HERS Blower Door & Air Infiltration 2 — Questions and Answers
Question 1: A blower door test is conducted at a pressure difference of 50 Pascals. What does the CFM50 value represent?
- The airflow in cubic feet per minute needed to maintain 50 Pa across the building envelope (Correct answer)
- The cubic feet of air volume inside the conditioned space
- The pressure drop across the HVAC filter at design conditions
- The minimum ventilation rate required by ASHRAE 62.2
Correct answer: The airflow in cubic feet per minute needed to maintain 50 Pa across the building envelope
CFM50 is the volumetric airflow (in cubic feet per minute) required to maintain a 50 Pa pressure differential across the building envelope during a blower door test.
Question 2: Which component of the blower door system measures the pressure difference between inside and outside the building?
- Manometer (Correct answer)
- Flow ring
- Speed controller
- Pressure pan
Correct answer: Manometer
The manometer measures the pressure differential across the building envelope and across the blower door fan during testing.
Question 3: When converting CFM50 to Equivalent Leakage Area (ELA), which pressure reference is typically used in the US?
- 4 Pascals
- 10 Pascals
- 25 Pascals (Correct answer)
- 50 Pascals
Correct answer: 25 Pascals
In the US, ELA is referenced at 4 Pa for Canadian standards, but the most common US reference for Effective Leakage Area is 4 Pa while ELA at 10 Pa is used in some protocols; however, RESNET typically uses 4 Pa — but the standard US ELA reference in blower door analysis is 4 Pa.
Question 4: A home has a volume of 15,000 cubic feet and measures 1,200 CFM50. What is the Air Changes per Hour at 50 Pascals (ACH50)?
- 4.8 ACH50 (Correct answer)
- 8.0 ACH50
- 12.5 ACH50
- 0.08 ACH50
Correct answer: 4.8 ACH50
ACH50 = (CFM50 × 60) / Volume = (1,200 × 60) / 15,000 = 72,000 / 15,000 = 4.8 ACH50.
Question 5: Which of the following building penetrations is MOST commonly identified as a major source of air leakage during blower door testing?
- Window glazing
- Top plates and attic bypasses (Correct answer)
- Exterior door panels
- Foundation footings
Correct answer: Top plates and attic bypasses
Top plates and attic bypasses — including gaps around ceiling electrical boxes, plumbing chases, and partition wall top plates — are among the largest sources of air leakage in residential buildings.
Question 6: During a blower door test, a technician notices the manometer reading is unstable and fluctuating. What is the MOST likely cause?
- The fan speed is set too high
- Wind-induced pressure fluctuations on the exterior (Correct answer)
- The house is too airtight for measurement
- The flow ring selected has too large an opening
Correct answer: Wind-induced pressure fluctuations on the exterior
Wind-induced pressure fluctuations on the building exterior cause unstable manometer readings, which is why blower door tests should be conducted on calm days (wind speeds under 15 mph).
Question 7: What is the purpose of using multiple pressure rings (flow rings) in a Minneapolis Blower Door system?
- To test different floors of the building simultaneously
- To allow accurate flow measurement across a wide range of leakage rates (Correct answer)
- To pressurize and depressurize simultaneously
- To measure duct leakage at different system pressures
Correct answer: To allow accurate flow measurement across a wide range of leakage rates
Different flow rings restrict airflow to different degrees, allowing the system to accurately measure both very tight and very leaky buildings within the fan's accurate measurement range.
HERS Blower Door & Air Infiltration 2
A blower door test is conducted at a pressure difference of 50 Pascals.
What does the CFM50 value represent?