Questions - A P.E.'s Technical Forum for Recovery Readiness
Week of June 15, 2020
Are you recommending to run HVAC systems 24/7 or just 2 hrs prior to occupancy and 2 hrs after? With a larger scale client who has air monitoring stations, they should run the OA based on the particulate size that is getting through. During the unoccupied times, the driving control would be the AMS vs. temp or CO2.
Have there been any studies that prove that particle counts in spaces will be reduced by using 6 A/C vs 2 A/C? I don’t have the specific study at my fingertips, but I believe there was one that indicated there was no appreciable reduction in particle count by increasing from 2 A/C to 6 A/C. Please refer to the Quanta graph in the Re-opening Schools Presentation
Week of June 8, 2020
Outside Air: Increasing outside air to increase dilution ventilation has been proposed. This seems like a good idea, as long as the space temperature and humidity are still within acceptable ranges (such as 68-76 deg F dry bulb and 40-60% RH). Some guidance would be appreciated with respect to how much outside air should be provided.
Is Code required values at maximum occupancy the criteria, which suggests disabling demand controlled ventilation is adequate?- Yes it is suggested to disable DCV in “ Pandemic Mode” and introducing the maximum possible OA. Because most of the buildings will have reduced occupancy, you will need to resent the CO2 ppm to ensure the OA is brought in.
Is more than Code OA suggested? If so how much more – a certain percentage above Code? Minimum air changes per hour of OA? Different criteria based on risk and occupant density? There is no “ quantifiable “ value , for now apply no more than OA code /standard( such as ASHRAE 62.1), but if additional OA can be delivered without compromising thermal conditions in spaces , it should be considered. Again, because the occupancy is reduced in our area, run the OA as if it is at normal occupancy.
Different system types have drastically different abilities to increase OA, so without guidance buildings may be providing vastly different OA values. For instance a lab building may be capable of providing 8 or more outside air changes per hour. labs are typically 100 % OA and driven by HVAC Guidelines for laboratories (for example BSL-2 as shown in 2019 ASHRAE Handbook page 17.17 shows 100% OA , 6 to 15 ACH) and in many cases the OA flow is driven by fume hoods which can be greater, these large quantities of OA are significantly larger than commercial applications where the OA ventilation is driven by the occupants IAQ. , a higher education building with an all-air VAV system and airside economizer may be able to provide 6 or more outside air changes per hour - Yes, this is correct, it is recommended to apply economizers as much as possible , but without compromising space thermal conditions Temp and humidity, proper operation of the economizer systems like controls, actuators and dampers should be verified , and an office building with a dedicated outside air ventilation system may only be able to provide 2 outside air changes per hour. Yes, this is correct , DOAS systems should remain and operate as designed ( in terms of air flow and operating conditions),if the DOAS unit can deliver higher flow than the design OA flow and the delivery system ( ductwork , diffusers and grills can distribute this extra OA it should be considered. it should be noted that DCV should be disabled to allow the DOAS supply the maximum air for ventilation regardless of the number occupants. CO2 setpoints can be modified to “ force” the DOAS to introduce the maximum possible OA to each space.
Should existing ASHRAE guidelines be consulted and applied to more common spaces, such as Standard 170 that suggests minimum 4 air changes per hour total air (outside and recirculated air), with half of that or 2 air changes per hour of outside air for patient rooms? Perhaps Standard 62.1 ventilation criteria for more critical space categories could help inform guidance, such as daycare sickroom, science laboratories, etc.?. Yes, as indicated previously, the design OA flow should remain as designed ( to avoid thermal conditions compromise), it suggested to check and inspect the systems to assure proper amounts of OA per the applicable code or ASHRAE 62.1.In cases where it is possible to increase the OA , again without compromise , OA increase can be considered. What we are doing with our clients is to gage there level of risk at each space. So with nurse’s suites in a k-12 school, we are pushing for ASHRAE 170 to govern. This is not the case for the classroom.
