Reducing Mold Growth in HVAC Systems

Mold growth in buildings and HVAC systems has been implicated as a contributor to poor Indoor Air Quality (IAQ) and the so-called Sick Building Syndrome. While HVAC components themselves are not the cause of mold proliferation in buildings, the very nature of HVAC system design and their typical operating limits may provide an acceptable niche where mold growth can occur. Once unchecked mold growth occurs, the HVAC system acts as a perfect vehicle to spread mold spores in the building through the air distribution system. Once this occurs, costs associated with mold remediation have been known to be quite exorbitant. To control mold growth and keep it from becoming problematic, one must understand the basic premise behind its growth and control.

Mold

Mold and mildew are a type of fungi, which can cause respiratory irritation and allergic reactions in those susceptible. Mold proliferates through the production of spores. Spores are capable of surviving very harsh environments and may lie dormant for many years until the right conditions exist for mold growth. Fungal spores range in size from 1 micron to slightly less than 100 microns. Studies have shown that with adequate filtration a majority of particles in this size range can be captured.

Many thousands of species of fungi exist, but only a small number of fungal species are generally implicated in IAQ problems. Some of those of concern are Aspergillus spp., Penicillium spp., and Stachybotrys spp.

Various studies have shown that when an indoor fungal bloom exists, the indoor spore concentrations can easily exceed those found in outdoor environments. When mold problems become prevalent, certain chemical compounds can be produced by the mold, resulting in the musty odors and poor air quality commonly associated with mold growth.

Design for Mold Control

Nutrient Control
To reduce the nutrient levels within the unit, fibrous glass insulation should be eliminated and filters carefully selected. Interior surfaces of the unit should be smooth, durable, and washable.

Insulation
Closed cell foam insulation should be used exclusively in the HVAC equipment. This material has a low water vapor permeability and will not support moisture absorption. By design, the closed cell structure of the insulation acts as its own vapor barrier. The smooth integral skin is a tough impermeable surface, meeting the requirements for ASTM G-21 for mold growth resistance. Studies have shown that fungal levels on the exterior surfaces of closed cell foam insulation are many thousands of times less than those found on fibrous glass insulation.

Filtration
Filters should be selected with construction consisting of synthetic materials rather than cardboard, cotton, or other organic elements; thereby eliminating another potential food source for the fungi. Additionally, by using filters that are comprised entirely of synthetic media and frames, there are no moisture absorbing materials that can support mold growth.

Moisture Control
Exterior moisture must be prevented from entering the building by using properly located vapor barriers, treating makeup air through dehumidification devices and providing positive air pressure to prevent infiltration of humid exterior air. Additionally, water vapor present in the interior air must not be allowed to condense on cold interior surfaces, and plumbing systems must be designed, installed and maintained to prevent liquid water from wetting building materials and furnishings. It is recommended that units operate in a conditioned space to prevent condensation on the unit casing exterior.

All condensation must be controlled so that it freely drains from the unit. Standing water in condensate pans is one of the largest contributors to mold proliferation within HVAC systems.

IAQ Drain Pan
To eliminate the potential for standing water, the condensate drain pan should:

1. Be sloped toward the drain connection to allow for positive drainage
   
2. Be fabricated from stainless steel for corrosion protection and cleanliness.
   
3. Be externally lined with closed cell insulation to prevent surface condensation.
   
4. Have a drain connection located on the bottom of the pan to facilitate drainage
   
5. Have a large diameter P-trap, removable for cleaning and service.

Coil Design
Large coil face areas result in low coil face velocities thereby eliminating the potential for condensation blow-off further increasing the effectiveness of moisture control. Coils designed with adequate dehumidification capacity to remove water vapor from the conditioned space are also recommended.

Dehumidification Control
Indoor relative humidity should be maintained below 60% to minimize the potential for fungal growth. The unit design must allow for dehumidification during part load conditions. Typical features used to accomplish this include:

1. Variable Speed Fan Control
   The controller (thermostat / humidistat) automatically adjusts fan speed based on cooling capacity and or humidity setpoint. The fan speed adjustment reduces airflow through the cooling coil lowering the supply air temperature at a given load thus removing more moisture from the return air. Fan speed control based on dehumidification setpoint, allows the unit to operate longer thus reducing the humidity swings normally encountered when strictly using sensible temperature control schemes.
2. Bypass Damper Control
   While in bypass operation, the coil leaving air temperature and humidity ratio are lowered, better drying the air. The mixed air temperature is above the room dewpoint, preventing condensation from forming on supply grilles. Reduced coil sensible heat capacity reduces valve cycling and keeps the coil active longer at part load.

Cleaning and Maintenance
Proper maintenance is essential to maintain the effectiveness of mold control features designed into HVAC equipment. Filters must be replaced on a monthly or bi-monthly schedule. Cooling coils should be inspected quarterly. The condensate drain pan and P-trap must be inspected quarterly for signs of debris and drain line blockage. Cabinets should be inspected annually for signs of water and air leaks, insulation degradation, and gasket damage. Repairs must be made immediately when necessary.

For more information, refer to the Reducing Mold Growth in HVAC Equipment and Closed Cell Foam Insulation white papers. Also see the ENVIRO-TEC Model VH Vertical Hi-Rise Fan Coil Catalog (Model VH is available with many anti-mold features).

Bibliography

1. ANSI/ASHRAE Standard 62.1-2001 - Ventilation for Acceptable Indoor Air Quality
   
2. Harriman, Lew, Brundrett, Geoff, Kittler, Reinhold. - Humidity Control Design Guide for Commercial and Institutional Buildings. ASHRAE 2001
   
3. Department of Architectural Engineering. Pennsylvania State University. Aerobiological Engineering, Fungi and Bacteria in Ventilation Systems. State College, PA.
   
4. Department of Biology. Pennsylvania State University. Aerobiological Engineering, Airborne Pathogen Database, Fungi. State College, PA.
   
5. Bahnfleth, William, Kowalski, W. "Airborne Respiratory Diseases and Mechanical Systems for Control of Microbes." HPAC Engineering. July 1998: 34 - 48
   
6. Lstiburek, Joseph. "Moisture Control for Buildings." ASHRAE Journal, February 2002: 36 -41
   
7. Schoen, Lawrence J. Controlling Moisture in Building Mechanical Systems. ASHRAE IAQ 2001
   
8. Thomann, W., Tulis, J. Fungal Contamination of HVAC Surfaces: The Role of the Dew Point in Microbial Amplification. Proceedings of the 1996 ASHRAE IAQ Conference, Baltimore, MD October 6-8 1996. ASHRAE Atlanta, GA ISBN 1-883413-41-9.
   
9. Burge, H. "Fungi: How they grow and their effects on human health." HPAC 69(6), June 1997
   
10. Ellringer, Paul, J., S.C. Hendrickson, Chin S. Yang, and Katy Boone. Fungal Levels on Interior Surfaces of Ventilation Ductwork: Closed Cell Foam Insulation Versus Fibrous Glass Insulation and Galvanized Metal. ASHRAE IAQ 2001
    
11. Buttner, Mark, P., and Linda D. Stetzenbach. Fungal Spores Dispersed from Fiberglass Ductboard, Fiberglass Liner, and Galvanized Metal Air Handling System Duct Material. ASHRAE IAQ 2001
    
12. Cheong, C. D., H. G. Neumeister-Kemp, Peter W. Dingle, G.E. St. J. Hardy. The Use of HEPA Air Filters to Control Airborne Indoor Fungi. ASHRAE IAQ 2001.

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