HVAC Systems and Moisture Related Damage
Can an HVAC System Cause Moisture Related Damage or Mold?
I have been involved in numerous cases or claims where mold damage to a building or home was suspected to be caused by the heating, ventilation, and air-conditioning (HVAC) system. The initial phone calls typically include a brief description of the property owner’s account of the damage and why the HVAC system is suspected. The description of the loss is often followed by the question, ‘Can an HVAC system cause moisture related damage or mold?’ The correct answer is, maybe. This article will provide an overview of how air and moisture behave and how a structure’s HVAC system may interact.
What is Moisture Related Damage?
To start, I need to clarify what I mean by moisture related damage. Everybody understands that mold likes to grow in wet places. The distinction to be made is how the water came to provide an environment favorable to mold growth. Simply put, if the damage-causing water entered the building in liquid form, it is considered water damage. If the water entered the building as moisture in the air and later condensed into liquid water, it is moisture damage.
What is Relative Humidity?
Air always carries some amount of moisture or water vapor, typically referred to as humidity. When liquid water molecules absorb enough heat, liquid water evaporates into vapor form. Therefore, the greater the temperature of the air, the more moisture the air can hold. This gives rise to the concept of relative humidity. If the air on a certain day is at 70% relative humidity, it means that the air is holding 70% of the total amount of moisture that could be held in the air at that temperature. So, 90 degree air at 70% relative humidity contains a great deal more moisture than 40 degree air at the same 70% relative humidity. At 100% relative humidity, the air has reached the maximum amount of water vapor that it can hold at that temperature. If the temperature drops, then there is no longer enough heat to hold all the water vapor in gas form, so some of the water must turn back into a liquid. The amount of water vapor that changes back to a liquid increases as the temperature decreases.
What is the Dew Point?
The physical change of water from vapor to liquid is known as condensation. The temperature at which the water in the air condenses is known as the dew point. The dew point changes depending on the temperature and humidity of the air.
Condensation in Daily Life
Condensation manifests itself in various ways in daily life. At night, when the temperature of the air drops sufficiently, the moisture in the air loses heat and some of the water vapor in the air condenses and falls to the ground as dew. The outside of a glass containing a cold drink may begin to sweat because the temperature of the glass is below the dewpoint temperature of the surrounding air. Any air that comes in contact with the glass is cooled below the dew point and the water vapor in the air condenses on the glass.
Condensation in HVAC Systems
In the same way, condensation occurs in a common direct expansion (DX) air-conditioning system like those found in most homes. Air is conditioned as it is blown through the air-conditioner’s cooling coil located at the furnace or air-handler inside the house. The cooling coil is typically cooled below the dew point of the air passing over it. As a result, as the air temperature drops, some of the moisture in the air condenses on the cooling coil. The liquid water or condensate, drips from the cooling coil and collects into a pan before flowing out a condensate drainpipe and discharging into a floor drain or outside the building. If an air-conditioning system is producing condensate, then it is removing moisture from the air, a process called dehumidification.
I often hear homeowners or building owners attributing their moisture damage or mold problems to the HVAC system because they claim that the air coming out of the supply air registers felt cold and wet. Understanding how condensation is formed provides an understanding that, when the HVAC system is operating normally, one should expect the air coming from the registers to feel cold and wet. If condensation is occurring at the cooling coil, that means that the air leaving the coiling coil is at 100% relative humidity. The air is going to warm a bit as it travels through the ductwork, but the overall moisture content of the air would stay the same. As a result, the relative humidity will drop a bit, but would still be very high at the point where the air is discharged from the supply register. Even though the air at the register may be at a low temperature and a high relative humidity, the HVAC system is still dehumidifying the air before it is discharged into the occupied space or living space. Once the cool, conditioned air mixes with the much warmer air in the occupied space, the relative humidity of the air in the occupied space would decrease a bit. As the air in the room is continually circulated through the air-conditioner, the relative humidity in the occupied space should continue to be reduced.
When Can HVAC Systems Cause Moisture Related Damage and Mold
At this point you may be asking, ‘if a properly functioning HVAC system removes moisture from the air, when might an HVAC system cause moisture related damage or mold?’ In a properly running system, most of the problems occur when the conditioned air cools building components below the dew point temperature of the surrounding air.
A common example would be when supply diffusers or registers are placed too close to walls and large amounts of cold air are directed at the surface of the wall. When the room reaches the desired temperature and the air-conditioning system shuts off at the end of its cycle, the air in the room comes in contact with the cold wall. If the wall is repeatedly cooled below the dew point of the room air, condensation will form on the wall, which may result in the appearance mold.
