This post provides a brief overview of how heating, ventilation and air conditioning (HVAC) systems can affect interior relative humidity levels, causing mold growth within the building.
The role of an HVAC system is to remove water vapor from the air. The HVAC system generally includes an exterior mounted compressor, accumulator, condenser coil and compressor fan and an interior mounted air handler unit (AHU) with an evaporator coil, evaporator fan/blower motor and expansion valve. The HVAC system removes water vapor by cooling the evaporator coils, located inside the AHU with refrigerant, which changes state/temperate by compression or expansion during the repeated phase transition from a liquid to a gas. Simply put, as warm return air passes over the cooled evaporator coils, the air reaches its dew point temperature and condenses into liquid. This water is collected in a condenser pan beneath the coil and subsequently removed from the system via condensate drain lines.
Many circumstances can affect this process, leading to increased relative humidity levels within a space and contributing to an environment where mold can develop. Below are a few of the scenarios that develop within the systems themselves and how they are operated and maintained:
- Condensate drain line clogged/annular float switch: If a condensate drain line is clogged, water can back up into the secondary drain pan. If this occurs, the annular float switch could repeatedly be activated, shutting off the system. This causes the system to short-cycle, or not operate for the duration needed to remove moisture from the air.
- Air Filters: When air filters are installed incorrectly, not maintained or not present at all, dust/debris may buildup on the AHU evaporator coils. As a result, the AHU has to work harder to draw in return air. The design and operation of an HVAC system depends upon a specific volume of air and airflow velocity across the evaporator coil. The dust/debris buildup can lead to the freezing over of the evaporator coils; as a result, the system may not operate for the duration needed to remove moisture from the air.
- Oversized HVAC system: HVAC systems are selected based on factors that produce heat (sensible heat load) and factors that produce moisture (latent heat). While an oversized system may sound like a good idea, an oversized system will cool the interior environment more rapidly than a correctly sized unit would. As a result, short cycling occurs and the system will not operate for the duration needed to remove adequate moisture from the air.
- Air Handler Fan in the “On” Position: Setting the fan mode on the HVAC thermostat to the “On” setting as opposed to the “Auto” setting allows airflow over the evaporator coils even when the system is not in cooling mode. This allows the return air to pick up moisture from the damp evaporator coils and circulate/distribute it throughout the interior space via the ductwork, which can lead to increased relative humidity levels. Having the fan set in the “Auto” position allows for the airflow across the wet evaporator coils to be interrupted when the set temperature is reached.
In addition to the above HVAC related issues, other exacerbating circumstances may affect the performance of the HVAC system, increasing relative humidity levels in an interior space and leading to the development of mold. Examples include:
- Vapor Drive: An HVAC system can create an abrupt temperature gradient between conditioned (cool and dry) and non-conditioned (hot and humid) spaces. If the exterior building envelope is not secure, water vapor may diffuse into the conditioned space through floor sheathing via a damaged vapor barrier, or through cracks in the exterior stucco, increasing interior relative humidity levels. Having a sound building envelope free of cracks and holes and a crawl space vapor barrier with intact insulation can prevent vapor drive from occurring.
- Negative Pressure Environment: Duct leaks can create a negative pressure interior environment. Duct leaks allow air to escape from the HVAC system. In response to this, the system will draw unconditioned air into the interior space from underneath door thresholds, around windows and from bathroom exhaust vents, leading to increased interior relative humidity levels.
- Thermostat Utilizing a Humidistat: Depending upon how the thermostat is wired, once the desired humidity level is obtained (most commonly by adding moisture to the interior air column during the dry winter periods), the HVAC system blower motor (fan) may continue to run, introducing additional moisture to the indoor air.
- Natural Disasters: In areas prone to hurricanes or other unpredictable natural incidents, such as earthquakes, tornadoes, wildfires, etc., power outages are common in the warm summer/fall months. In these areas, power outages could result in days or even weeks when the HVAC system is inoperable. To combat rising indoor temperatures and provide some relief, many homeowners decide to open exterior windows. The combination of no power and the introduction of warm/humid outdoor air can increase interior relative humidity levels within a short period.
The Vertex Companies, LLC (VERTEX), with an experienced staff of professional engineers, certified industrial hygienists, certified indoor environmental consultants and environmental scientists, can provide assistance in investigating the cause, origin and duration of pollution policy insurance claims for mold growth in cases where relative humidity/mechanical system issues are suspected. For more information, please contact Patricia Riser at priser@vertexeng.com.
