Many experts believe that HVAC systems, particularly those in most non-medical buildings, are typically a small factor when it comes to transmission of infectious diseases. However, this hypothesis is simply based on what appears to be reasonable assumptions based on available data, which has not yet been validated. Their main intent appears to be to reduce disease transmission from airborne particles by increasing outdoor air ventilation and filtration efficiency. A facility located in Boston, for example, should not be designed the same if it were located in Miami.
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Many experts believe that HVAC systems, particularly those in most non-medical buildings, are typically a small factor when it comes to transmission of infectious diseases.
However, this hypothesis is simply based on what appears to be reasonable assumptions based on available data, which has not yet been validated. Their main intent appears to be to reduce disease transmission from airborne particles by increasing outdoor air ventilation and filtration efficiency.
A facility located in Boston, for example, should not be designed the same if it were located in Miami. Weather — and, more importantly, moisture — in outdoor airflow is drastically different, particularly in warmer southern climates. Therefore, what may work for one location will not work the same for another. This may include some or all of the following activities: Increase ventilation rates.
Increase the percentage of outdoor air that circulates into the system. Changes to building operations, including the operation of heating, ventilating, and air-conditioning systems, can reduce airborne exposures. Ventilation and filtration provided by heating, ventilating, and air-conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air.
Unconditioned spaces can cause thermal stress to people that may be directly life threatening and that may also lower resistance to infection. In general, disabling of heating, ventilating, and air-conditioning systems is not a recommended measure to reduce the transmission of the virus.
Increasing Outdoor Air Ventilation While studies have shown that increasing ventilation within a building can be an effective way of reducing the potential for airborne transmission, simply doing so may have some unintended consequences.
Depending on the climate, increasing ventilation into a building — either through mechanical means or simply opening a window — increases moisture infiltration into the building as well.
The effects of moisture within a building depends on a variety of factors, which include but not limited to, current HVAC design and operation, exterior climate conditions, construction type, and the types of building materials used.
Before one can fully understand the effects of moisture, one needs to understand that moisture will still infiltrate into a building, even if an HVAC system is off. Vapor Pressure and Relative Humidity Vapor pressure is essentially the portion of atmospheric air pressure attributable to water vapor. In short, air is primarily made up of nitrogen and oxygen; however, several other small amounts of gases usually less than 1 percent also exist.
In addition, the air molecules also include a portion of water vapor, the amount of which depends on the atmospheric conditions i. As air temperature increases, so does its ability to hold more moisture.
The specific amount of moisture that air can hold at a particular environmental condition is typically referred to as specific humidity. Relative humidity is the amount of moisture air can hold compared to the maximum amount of moisture the air can hold at a specific temperature. Just as the ability to hold more moisture when air temperature is increased, if you decrease air temperature, the ability of the air at the cooler temperatures to hold moisture decreases also; however, the relative humidity actually goes up, while the specific humidity remains the same.
When relative humidity reaches percent, the air is considered saturated and condensation will form. Dew point temperature is the temperature at which water vapor condenses in the air. In addition, warmer air also has a higher vapor pressure than cooler air. The higher pressure of warmer air tends to move toward areas that have lower vapor pressures. This is why one typically insulates buildings in south Florida — to keep out the heat and moisture from the exterior and the indoor conditioned air within the interior spaces.
Insulation and air conditioning will never stop the occurrence of vapor pressure and its tendency to want to infiltrate from the exterior, as this occurs naturally in nature. The diffusion process depends on many factors. Some of these include the construction of the building envelope, insulation properties, and quality of the construction itself. In addition, the exterior environmental conditions and interior space conditions play an important role as well.
As previously indicated, the higher the temperature and humidity, the higher the vapor pressure is at those conditions. The vapor pressure differential between the exterior and interior environments affects the rate and ability for warmer air to infiltrate into cooler spaces. The higher the pressure differential, the easier it is for warmer air — and subsequent higher moisture quantities — to infiltrate a space. It is possible to elevate the vapor pressure differential by increasing the outdoor environmental conditions and maintaining a stable indoor environment.
