In April, Tom Durkin hosted an extremely popular ES webinar tackling humidity in schools. We ran out of time and had a slew of leftover questions, so here, his engineering team joins him in addressing them. Caveat: The questions often solicit professional opinions based on experience, but that candid exchange is part of the beauty of the webinar, right?
You started by saying that high summer humidity was the number one IAQ issue. What about lack of fresh air?
Response:As a firm, we have investigated over 50 IAQ problems in buildings. We have found all calls to be a result of high indoor humidity levels, not due to a lack of fresh air. Invariably, once we were on site doing our IAQ testing, we also often found elevated CO2levels indicating low ventilation rates. While lack of fresh air is a significant IAQ issue, it does not seem, to us, to be what generates the initial complaint.
2. Question:Are parts of the country more prone to humidity problems than others?
Response: Any area east of the Rockies and the Pacific Northwest is most likely to be susceptible to high humidity problems.
3. Question:How do you balance humidity control and energy efficiency?
Response: This is the single biggest issue facing the engineer trying to control humidity. The prerequisites for acceptable design are IAQ, humidity control, and occupant com-fort. Only after those items are thoroughly addressed, should energy efficiency be considered.
4. Question: What’s your opinion on varying chilled water supply temp to aid humidity control?
Response: The lowest chilled water temperature is needed when the building has the greatest demand for dehumidification. If your motivation for varying chilled water supply tem-perature is to save energy by delivering higher chilled water temperature, it’s important to remember not to compromise humidity control for the sake of operating costs. If the question concenrs improving dehumidification by lowering CHW temperature, we don’t think that would help much, unless the air-handling equipment cannot get to design leaving air temperature.
5. Question: If the weather conditions permit, why not use the HVAC system in full economizer to dry the carpet in lieu of dehumidifiers?
Response:If weather conditions permit (outdoor air dewpoint under 52°F), this might be an acceptable strategy. However, during the summer, when most school carpeting gets cleaned, the outside air conditions would probably not be conducive to air drying.
6. Question:We are looking to do demand shaving by resetting the send-out central chiller plant for an event that may occur for 6 hrs, 10 times during the cooling season. Any thoughts on mold control?
Response: Refer to the chart on slide 16 of the webinar (2007 AHSRAE Handbook – Applications, page 21-3). The chart shows that as long as the duration of high humidity is not significant, mold growth is less likely because mold occurs with protracted exposure to high rh. We recommend staying well below the marginal area on the chart.
7. Question: My building only has a mold issue at the first register out of a rooftop unit. It is an office building. Would this suggest that it is most likely a condensate carryover issue? What might be some likely causes? I’m thinking improper cleaning, but what may be others? We have done a mold study, and only this area currently is showing mold. I have two proposals on the table - hot gas bypass and a DOAS for this plus five other RTUs. I am wondering if it is an equipment issue or a building issue and how to proceed. I’m planning a summertime study of humidity with dataloggers.
Response:We would check the unit to ensure that no air is bypassing the filters since dirt in the airstream will be a source of food for mold spores. However, there may potentially be other sources of high latent air causing this problem. High outside air concentrations could be coming in via an adjacent vestibule introducing outside air near this diffuser, or via envelope leakage, resulting in the condensation and subsequent mold. We would not rule out other issues outside of the ductwork and rooftop unit.
8. Question: Do you design reheat systems for school A/C systems?
Response:If there is adequate justification for it. However, if we elect to use reheat for humidity control, we advocate the use of a heat recovery chiller so that we reheat without “new” energy.
9. Question:What are your thoughts on the use of heat pipes and runaround coils?
Response: It depends upon the application. Heat recovered from a heat pipe to be used at an AHU may make sense in some applications. However, it is impractical to use the recovered heat at terminal units. Runaround coils are typically inefficient and require exacting economic analysis to verify their cost-effectiveness. A large disadvantage of both of these applications is that the greatest need for reheat often occurs at low load, when there isn’t much energy available for them to recover.
10. Question: With today’s energy costs, who will design a 24-hr operation?
Response: It’s not a question of who will design it, but if it inadvertently happens. If the building HVAC occupancy is not correctly scheduled, or if someone thinks that humidity problems will be better helped by allowing the equipment to run constantly, there will most likely be humidity problems as a result.
