Chapter 20 Quiz: Ductwork, Airflow, and Ventilation

Part A: Multiple Choice

1. The U.S. Department of Energy estimates that typical American homes lose what percentage of conditioned air through duct leakage?

A) 5–10% B) 20–30% C) 40–50% D) Less than 5% in modern construction

2. A room has a supply register but no return grille and no transfer grille. When the HVAC system runs with the door closed, the room will likely:

A) Receive more conditioned air, since it has no competing return path B) Pressurize, pushing conditioned air into wall cavities and the attic through gaps C) Depressurize, drawing outdoor air in through window and door gaps D) Maintain neutral pressure because supply and return automatically balance

3. The correct material for sealing ductwork connections is:

A) Standard duct tape (silver, fabric-backed) B) UL 181-rated duct mastic or aluminum foil tape C) Fiberglass mesh tape with drywall compound D) Silicone sealant

4. Flex duct that is compressed to 80% of its design diameter loses approximately what fraction of its airflow capacity?

A) About 10% B) About 25% C) About 60% D) Compression has no effect on airflow capacity

5. A bathroom exhaust fan in an older home is ducted with a short flex duct run that terminates in the attic above the bathroom. This is:

A) Acceptable if the attic is well-ventilated B) Acceptable for bathrooms with low moisture generation (powder rooms) C) A code violation that causes moisture damage to the attic over time D) The standard installation method for cold climates where exterior termination would freeze

6. A Heat Recovery Ventilator (HRV) installed in a tight home primarily:

A) Recovers waste heat from HVAC exhaust and returns it to the refrigerant cycle B) Transfers heat from stale outgoing house air to incoming fresh outdoor air, reducing ventilation energy loss C) Filters incoming air through a high-MERV media filter while recovering heat D) Stores heat in a thermal mass during off-peak hours for release during peak demand

7. The difference between an HRV (Heat Recovery Ventilator) and an ERV (Energy Recovery Ventilator) is:

A) HRVs are designed for cooling climates; ERVs are designed for heating climates B) ERVs also transfer moisture between the airstreams in addition to heat; HRVs transfer heat only C) HRVs are whole-house systems; ERVs are single-room devices D) ERVs have higher efficiency ratings but shorter service lives than HRVs

8. Dave Kowalski had his home's duct leakage measured at 32% before professional sealing. After sealing, it dropped to 8%. His annual HVAC cost was approximately $2,000. Roughly how much did he save annually?

A) About $80 B) About $240 C) About $480 D) About $640

9. An "encapsulated crawlspace" differs from a "vented crawlspace" in that:

A) It uses mechanical equipment to continuously dry the air, while a vented crawlspace relies on outdoor air B) Foundation vents are sealed, a vapor barrier covers the floor, and the space is insulated and conditioned with the house C) It uses a heavy concrete vapor barrier instead of plastic sheeting D) The floor framing is left exposed on the bottom side to allow natural airflow through the joists

10. A blower door test measures which of the following?

A) The flow rate of air through the duct system at standard pressure B) The efficiency of the HVAC blower motor at various speeds C) The overall air leakage of the building envelope at a standard pressure difference D) The static pressure in the duct system at maximum blower speed

Part B: Short Answer

11. A homeowner has a 2,000 sq ft house with an annual heating and cooling bill of $2,800. Their duct system is estimated to have 25% leakage to outside. A contractor quotes $2,000 for professional duct sealing, which they estimate will reduce leakage to 6%. Calculate the estimated annual savings and simple payback period.

12. Explain why venting a bathroom exhaust fan into the attic causes damage, using the physical principles of heat, moisture, and condensation. What is the correct solution?

13. A house has a blower door test result of 1.2 ACH50 after a deep energy retrofit. The homeowner notices that despite good energy performance, the air feels stale and carbon dioxide levels are elevated when the house is occupied. Explain the cause and the appropriate solution.

14. A homeowner is told by a contractor that their HVAC system's static pressure is 0.82 inches water column (in.w.c.). The design maximum is 0.50 in.w.c. What problems might this cause, and what are three possible causes of elevated static pressure?

15. Explain the difference between supply-side duct leakage and return-side duct leakage. Which is worse for energy efficiency, and why? Is there a type that presents a safety risk?

Answer Key

Part A: 1. B 2. B 3. B 4. C 5. C 6. B 7. B 8. C — Before: 32% of $2,000 = $640 in waste; After: 8% = $160 in waste; Annual savings = $480 9. B 10. C

Part B — Model Answers:

11. Current waste: 25% × $2,800 = $700/year. After sealing to 6%: 6% × $2,800 = $168/year. Annual savings: $700 − $168 = $532/year. Simple payback: $2,000 ÷ $532 = 3.76 years. This is an excellent investment — short payback, no ongoing maintenance costs, and the improvement is permanent as long as the mastic doesn't fail (it generally doesn't for decades).

12. Warm, humid air from a shower travels through the exhaust duct. When this humid air reaches the uninsulated attic space, it contacts cold surfaces (roof sheathing, attic framing) that are below the dew point of the warm air. Moisture in the air condenses on these surfaces, just as water droplets form on a cold glass. Over a heating season, this can deposit many gallons of water in the attic. The result is soaking insulation (which loses R-value), wood rot in the sheathing and rafters, and eventually mold growth. The correct solution is routing an insulated flex duct from the fan housing through the attic to a properly flashed and dampered roof cap or sidewall cap — the humid air is carried entirely to the exterior without contacting attic surfaces.

13. A 1.2 ACH50 result means the house is very tight — well below the threshold for adequate ventilation through air infiltration alone. The house is not getting enough fresh air through normal infiltration, so CO₂ from occupants accumulates and air feels stale. The solution is mechanical ventilation: an HRV or ERV sized to meet ASHRAE 62.2 requirements (approximately 60–75 CFM continuous for this house size), connected to the forced-air system return or to its own dedicated duct network. Simply opening windows is a workaround, not a solution, because it can't be reliably done in all weather and provides no heat recovery.

14. Static pressure of 0.82 in.w.c. against a 0.50 in.w.c. design maximum means the blower is working against nearly double its design resistance. Consequences: dramatically reduced airflow (the blower delivers far less CFM than designed), reduced heating and cooling capacity (less air over the coil), possible coil icing in cooling mode (low airflow causes low evaporator temperature), and premature blower motor wear from sustained high-amp operation. Three possible causes: (1) A high-MERV filter that wasn't specified in the design — replacing it with a lower-MERV or larger filter reduces resistance significantly. (2) Dirty evaporator coil adding flow resistance — professional coil cleaning needed. (3) Undersized duct branches or excessively long flex duct runs — a design issue requiring duct modification.

15. Supply-side duct leakage is conditioned air (heated or cooled) escaping from supply ducts into unconditioned space. You paid to condition that air; it never reaches a room. It's pure waste. Return-side duct leakage is unconditioned, potentially dirty air (from attic, crawlspace, or wall cavities) being drawn into the return duct and treated as house air. It's wasteful (you must condition this air) and introduces pollutants, humidity, and particulates into the house. Return-side leakage also presents a combustion safety risk: if the return is in or near the utility room and draws air from near the furnace or water heater, it can create negative pressure that backdrafts combustion appliances, potentially pulling carbon monoxide into the house airstream. This is why combustion safety testing should follow any significant duct sealing work.