Another similar example occurs when 4-way blow pattern ceiling diffusers are installed in a corridor. Normally a 2-way blow pattern should be used to direct the air down the corridor. If a 4-way blow pattern is used, half of the air is directed at the corridor walls, which can lead to cooling below the dew point of the ambient air.
Ductwork is often run through unoccupied areas or unconditioned spaces such as attics or between floor joists in multi-story buildings. If the ducts are not properly insulated, the exterior surface of the ducts can be cooled below the dew point of the surrounding air. The result would be condensation forming on the surface of the duct. As the condensation builds, it may drip down onto the backside of the ceiling materials causing moisture damage and mold.
Certain high moisture areas like bathrooms, showers, or laundry facilities require careful HVAC system considerations to remove the extremely humid air that can occur. Normally, exhausting the air in these areas is required, and accommodations must be made to resupply the area with conditioned make-up air. If exhaust fans are omitted and/or an adequate supply of conditioned air is not available, the high humidity in the area may reach a level favorable for mold growth.
Finally, some commercial buildings use a system of chilled beams to cool the occupied space. A chilled beam is a device that typically resides in the ceiling of a room. Chilled water passes through the wet side of the beam’s coil, while air passes through the dry side. Chilled beams do not typically have a means of draining condensation. Therefore, it is imperative that the design engineers and building operators make sure that the coils of the chilled beams never reach the dew point of the ambient air in the rooms or water will drip from the beams. In humid climates, chilled beams located near exterior doors with heavy traffic are of particular concern. As humid outdoor air infiltrates the building and mixes with the indoor air, the dew point temperature of the air can rise, quickly approaching the temperature of the chilled beams. The result may be a light rain in the conference room or office area, which can diminish productivity and lower employee morale.
Hopefully, in reading this article you have gained a better understanding of how moisture in the air relates to relative humidity and how it reacts to changes in temperature. The effect of an HVAC system on the moisture in the air with which it interacts can improve indoor air quality or result in moisture related damage and mold. It is best to engage an expert in the field of HVAC engineering to evaluate a system’s design and function to understand any links to claims of moisture damage. The engineers at EIS stand ready to answer your questions or investigate the root cause of your moisture related damage.
Can an HVAC System Cause Moisture Related Damage or Mold?
I have been involved in numerous cases or claims where mold damage to a building or home was suspected to be caused by the heating, ventilation, and air-conditioning (HVAC) system. The initial phone calls typically include a brief description of the property owner’s account of the damage and why the HVAC system is suspected. The description of the loss is often followed by the question, ‘Can an HVAC system cause moisture related damage or mold?’ The correct answer is, maybe. This article will provide an overview of how air and moisture behave and how a structure’s HVAC system may interact.
What is Moisture Related Damage?
To start, I need to clarify what I mean by moisture related damage. Everybody understands that mold likes to grow in wet places. The distinction to be made is how the water came to provide an environment favorable to mold growth. Simply put, if the damage-causing water entered the building in liquid form, it is considered water damage. If the water entered the building as moisture in the air and later condensed into liquid water, it is moisture damage.
What is Relative Humidity?
Air always carries some amount of moisture or water vapor, typically referred to as humidity. When liquid water molecules absorb enough heat, liquid water evaporates into vapor form. Therefore, the greater the temperature of the air, the more moisture the air can hold. This gives rise to the concept of relative humidity. If the air on a certain day is at 70% relative humidity, it means that the air is holding 70% of the total amount of moisture that could be held in the air at that temperature. So, 90 degree air at 70% relative humidity contains a great deal more moisture than 40 degree air at the same 70% relative humidity. At 100% relative humidity, the air has reached the maximum amount of water vapor that it can hold at that temperature. If the temperature drops, then there is no longer enough heat to hold all the water vapor in gas form, so some of the water must turn back into a liquid. The amount of water vapor that changes back to a liquid increases as the temperature decreases.
What is the Dew Point?
The physical change of water from vapor to liquid is known as condensation. The temperature at which the water in the air condenses is known as the dew point. The dew point changes depending on the temperature and humidity of the air.
Condensation in Daily Life
Condensation manifests itself in various ways in daily life. At night, when the temperature of the air drops sufficiently, the moisture in the air loses heat and some of the water vapor in the air condenses and falls to the ground as dew. The outside of a glass containing a cold drink may begin to sweat because the temperature of the glass is below the dewpoint temperature of the surrounding air. Any air that comes in contact with the glass is cooled below the dew point and the water vapor in the air condenses on the glass.