Outside Air vs. Humidity: Many buildings do not have humidification and it is also frequently difficult to retrofit within existing air handling units. If building owners increase ventilation during colder weather without humidification, will the resultant lower indoor relative humidity be more detrimental than the benefit from increased outside air? It will be difficult to quantify the tradeoff of low humidity ( below 40 % RH) Vs increased ventilation in terms of COVID -19. Due to the extreme costs associated with introducing humidifiers at the central air handling stations, we are putting in portable smaller terminal units with manual water fills.
Should the increased outside air guidance be qualified that OA should not be increased beyond Code if it results in space humidity outside the recommended rage of 40-60% RH? Yes Maintaining humidity in the desired range appears to be one of the few HVAC strategies that can have a drastic impact on the spread of disease (40-60% RH reportedly reduces pathogen transmission and infectivity, and increases the effectiveness of the human immune system).
3. Enthalpy Wheels: The ASHRAE information states to disable and bypass enthalpy wheels in one area, and says to get the wheels evaluated for proper operation before using in another area. We believe the latter statement is the prudent approach: Yes, the latter approach of continue to run the enthalpy wheel after careful evaluation and approval ( and modifications if required) is the current suggested approach. This recommendation is per ASHRAE TC 5.5 ( Energy Recovery ETF , still waits for ASHRAE ETF approval). If there is a possibility of cross contamination than stop the wheels. We also recommend a pandemic mode where if there Is an infected occupant, wheels stop.
ASHRAE Standard 90.1 requires enthalpy wheels for many systems with outside air.
Many systems are not designed with capacity to deliver required conditions without wheel operation.
There are criteria in ASHRAE Standards we can consult to provide guidance for wheel operation, such as 62.1. The 62.1 air classifications (Classes 1-4) are useful – there are a few class 4 sources listed, such as fume hoods, kitchen grease hoods, and paint spray booths. Class 3 areas include daycare sick rooms, janitor closets, trash rooms, recycling, and animal facilities. Toilet areas are Class 2. Most of the common occupied spaces ae Class 1.
Standard 62.1 also permits Class 3 air to be reclassified as Class 1 if no more than 5% of the Class 3 air is in the airstream when using energy recovery (Exception to 5.18.3.3.1). If the wheel is inspected for proper operation and has no more than 5% leakage, why should the wheel not be used? This is especially important when a system is not designed for the wheel being off (resulting in hot humid indoor conditions in the summer and cold dry indoor conditions in the winter), or outside air quantities are reduced perhaps below Code levels to prevent system damage. Since the wheel transfers latent energy, if a systems does not have humidification wheel operation will result in higher winter indoor relative humidity.
Wheel Construction: To minimize cross-contamination, there are key features that should be present, including purge air, seals, and wheel media. Each wheel should have a purge section that transfers air from supply to exhaust and blows contaminants off the wheel surface just before it moves into the supply air – note for this to work the supply pressure must be higher than the exhaust pressure. The pressure differential can be monitored for safety on the BAS and locations of fans help assure proper operation. Seals at the supply-exhaust border help minimize cross-contamination. Finally the media itself plays a role – if the media is smaller than the virus/bacteria/gas, then it will not pass through the media. For instance the COVID-19 particles have been stated to be 0.1 micrometers, which seems small but converts to 1,000 Angstroms. Lab-grade wheels are frequently 3 or 4 Angstroms or 0.0004 micrometers, much smaller than the virus and in fact there are very few gasses smaller than 3 Angstroms.
Bypassed Wheels Still Leak: Wheel bypass dampers are often adjacent to the wheel and when open the air flows through the bypass damper and the wheel. Unless you completely isolate the wheel when bypassed, the wheel will still leak through the seals.
If UGVI were provided downstream of the wheel, wouldn’t that further alleviate concerns?