However, the opposite is also true. The vapor pressure differential may also be increased by decreasing the interior space temperature conditions while maintaining a stable exterior environment.
Many people do not typically think about the latter; however, many indoor environmental problems occur for spaces that are potentially too cool, just as they do when they are too warm. This correlates to a vapor pressure differential of approximately 0. See Figures 1 and 2 below. It is important to realize that if one were to increase ventilation within a space, it creates the potential for increasing relative humidity. Therefore, the operation of HVAC systems should be controlled to achieve levels below the recommended dew point temperature at all times, regardless of COVID or other potential similar threats, to prevent the potential for other adverse effects relating to moisture and condensation to occur.
Most occupants unfamiliar with air properties would tend to lower the space temperatures via a thermostat or have their air conditioning systems run longer to remove the excess moisture.
However, this sometimes can create an adverse effect, as it may end up pulling more moisture into an interior space due to diffusion. This may lead to the same or additional problems with regard to maintaining proper interior temperature and humidity levels and controlling interior moisture levels.
Consider Your Environment and Building Systems In summary, moisture will get into buildings at some point, whether you want it to or not. Before running around the building and opening every outdoor damper percent, consider the location of your building.
Some HVAC systems are better than others with regards to controlling moisture. However, each system within a building was likely designed to handle a specific quantity of outdoor air ventilation per ASHRAE guidelines.
Increasing the outdoor air ventilation for a specific air handling unit AHU system increases the mixed air temperature being delivered to the cooling coil, which in turn increases the supply air temperature leaving the AHU that is delivered into the space — along with any excess moisture the AHU system was not able to remove.
Specific AHU systems may have some ability to be adjusted in order to handle some excess outdoor air, but this should be evaluated by a licensed mechanical engineer or vendor familiar with such systems before any changes to the systems are made.
In addition, should additional outdoor air ventilation be introduced into a building and controlled properly by the existing HVAC system, this excess outdoor air ventilation will need to go someplace. Therefore, the capacity of the exhaust system to handle this additional ventilation should be evaluated as well.
It is important to remember that excess moisture from a warm environment into a cooler environment also brings the added potential for condensation to form. This is particularly apparent around areas where, for example, duct leakage may be occurring. Therefore, it is imperative to verify that insulation around ductwork, air devices, piping, and air handling equipment is properly installed.
Any added moisture from additional outdoor air that is not properly controlled can potentially make these conditions worse. However, it is understood that existing building configurations may not allow for such a system to be constructed cost-effectively.
Outdoor air ventilation directly into a plenum space — above a ceiling, for example — should be avoided for many of the reasons noted above. This way, the outdoor air ventilation and its associated moisture will have a better chance to be conditioned before it enters the occupied spaces.
However, careful consideration needs to be made when selecting the types of filters used for existing AHU systems. A plenum system is a space within a building utilized for air circulation for HVAC systems. Typically plenum spaces are located above ceiling spaces, below floors or within mechanical rooms and are used as mixing chambers typically for return, outdoor air, or both. Photo of microbial growth along interior wall of a closet primarily due to vapor pressure diffusion from attic into plenum space above.
Photo of outdoor air duct stubbed into apartment AHU plenum closet. Not ideal but would be better to duct down to return air opening of AHU and provide volume damper for control. In general, filters with higher MERV ratings have smaller openings in them, which allow the higher MERV filters to capture more and smaller airborne particles.
However, this can restrict the airflow passing through them. Increasing filter efficiency means increased resistance or static pressure within a system, which relates to a drop in airflow velocity across the filter itself.