Condensation in HVAC Systems
In the same way, condensation occurs in a common direct expansion (DX) air-conditioning system like those found in most homes. Air is conditioned as it is blown through the air-conditioner’s cooling coil located at the furnace or air-handler inside the house. The cooling coil is typically cooled below the dew point of the air passing over it. As a result, as the air temperature drops, some of the moisture in the air condenses on the cooling coil. The liquid water or condensate, drips from the cooling coil and collects into a pan before flowing out a condensate drainpipe and discharging into a floor drain or outside the building. If an air-conditioning system is producing condensate, then it is removing moisture from the air, a process called dehumidification.
I often hear homeowners or building owners attributing their moisture damage or mold problems to the HVAC system because they claim that the air coming out of the supply air registers felt cold and wet. Understanding how condensation is formed provides an understanding that, when the HVAC system is operating normally, one should expect the air coming from the registers to feel cold and wet. If condensation is occurring at the cooling coil, that means that the air leaving the coiling coil is at 100% relative humidity. The air is going to warm a bit as it travels through the ductwork, but the overall moisture content of the air would stay the same. As a result, the relative humidity will drop a bit, but would still be very high at the point where the air is discharged from the supply register. Even though the air at the register may be at a low temperature and a high relative humidity, the HVAC system is still dehumidifying the air before it is discharged into the occupied space or living space. Once the cool, conditioned air mixes with the much warmer air in the occupied space, the relative humidity of the air in the occupied space would decrease a bit. As the air in the room is continually circulated through the air-conditioner, the relative humidity in the occupied space should continue to be reduced.
When Can HVAC Systems Cause Moisture Related Damage and Mold
At this point you may be asking, ‘if a properly functioning HVAC system removes moisture from the air, when might an HVAC system cause moisture related damage or mold?’ In a properly running system, most of the problems occur when the conditioned air cools building components below the dew point temperature of the surrounding air.
A common example would be when supply diffusers or registers are placed too close to walls and large amounts of cold air are directed at the surface of the wall. When the room reaches the desired temperature and the air-conditioning system shuts off at the end of its cycle, the air in the room comes in contact with the cold wall. If the wall is repeatedly cooled below the dew point of the room air, condensation will form on the wall, which may result in the appearance mold.
Another similar example occurs when 4-way blow pattern ceiling diffusers are installed in a corridor. Normally a 2-way blow pattern should be used to direct the air down the corridor. If a 4-way blow pattern is used, half of the air is directed at the corridor walls, which can lead to cooling below the dew point of the ambient air.
Ductwork is often run through unoccupied areas or unconditioned spaces such as attics or between floor joists in multi-story buildings. If the ducts are not properly insulated, the exterior surface of the ducts can be cooled below the dew point of the surrounding air. The result would be condensation forming on the surface of the duct. As the condensation builds, it may drip down onto the backside of the ceiling materials causing moisture damage and mold.
Certain high moisture areas like bathrooms, showers, or laundry facilities require careful HVAC system considerations to remove the extremely humid air that can occur. Normally, exhausting the air in these areas is required, and accommodations must be made to resupply the area with conditioned make-up air. If exhaust fans are omitted and/or an adequate supply of conditioned air is not available, the high humidity in the area may reach a level favorable for mold growth.
Finally, some commercial buildings use a system of chilled beams to cool the occupied space. A chilled beam is a device that typically resides in the ceiling of a room. Chilled water passes through the wet side of the beam’s coil, while air passes through the dry side. Chilled beams do not typically have a means of draining condensation. Therefore, it is imperative that the design engineers and building operators make sure that the coils of the chilled beams never reach the dew point of the ambient air in the rooms or water will drip from the beams. In humid climates, chilled beams located near exterior doors with heavy traffic are of particular concern. As humid outdoor air infiltrates the building and mixes with the indoor air, the dew point temperature of the air can rise, quickly approaching the temperature of the chilled beams. The result may be a light rain in the conference room or office area, which can diminish productivity and lower employee morale.
Hopefully, in reading this article you have gained a better understanding of how moisture in the air relates to relative humidity and how it reacts to changes in temperature. The effect of an HVAC system on the moisture in the air with which it interacts can improve indoor air quality or result in moisture related damage and mold. It is best to engage an expert in the field of HVAC engineering to evaluate a system’s design and function to understand any links to claims of moisture damage. The engineers at EIS stand ready to answer your questions or investigate the root cause of your moisture related damage.