Note some DOAS have passive desiccant wheels downstream of the cooling coil (and therefore likely downstream of UVGI). Disabling the passive desiccant wheel may cause space humidity levels to rise and condensation to form, especially in sensible cooling systems such as chilled beams. Many passive desiccant wheels are similar construction to enthalpy wheels. As indicated previously, energy recovery wheels should continue to run after careful evaluation and approval. There are desiccant based DOAS system utilizing ACTIVE desiccant wheels ( or thermally operated , which I suspect you are referring to) where cooling coil(s) installed upstream the active desiccant wheel as shown below. The DX cooling coil precools/pre-dehumidify the entering OA ( typically to around 70 Deg F /90-95 % RH) and the active desiccant wheel post -dehumidify and reheats the precooled/dehumidified air to the required supply conditions ( as shown below , 75 Deg F DB /45 Deg F DP). Waste heat ( typically low temperature reactivation) from the condenser is used for reactivation. It should be noted the adsorbent desiccant material specifically designed for low reactivation temperature.
https://www.munters.com/globalassets/inriver/resources/pg0018-14-drycool-standard.pdf
As Shown in the schematic the desiccant wheel uses outside air for reactivation and not exhaust air from the building , therefore there is no contamination risk ( the two air streams are from outside) and I don’t see a reason for UV systems . As suggested for passive enthalpy wheels , these systems also need to be inspected for proper operation.
Air Circulation: What minimum air circulation rate (recirculation air and outside air) should be provided in occupied spaces? 6 ACH is recommended Should all this air be treated (either filtered, disinfected, or diluted with OA) before being re-introduced in the space? Yes, the goal is to have the air see a MERV 13 filter, disfinected (UV-C or PCO) or diluted every 10 minutes.
Units in Occupied Spaces: For localized systems that recirculate air within the occupied spaces, such as fan coil units, fan-powered VAV boxes, and chilled beams, what strategies should be employed – UVGI? MERV-13 or above? Both?. If Possible, both. But in many cases, it will be impossible. Every case is unique and needs special evaluation of a PE. For example if a fan coil installed outside the classroom, distribute the air via ductwork to the classroom, the diffusers are properly located , OA is delivered independently ( from DOAS) to the space at space conditions ( or better, such as 60-65 Deg F and 40 to 60 gr/Lb in the summer ). Many of the terminal units do not have the static to go to MERV 13, so the disinfectant method must be accomplished with smaller independent units. There are many portable and light troffer based UV-C and HEPA machines under 500 cfm. This will be more cost effective than changing out terminal units motors.
Combined Strategies: Is there a way to evaluate the combined effectiveness of strategies to help inform decision making? For instance, if filtration is provided at MERV-13 or above, does also increasing the outside air beyond Code reduce the spread of disease by any appreciable amount? If UVGI is used to standard disinfection level (90% Influenza A first pass), does MERV-13 or above filtration provide an appreciable added benefit? What about combining UVGI and increased ventilation? Combining UVGI, MERV-13 or above, and increased ventilation? This is a good question. The theory is about the the particulate size in terms of nm that we want to capture. The higher the merv, the smaller particles we are capturing. Most of the viruses are in the 100 nm and above size. So MERV’s in that range work well. If you deploy ionization tech, you can then artificially increase the size of the particles, thereby making them easier to capture in the filter.
Occupant Density: Should guidance be provided for amount of room volume (cubic feet) per person? Does spacing people 6 feet apart in a room with say 8 ft ceiling provide adequate room volume to mitigate disease transmission? Perhaps criteria changes based on duration of occupancy? Because ASHRAE is confident that the viruses will spread in the air, the diffusers in a room are designed to move the air. So, event though the occupants are apart 6 feet, the air movement will ensure there is mixture. Furthermore, if they are sitting in a say a classroom for over 1 hour in place, the 6 foot barrier will be ineffective.
Diffuser Type and Placement: Should guidance be provided based on occupant density? What are suggested schemes – mixing supply high and return low, mixing supply low and return high, displacement ventilation, others? The diffuser selection is done to ensure good mixing of air. The only way to avoid cross air flow is to have return grilles above each student and supply low. In essence creating a 6 foot air flow vertical pattern to avoid horizontal migration.