CMI HVAC Systems and Mold Growth 2 — Questions and Answers
Question 1: An inspector discovers that an HVAC system is sized significantly oversized for the building load. How does this contribute to mold risk?
- Oversized systems run longer cycles, drying out the coil more effectively
- Oversized systems short-cycle, failing to run long enough to adequately dehumidify the air (Correct answer)
- Oversized systems always use higher MERV filters that restrict airflow
- Oversized systems create positive building pressure that prevents mold spore infiltration
Correct answer: Oversized systems short-cycle, failing to run long enough to adequately dehumidify the air
Oversized systems cool the space rapidly and shut off before the refrigerant coil has run long enough to condense and drain significant moisture from the air. This short-cycling keeps indoor relative humidity elevated, which is the primary driver of mold growth. Longer, properly sized run cycles are needed for effective dehumidification.
Question 2: Mold growth on supply air diffusers near the ceiling is often caused by which phenomenon?
- Insulation failure in the duct causing heat gain
- Cold supply air causing condensation on the diffuser face when room humidity is high (Correct answer)
- Excessive filter loading reducing air velocity
- Return air mixing with supply air at the ceiling plenum
Correct answer: Cold supply air causing condensation on the diffuser face when room humidity is high
Cold supply air chills the metal diffuser face below the room air's dew point, causing water vapor to condense directly on the surface. Dust carried in the air adheres to the wet surface, providing nutrients for mold. This is a surface condensation problem driven by high indoor RH, not a duct or filter issue.
Question 3: Which NADCA standard provides guidelines for the assessment and cleaning of HVAC systems suspected of mold contamination?
- NADCA ACR (Assessment, Cleaning and Restoration of HVAC Systems) (Correct answer)
- ASHRAE 62.1 Ventilation Standard
- IICRC S500 Water Damage Standard
- EPA IAQ Tools for Schools Framework
Correct answer: NADCA ACR (Assessment, Cleaning and Restoration of HVAC Systems)
NADCA's ACR (Assessment, Cleaning and Restoration of HVAC Systems) is the industry-recognized standard specifically for evaluating and cleaning HVAC systems, including mold-contaminated ones. ASHRAE 62.1 addresses ventilation rates, IICRC S500 covers structural water damage, and EPA IAQ Tools for Schools is an educational framework — none are HVAC cleaning protocols.
Question 4: When inspecting a rooftop package unit, what is the primary concern related to mold growth from the unit casing seams and roof penetrations?
- Wind uplift of the unit causing refrigerant leaks
- Water infiltration through failed seals introducing outdoor moisture directly into the unit and ductwork (Correct answer)
- Excess UV radiation degrading internal insulation lining
- Thermal expansion cracking the supply plenum connection
Correct answer: Water infiltration through failed seals introducing outdoor moisture directly into the unit and ductwork
Failed caulking or sealant at casing seams and roof penetrations allows rainwater and outdoor humid air to enter the unit and connected ductwork without passing through the dehumidification process. This moisture directly wets internal insulation and liner surfaces, creating ideal mold conditions. Refrigerant leaks, UV degradation, and thermal cracking are secondary concerns unrelated to moisture infiltration.
Question 5: A building inspector finds mold on the interior liner of a supply trunk that tested positive for Aspergillus and Penicillium. The best remediation approach per IICRC S520 is:
- Apply biocide spray to the liner and return to service
- Remove and replace contaminated lined ductwork sections that cannot be effectively cleaned (Correct answer)
- HEPA vacuum the liner and seal with encapsulant
- Increase MERV filter rating upstream to capture spores
Correct answer: Remove and replace contaminated lined ductwork sections that cannot be effectively cleaned
IICRC S520 requires removal and replacement of porous materials like duct liner when they cannot be effectively cleaned because mold hyphae penetrate the substrate and cannot be fully eliminated. Biocide sprays and encapsulants are not acceptable standalone remediation methods per S520, and increasing the upstream filter rating does nothing to address existing colonization downstream.
Question 6: In a variable air volume (VAV) HVAC system, which scenario increases the risk of duct condensation and subsequent mold growth?
- VAV boxes modulating to minimum airflow, reducing supply air velocity and allowing cold duct surfaces to accumulate condensation (Correct answer)
- The main AHU operating at full design capacity
- Supply air temperature reset to a warmer setpoint during mild weather
- Return air temperature sensors accurately tracking space conditions
Correct answer: VAV boxes modulating to minimum airflow, reducing supply air velocity and allowing cold duct surfaces to accumulate condensation
At minimum airflow, supply air velocity drops significantly, allowing cold air to dwell in ducts longer and increasing the time cold duct surfaces are exposed to any infiltrating ambient air. If duct surfaces drop below the dew point of surrounding air, condensation forms on the interior, providing the moisture mold needs to grow. Full AHU capacity, warmer supply air reset, and accurate sensors all reduce rather than increase this risk.
CMI HVAC Systems and Mold Growth 2
An inspector discovers that an HVAC system is sized significantly oversized for the building load.
How does this contribute to mold risk?