HERS Blower Door & Duct Testing — Questions and Answers

Question 1: During a blower door test, what effect does a large open fireplace damper have on the result?

It has no measurable effect
It increases the measured CFM50, making the house appear leakier (Correct answer)
It decreases the measured CFM50 by creating a backdraft
It causes the fan to stall

Correct answer: It increases the measured CFM50, making the house appear leakier

An open fireplace damper is a large pathway to outside and will increase the measured infiltration (CFM50), inflating the apparent leakage of the building shell.

Question 2: What is the primary reason HERS raters must depressurize ducts to a specific pressure (typically 25 Pa) during a duct leakage test?

To replicate worst-case wind conditions
To standardize results so tests from different homes and raters are comparable (Correct answer)
To maximize airflow through the ducts
To satisfy fire code requirements

Correct answer: To standardize results so tests from different homes and raters are comparable

Standardizing at 25 Pa ensures that duct leakage measurements are consistent and comparable across different test conditions, homes, and raters.

Question 3: In a duct leakage test, CFM25 total is measured at 300 cfm and CFM25 out is measured at 180 cfm. What is the duct leakage to outside?

480 cfm
300 cfm
120 cfm (Correct answer)
180 cfm

Correct answer: 120 cfm

Duct leakage to outside equals total leakage minus leakage to conditioned space: 300 − 180 = 120 cfm.

Question 4: Which condition must be established before starting a blower door test according to RESNET standards?

All interior doors must be open (Correct answer)
The HVAC system must be running at full capacity
Combustion appliances must be turned off and flues sealed
Windows must be left partially open for pressure relief

Correct answer: All interior doors must be open

RESNET requires all interior doors to be open so that the entire conditioned envelope is pressurized or depressurized uniformly during testing.

Question 5: What does a high ELA (Effective Leakage Area) value indicate about a home?

The home has excellent air sealing
The home has a large equivalent hole size and poor air sealing (Correct answer)
The HVAC system is oversized
The windows are energy efficient

Correct answer: The home has a large equivalent hole size and poor air sealing

A high ELA means the combined leakage pathways are equivalent to a large hole, indicating poor envelope air tightness.

Question 6: During duct leakage testing, why is it important to seal all supply and return grilles before testing?

To prevent dust from entering the duct system
To isolate duct leakage from building envelope leakage (Correct answer)
To increase duct pressure above 25 Pa
To comply with OSHA confined space rules

Correct answer: To isolate duct leakage from building envelope leakage

Sealing grilles ensures that airflow measured by the duct blaster represents only leakage through duct seams and connections, not intentional supply/return openings.

Question 7: A home tests at 6 ACH50. Which HERS reference home air leakage rate would this most closely correspond to for a typical new construction standard?

It exceeds the typical new construction benchmark of 3 ACH50 (Correct answer)
It meets the ENERGY STAR requirement of 7 ACH50
It is at the Passive House standard of 0.6 ACH50
It equals the IECC 2021 mandatory maximum

Correct answer: It exceeds the typical new construction benchmark of 3 ACH50

A common new construction benchmark is 3 ACH50 or less; 6 ACH50 is twice that limit, indicating the home is leakier than typical code expectations.

Question 8: What is the purpose of the pressure pan test in duct diagnostics?

To measure total duct leakage at 25 Pa
To locate which registers have the greatest duct leakage to outside (Correct answer)
To verify that the air handler is operating at design airflow
To calibrate the duct blaster fan

Correct answer: To locate which registers have the greatest duct leakage to outside

A pressure pan placed over each register measures the pressure difference behind it, identifying registers associated with ducts that leak outside the conditioned space.

Question 9: During a blower door test, which measurement channel on the manometer typically reads building pressure with respect to outside?

Channel A
Channel B
The differential channel (B-A or A-B) (Correct answer)
The absolute pressure channel

Correct answer: The differential channel (B-A or A-B)

The differential channel measures the pressure difference between inside and outside (or across the blower door fan), which is the key value for calculating air leakage.

Question 10: A rater finds that CFM25 out (duct leakage to outside) exceeds the RESNET threshold for the home size. What is the most appropriate next step?

Report the result and recommend duct sealing, then retest (Correct answer)
Increase fan speed and retest immediately
Seal the blower door and cancel the test
Replace all ductwork before filing the HERS rating

Correct answer: Report the result and recommend duct sealing, then retest

When duct leakage exceeds thresholds, the rater reports the finding, recommends corrective duct sealing by the contractor, and then conducts a retest to verify compliance.

Question 11: What term describes duct leakage that exits into an unconditioned space such as an attic or crawlspace?