11. Question:What manufacturers have you found to be most effective for DHRC applications?
Response:The right DHRC equipment is any water cooled chiller that can make 130° condenser water and has the control capability to seamlessly switch between evaporator and condenser control.
12. Question: For museum applications that require tight temperature and humidity control, what is the best system?
Response:It is best to follow the guidelines outlined in the ASHRAE Handbook - Applications. However, we recommend that any reheat required comes from recovered heat.
13. Question: Any comment on reliability and accuracy of CO2and humidity sensors?
Response: They are much better than they were 10 years ago, but our experience is that they will still require frequent recalibration. We try to limit the application of CO2sensors to large spaces such as auditoriums, gymnasiums, and cafeterias so that their use is minimized and the maintenance burden (e.g., calibration) on the owner is kept to a minimum. Humidity sensors, when needed, are used to monitor several clustered areas with similar characteristics to minimize their use.
14. Question:In your opinion, what are the acceptable limits for temperature and humidity during unoccupied periods with the systems disabled?
Response: We would start by looking at the dewpoint in lieu of using a specific temperature or humidity level. Any resulting temperature and relative humidity with a dewpoint temperature below 59° would be a good choice as long as the space temperature is above 72°. Refer to webinar slide 16 and stay in the “Safety Region.”
15. Question: You mentioned having a summertime humidity analysis; what is included in this?
Response:A thorough humidity analysis is needed when selecting equipment for dehumidification. Multiple temperature and humidity conditions for outside air should be considered in equipment selection. The equipment shall not just be evaluated at peak cooling (drybulb and wetbulb design) and heating loads, but also at part-load conditions with high humidity such as 70° and high humidity often found on rainy days with partial building occupancy.
16. Question:Regarding DOAS, any thoughts on final discharge temperature, i.e., neutral-to-space usually requiring reheat or lower?
Response: It depends upon the connection and arrangement of the DOAS system. If DOAS air is supplied directly to the space, we would likely make the DOAS temperature room neutral. However, if the DOAS air is supplied in conjunction with a terminal unit such as two-pipe system, we recommend it be supplied to deliver warm air during cooling season and cool air during the heating season to address the concurrent heating and cooling demands of the space.
17. Question:What about duty cycling the central air handler fans every 15 min for a peak shaving event? Would this wet and dry the cooling coil too much?
Response: We do not advocate any strategies that would compromise IAQ, occupant comfort, or humidity control. However, if the realities of the situation are such that duty cycling is necessary, cycling the fan would compromise all three. Closing the OA damper would significantly reduce cooling load, but would only compromise IAQ. And if combined with CO2monitoring, it might not even do that.
18. Question:Is mold growth more a function of dewpoint or rh?
Response: Mold growth is a function of rh.
19. Question:What is the impact on the humidity levels when there is a significant breech in the building’s vapor barrier?
Response: It would be impossible to predict the humidity impact, other than to say it would be unacceptable. The best strategy would be to repair the breech. Trying to make the HVAC system compensate would be an “iffy” solution and probably would significantly increase cooling cost.
20. Question:Of the four systems - DOAS, reheat, VAV, and face and bypass - which is least expensive to install and operate over 20-yr duration for a central Missouri location?
Response: In our opinion, the least expensive system to operate and install for most climate zones would be a two-pipe face and bypass system. All of our studies and experi-ence show us that it is the least expensive to install and operate when properly designed and controlled.
21. Question: Do you have a recommended minimum OA quantity for systems when using CO2control for OA flow variation?
Response: We advocate following ASHRAE Standard 62 when choosing the OA quantity for a system with CO2control for outside airflow variation.
22. Question:What do you consider the optimal range for CO2for student learning?
Response:We recommend at least 700 ppm above ambient as recommended by ASHRAE Standard 62, Appenix C. Ambient CO2levels are usually in the 300 to 500 ppm range, but in urban environments it might be higher.
23. Question:When we reduce the fan airflow, what will happen to fresh air?
Response: As the total airflow decreases, fresh air will also decrease. This is corrected by opening the OA damper to maintain fresh air minimums per ASHRAE Standard 62.ES
Q&A: Dehumidification In Schools
August 1, 2008