- THE MAGAZINE
This is especially true for those that perform mold remediation in hot environments, as the combination of PPE and isolation of the work area from the HVAC system, for containment purposes, can easily create stressful and even dangerous thermal conditions.
Last month's column concentrated on personal protective equipment (PPE) that can help protect against heat stress, with coolant vests perhaps being the most efficient and practical type of PPE for this purpose. Also addressed were some of the other engineering controls and work practices that can help reduce heat stress.
This month's column specifically addresses methods by which sensible heat (dry-bulb temperature) in the work area can be reduced, thereby allowing the worker to shed body heat more efficiently. There are two basic ways of reducing temperature in the work area. One uses the HVAC system to control temperature; the other isolates the work area from the HVAC system and uses auxiliary cooling and/or dehumidifying equipment to control conditions.
Controlling Temperature in the Work Area with
the HVAC System
When performing remediation, the supply and return vents in the work area are normally sealed to prevent cross-contamination and to allow sufficient control of infiltration so that negative pressure can be maintained. When the HVAC system is shut down, possible humidity and/or heat problems can be created not only in the work area but in other parts of the structure, including areas that have not been affected by the mold contamination.
So, assuming that the system has not become contaminated, is it possible to keep the HVAC system operating to control conditions in the rest of the building?
The answer is that it is possible to keep an uncontaminated system operating without spreading contaminants or causing other problems, if you know what you are doing. Careful consideration must be given to the fact that when you seal off the supply or return vents in the work area, you are likely to create airflow restrictions that may damage the system.
In order to keep the HVAC system in operation, modifications may be necessary. Any modifications to the system must be designed and installed by qualified HVAC contractors or engineers. Depending upon which state you are working, you may be required to have a mechanical contractor's license and a permit. If you are able to keep the HVAC system in operation in the unaffected part of the building, it may be possible to pull makeup air from the conditioned portion of the structure, thereby maintaining comfortable working conditions, and still vent to the exterior, especially if the work area makes up a relatively small portion of the building. Venting to the exterior is always safer as it eliminates the chance of cross-contamination if the air filtration device is not filtering properly.
When you exhaust to the exterior you may depressurize the entire structure. This can result in so much exterior air being pulled into the building that the HVAC system is unable to maintain appropriate conditions. Especially if the exterior air is hot and/or humid, the system may be unable to keep up. In these circumstances, it may be possible to recycle the conditioned air from the work area by exhausting it back into the building rather than to the exterior. This approach minimizes the depressurization of the structure.
Obviously, before using this approach, you must be sure that the HEPA air filtration device (AFD) you are using is operating efficiently, or you risk massive cross-contamination of the "clean" parts of the building. There are several ways you can be sure that you are not cross contaminating:
Using Auxiliary Cooling Equipment
There are many situations where controlling conditions in the work area with the HVAC system is not a realistic option. In these cases, if you wish to control heat stress with engineering controls, you must bring in auxiliary equipment.
For a small-sized work area, this can be as simple as temporarily installing a window air conditioner in the room. Even in somewhat larger areas, two or more of these units can be installed. Keep in mind that the negative pressures that must be maintained may sometimes interfere with the efficient operation of these systems.
More efficient 115- or 230-volt portable split systems are available which can be installed with the exterior compressor section connected through a window to the interior evaporator section via flexible insulated hoses.
Any parts of an AC unit that have been exposed to contaminants must be thoroughly decontaminated at the end of the project. Split systems are likely to be significantly easier to decontaminate, as the components of the system located outside do not become contaminated.
Temperature and humidity conditions on larger projects can often be controlled by bringing in larger temporary air conditioning equipment that is widely available from rental companies specializing in temporary HVAC equipment.
The simplest way to install the equipment is to create negative pressure in the work area and allow the make up air to come from the AC unit. This approach will work when the exterior conditions are not excessively hot or humid. Most AC units are designed to create a temperature drop of 14 to 18 degrees F across the coil. If exterior conditions are humid, the system may not be able to generate that much of a temperature drop. The system's capabilities may be used up removing humidity (latent heat) from the air, leaving less cooling capacity available to reduce temperature (sensible heat). When using this approach in humid conditions with exterior temperatures as low as 80 to 85 degrees F, the makeup air entering the work area after passing through the AC unit may still be warm and humid, contributing to worker heat stress.
Seek expert advice before renting or buying portable or temporary air conditioning equipment. Some of the readily available equipment may stop functioning when the temperature at the intake reaches about 95 degrees F, while others may continue to be effective up to an intake temperature of 115 degrees F or even higher.
Systems are available for temporary installations that are capable of generating a greater temperature drop and/or removing more humidity on a single pass through the system. However, these systems are likely to cost significantly more to rent or purchase than "standard" systems. Another cost-effective approach involves setting up a standard AC system so that most of the work area air recycles through the system repeatedly, allowing it to pull down the temperature and humidity gradually, similar to the way that the structure's HVAC system generally operates. It is possible to do this in a way that the AC system is not exposed to contaminants.
Multi-ton auxiliary AC units are often sited outside the structure. The air from the structure is filtered through an AFD to ensure that it is clean. Then the air is ducted out a window, into the AC unit to be cooled and then back into the structure. If a portion of this air is vented to the exterior after exiting the structure, but before entering the AC unit, it is possible to maintain adequate negative pressure in the work area and at the same time maintain comfortable working conditions. Chart 1 illustrates one way in which this goal can be achieved.
Similar approaches can be taken to installing the evaporator unit of a split system inside a mini-containment (inside the work area) that is then positively pressurized with filtered air. As long as pressure relationships are appropriately maintained, the evaporator unit will remain clean. (Note: Any auxiliary AC equipment used must be properly sized and its installation properly designed and implemented for it to function safely and effectively. If you are not expert in this area, make sure you get expert assistance.)
Heat stress is a critical issue for all restorers, and especially for remediators. Careful attention to a combination of personal protective equipment and work practice, in addition to administrative and engineering controls, will allow remediators to work safely, even under extreme conditions.