Duct leakage to inside
Duct leakage to outside (Correct answer)
Envelope leakage
Balancing bypass

Correct answer: Duct leakage to outside

Duct leakage that goes into unconditioned spaces (attic, crawlspace, garage) is called duct leakage to outside because it effectively leaves the conditioned envelope.

Question 12: Which instrument is used to simultaneously measure fan flow and building pressure difference during a blower door test?

Duct blaster only
Digital manometer connected to the blower door fan and building (Correct answer)
Infrared camera
CO2 meter

Correct answer: Digital manometer connected to the blower door fan and building

A digital manometer measures both the pressure difference across the building envelope and the flow through the blower door fan, allowing CFM50 to be calculated.

Question 13: For HERS rating purposes, what does 'verified duct leakage' mean when ducts are entirely inside conditioned space?

Only leakage to outside matters and is measured separately
Total duct leakage is zero by definition
The rater may use a default duct leakage value without testing (Correct answer)
Duct leakage to outside is assumed to equal total leakage

Correct answer: The rater may use a default duct leakage value without testing

When all ducts are confirmed to be within the conditioned envelope, RESNET allows raters to use a default duct leakage assumption rather than requiring a measured test.

Question 14: What is the typical baseline depressurization target for a blower door test per ASTM E779?

10 Pa
25 Pa
50 Pa (Correct answer)
75 Pa

Correct answer: 50 Pa

ASTM E779 and RESNET protocols specify 50 Pascals as the reference depressurization pressure, which is why results are expressed as CFM50.

Question 15: A rater suspects that wind is affecting blower door readings. Which technique helps compensate for wind-induced pressure fluctuations?

Run the test only at night
Take multiple fan pressure readings and average them (Correct answer)
Block all exterior openings before testing
Use a higher test pressure of 75 Pa

Correct answer: Take multiple fan pressure readings and average them

Averaging multiple readings or using a data-logging manometer to record many samples over time compensates for short-term wind pressure fluctuations.

Question 16: Which metric best represents the air tightness of a home independent of its volume?

CFM50
ACH50 (Correct answer)
ELA
CFM25 out

Correct answer: ACH50

ACH50 (air changes per hour at 50 Pa) normalizes CFM50 by home volume, allowing meaningful comparisons between homes of different sizes.

Question 17: During a duct blaster test, the technician notices pressure in the duct system won't reach 25 Pa even at maximum fan speed. What is the most likely cause?

The duct system is extremely tight with no leaks
There is a large unsealed duct opening or disconnected duct section (Correct answer)
The manometer battery is dead
The outside temperature is too cold

Correct answer: There is a large unsealed duct opening or disconnected duct section

If the duct blaster cannot reach 25 Pa, it usually means there is an extremely large leak—such as a disconnected duct section or an unsealed chase—allowing air to escape faster than the fan can pressurize the system.

Question 18: What is the formula for converting CFM50 to ACH50?

ACH50 = CFM50 × 60 / conditioned volume (ft³) (Correct answer)
ACH50 = CFM50 / floor area (ft²)
ACH50 = CFM50 × conditioned volume / 60
ACH50 = CFM50 / 50

Correct answer: ACH50 = CFM50 × 60 / conditioned volume (ft³)

ACH50 = (CFM50 × 60 min/hr) ÷ conditioned volume in cubic feet, converting the per-minute flow to an hourly air change rate.

Question 19: In a duct leakage test, what does a very low CFM25 out reading (near zero) with a relatively high CFM25 total reading suggest?

The duct system has significant leaks, all within the conditioned space (Correct answer)
The duct system has no leaks at all
The blower door fan is malfunctioning
The home fails ENERGY STAR duct criteria

Correct answer: The duct system has significant leaks, all within the conditioned space

High total leakage with near-zero outside leakage means ducts are leaking significantly but all the leakage goes into conditioned space, not to unconditioned areas.

Question 20: Which RESNET-recognized testing method allows duct leakage to be measured without pressurizing or depressurizing the entire duct system?

Duct pressurization test (duct blaster)
Pressure pan test
Delta Q method (Correct answer)
Tracer gas decay method

Correct answer: Delta Q method

The Delta Q method estimates duct leakage to outside by comparing house pressure with the air handler running versus off, without using a duct blaster.

Question 21: A blower door test is performed on a home with a conditioned floor area of 2,500 ft² and 8 ft ceilings. CFM50 = 1,200. What is the ACH50?

2.88 ACH50
3.6 ACH50 (Correct answer)
6.0 ACH50
0.48 ACH50

Correct answer: 3.6 ACH50

Volume = 2,500 × 8 = 20,000 ft³; ACH50 = (1,200 × 60) / 20,000 = 72,000 / 20,000 = 3.6 ACH50.

HERS Blower Door & Duct Testing

During a blower door test, what effect does a large open fireplace damper have on the result?

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