This can lead to an overall drop in total airflow being delivered to a space. In addition, if the velocity — particularly across a cooling coil — is too low, this can lead to coils freezing up. However, how the airflow behaves depends upon the type of HVAC system and — specifically — the fan serving it. In order to counteract this effect, the system must have a way to make up the difference of the greater pressure drop caused by the increased filter efficiency.
In some larger AHU systems, simply increasing the speed of the fan or the even the horsepower hp of the fan motor may make up for the added pressure drop within the system. However, for smaller AHU systems, such as those found in most residential, multi-family, and light commercial buildings, the installed AHU systems likely do not have these capabilities due to the size and hp limitations of the supply fans themselves. All filters have specific pressure drops associated with them, which varies depending upon the type of the filter itself as well as the quantity of airflow across it.
Pressure drops also increase as the filters become dirty. Installing a higher efficiency filter may work fine initially while the filter is clean but may not when the filter gets dirty, as the AHU fan may then be outside its performance range, resulting in a decrease in total airflow.
Verify the AHU performance including ductwork layout to determine if such filters are suitable for its intended application. Therefore, as noted above, it is important to have each system evaluated by a licensed mechanical engineer or vendor familiar with such systems before any changes to the systems are made.
This may include verifying the AHU system performance by a licensed test and balance or mechanical contractor to assure the system is performing in accordance with its intended design standards — and also to verify what, if any, excess capacity the respective systems have before making temporary or permanent changes to their current operation.
In addition to selecting the proper filter and respective filter efficiency, careful consideration and protocols should be employed to removing and disposing of used filters.
Consulting with a licensed engineering firm for the development of such protocols and standards can be an option for those facility managers wishing to improve their standard operating procedures.
Overcooling the space can bring detrimental effects to a building for many of the reasons noted above. Therefore, controlling the space temperatures by adjusting control settings — namely, night setback settings — may assist with maintaining a proper indoor environment.
Controlling energy usage, and the costs associated with it, has always been of importance to any building owner or facility manager. This may include evaluating whether the additional costs to implement such practices in the short term make economical sense versus shutting down the respective building s altogether, assuming that is an option. Unfortunately, many essential businesses do not have the luxury of the latter and need cost-effective ways to implement these additional safety measures without being detrimental to both their occupants and their business continuity plan.
However, like any new system, the incorporation of these type of systems should be evaluated prior to installation to determine how and if operating these new systems will negatively affect the performance of the existing HVAC systems currently serving the respective buildings. Conclusions The reality is that there is always a way to do something better. There are simply many factors related to HVAC performance that are not commonly thought of when trying to implement general recommendations, including building location, construction type, building envelope, and how well the building was constructed in the first place.
It is much more valuable to spend some time analyzing the proposed changes before spending the extra time and money to implement them and finding that the work performed to initially achieve a betterment in the short term may be detrimental in the long term to the facility. More effective filters with reasonable pressure drop are available, and some increased pressure drop often results in an acceptably small effect on system performance.
Either of these takes time to plan and install. This article was originally posted on www. Related Articles.
Searchsearch button Psychrometric humidity chart Locating air properites on a psychrometric chart. To understand how all that works, we need quantities and we need them to be well-defined. Having previously taught psychrometrics to architects, I always found it challenging to convincingly explain why the psychrometric chart used humidity ratio or absolute humidity in the vertical axis instead of the more familiar metric of relative humidity, and why it was so important to understand the various metric lines. We shall now see how the air behaves when it is subjected to changes in temperature and humidity to suit the various applications for which the air conditioning is meant. Psychrometric Chart Pdf vfix
COVID-19: Discussing the CDC and ASHRAE Recommendations for HVAC Systems
Easily plot HVAC and other psychrometric processes while out in the field, save the graphs, and then email the graph and results to yourself or clients. As you move your finger around the graph, the psychrometric properties at the top of the screen dynamically update. In addition, you can double-tap a point to display the point properties and then edit them. Create psychrometric processes by plotting points then connecting them with process lines. Also, create custom labels anywhere on the chart.