HERS Ventilation & Indoor Air Quality 2 — Questions and Answers
Question 1: An HRV (Heat Recovery Ventilator) differs from an ERV (Energy Recovery Ventilator) in that an HRV:
- Transfers both sensible heat and moisture between incoming and outgoing airstreams
- Transfers only sensible heat between airstreams without transferring moisture (Correct answer)
- Only provides conditioned supply air without any heat recovery
- Is specifically designed for hot-humid climate zones
Correct answer: Transfers only sensible heat between airstreams without transferring moisture
An HRV uses a heat exchanger core that transfers only sensible (temperature) energy, whereas an ERV core also transfers latent energy (moisture) between airstreams.
Question 2: In which climate condition is an Energy Recovery Ventilator (ERV) generally preferred over a Heat Recovery Ventilator (HRV)?
- Cold climates (IECC zones 6-8) with very low winter humidity
- Hot-humid climates (IECC zones 1-3) where moisture control is critical (Correct answer)
- High-altitude climates with extremely dry air year-round
- Mixed-dry climates where no humidity management is needed
Correct answer: Hot-humid climates (IECC zones 1-3) where moisture control is critical
In hot-humid climates, an ERV's ability to transfer moisture from the humid incoming air to the drier exhaust air reduces latent cooling loads, making it preferable over an HRV.
Question 3: A supply-only ventilation system in a hot-humid climate can create which building science concern?
- Depressurization that draws soil gases into the conditioned space
- Pressurization that drives warm humid air into wall cavities, risking moisture damage (Correct answer)
- Reduced blower door test results below program minimums
- Backflow of combustion gases from sealed-combustion appliances
Correct answer: Pressurization that drives warm humid air into wall cavities, risking moisture damage
Supply-only systems positively pressurize the home, which in hot-humid climates can push warm, humid outdoor air into wall assemblies, creating condensation and moisture damage risk.
Question 4: An exhaust-only ventilation system draws outdoor replacement air into the home primarily through:
- A dedicated supply duct connected to the air handler return
- Intentional envelope leakage paths and passive fresh air inlets (Correct answer)
- An HRV or ERV core mounted at the air handler
- The plumbing drain-waste-vent (DWV) stack
Correct answer: Intentional envelope leakage paths and passive fresh air inlets
Exhaust-only systems depressurize the home, causing outdoor air to infiltrate through envelope leakage paths and any intentionally installed passive makeup air inlets.
Question 5: What is the typical sensible heat recovery efficiency range for a quality residential HRV unit under rated conditions?
- 20–40%
- 45–65%
- 70–85% (Correct answer)
- 90–99%
Correct answer: 70–85%
Quality residential HRV units typically achieve 70–85% sensible heat recovery efficiency, significantly reducing the energy needed to condition incoming fresh air.
Question 6: A HERS rater measures a home's exhaust-only ventilation fan at 35 CFM, but the ASHRAE 62.2 design requirement is 45 CFM. This system is:
- Compliant, since it is within 25% of the design rate
- Non-compliant, since measured flow is more than 10% below the design rate (Correct answer)
- Compliant, since any rate above 30 CFM satisfies minimum requirements
- Compliant if operable windows are present in all sleeping areas
Correct answer: Non-compliant, since measured flow is more than 10% below the design rate
35 CFM is approximately 78% of the 45 CFM design rate, which is more than 10% below the required rate, making the system non-compliant per ASHRAE 62.2 tolerance limits.
Question 7: When verifying ventilation compliance during a HERS rating, the rater typically documents ventilation performance by recording:
- Static pressure at the air handler supply plenum
- Measured airflow rate in CFM using a flow hood, anemometer, or fan flow meter (Correct answer)
- Duct leakage to outdoors from the duct pressurization test
- Blower door CFM50 result from the air sealing test
Correct answer: Measured airflow rate in CFM using a flow hood, anemometer, or fan flow meter
HERS ventilation verification requires measuring actual airflow in CFM at the fan or grille using a calibrated flow measurement device, confirming the installed system meets design requirements.
HERS Ventilation & Indoor Air Quality 2
An HRV (Heat Recovery Ventilator) differs from an ERV (Energy Recovery Ventilator) in that an HRV: