditioning Repair Airflow and Duct Systems 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?

Accepted Answer

Reduced airflow may cause the evaporator coil to freeze or the heat exchanger to trip on high limit.

Answer

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?

Accepted Answer

To reduce air turbulence and minimize static pressure loss.

Answer

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?

Accepted Answer

A dirty or overly restrictive air filter.

Answer

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?

Accepted Answer

Undersized or inadequately supported ductwork experiencing "oil canning."

Answer

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?

Accepted Answer

In the supply plenum after the indoor coil and in the return plenum after the filter but before the blower.

Answer

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).

Air Conditioning Repair Practice Test

Air Conditioning Repair Practice Test

Free Air Conditioning Repair Airflow and Duct Systems Questions and Answers