Resilient Buildings: HVAC Design Considerations
A lot of discussion in the design and construction industry recently centers around the "new normal" and how we will adapt to a post COVID world. As many businesses open this month and we return to work and start returning to public spaces, the industry is examining some considerations for building owners that can be implemented now and considered for future building updates and design. Our design approach has always considered occupant health and safety as the primary goal. Now more than ever owners and operators will consider how they can ensure a new or renovated building will utilize the appropriate technology to meet those goals. The list below shows some of the methodology used to keep the air in the building clean and safe.
Also referred to as solution by dilution, is the first way we can reduce contaminants in the air in any building type. The amount of fresh air introduced into a building through the HVAC system is governed by ASHRAE 62.1-2010 Ventilation for Indoor Air Quality. The current standard provides tables to determine the amount of fresh air for different room types using room square foot and occupant loading. Not all rooms are clearly listed in the table and some engineering judgement is required to make an estimate of the actual air quantity. This standard is currently required by Ohio building code, although ASHRAE has more recently published guidelines that are used in other states and by those prioritizing ventilation above current standards. Ultimately, engineers must help owners gauge what is best for each building as too much fresh air results in costly temperature control and too little does not provide safe breathing air.
Fresh air quantities can be very significant and the HVAC equipment does not perform well when the HVAC system supply air requires 50% or greater fresh air. This is where an alternative to the solution by dilution can work by filtering the air with charcoal filters. Charcoal filters are commonly used all around us in coffee machines, aquariums, pitcher water filters and even whiskey making. They are used frequently in HVAC systems as well. With a wide range of impurities that can be filleted using charcoal filters including smoke fumes and odors it is easy to see how this application can be adapted for buildings. By reducing air impurities, charcoal filters reduce the amount of fresh necessary and provides the safe environment while saving energy. Another energy saving alternative is to use energy recovery transferring exhaust air energy over to the intake air and thereby keeping for high outside air flow rates.
Finally, in an era where viruses and bacteria are a primary concern, alternate filter techniques can reduce exposure through the HVAC air system. HEPA filters or High Efficiency Particulate Air filters can capture particles down to 0.3 microns in size at capture at rates of 99.79% effective. This is better than the KN95/N95 masks which only are effective to 95% at 0.3 micron size.
Another technique for cleaning conainiments for air is with UVGI (Ultra Violet Germicidal Radiation) This technique uses short wave ultra violet light to kill microorganisms by disrupting their DNA and rendering the organism unable to perform cellular function. This technique can be used in HVAC systems and is used in many water and food purification processes. For more on UVGI read our upcoming post post on GUV effectiveness.
Capture Contaminants at the Source
The technique of capturing a contaminant at the source of generation to control indoor pollution is common sense. Why would you want a contamination circulated through-out the building or even in the room? At times though, this can be difficult to implement. Ventilation hoods of varying types capture contamination at the source and the right hood will capture the highest percent of contamination. Some of the familiar ones include kitchen hoods and lab hoods. Bench top nozzle type hoods can also provide localized source capture with extender arms and flexibility for location of contamination capture. Increasing the use of hoods or ensuring the proper hood is in use and maintained can result in safer conditions for many workers.
Room pressurization techniques can used to control the direction of airflow within the building from one room to another. This can be used to control contamination keeping workers safe. The airflow in one room may be kept under a negative pressure by removing more air than is introduced to the room and an adjacent space is under positive pressure by introducing more air than is removed. The air will move from the positive pressure room to the negative pressure room controlling any contamination from proliferating into the building. Hospitals use this technique to control room contamination within the building and the standard ASHRAE 170 assigns each room in a hospital a positive or negative or neutral pressure relationship to assist with how the airflow is controlled. tis method may be considered in other healthcare spaces in the future as we consider the flexibility of future buildings for adaptation to pandemic scenarios.
It does take not much water for mold and mildew or sometimes worse to accumulate. Water can destroy a building from the inside out. Some design considerations to control the water helps maintain the longevity of the building. One of these is controlling the dewpoint within walls. If the dew point is not controlled, condensation can occur within a wall or on glass or even on the bottom of a roof deck. The building relative humidity should be held at levels that will not cause interior condensation. Occupied buildings have a window range that is represented in ASHRAE 55 on the psychometric chart for both temperature and humidity. While used for occupant safety and comfort, other moisture maintenance issues should not be ignore. This includes leaky pipes, cooling coil condensation pans and leaky roofs.
John Milenius, PE, LEED AP
Recognized as a pragmatic and skilled expert in HVAC engineering and plumbing design, John focuses on making the most of project budgets and schedules for our clients. A skilled troubleshooter and proponent of increasing energy efficiency, John manages and leads or mechanical engineering staff at Tec. In addition, he is always looking to help future AEC professionals along the way.
Contact John for HVAC expertise.