You may have seen recent guidance from the CDC on Keeping the Workplace Safe, which includes increasing ventilation at home, in the workplace, and at commercial establishments. This is great advice! In fact, HVAC equipment at hospitals and other health care facilities are often designed to provide 100% outdoor air to improve patient outcomes. Typically, it’s easy enough to have maintenance personnel override the minimum outdoor air damper positions using your building automation system, however there are some practical considerations to keep in mind.
Ben Tashjian, P.E.
The first, and most damaging thing to consider, is the potential to freeze heating and cooling coils in your HVAC equipment. Depending on your climate and your system type, the potential to freeze and crack coils can be a real concern. If the outdoor air temperature is below freezing and you’ve overridden units that weren’t designed for 100% outdoor air, this is virtually an inevitability. If you have hot water coils and/or chilled water coils in your units and these loops do not have sufficient glycol (most loops do not have any glycol), this is a recipe for disaster. Steam coils are also prone to freezing under these conditions. Freezing is generally not a concern with gas fired units and units with refrigerant coils (i.e. heat pumps). Although most units have freeze-stats on them which will shut down the unit if the discharge air temperature gets too low, this is a last resort and should not be relied upon – not to mention that shutting down your unit will drastically reduce the ventilation you’re providing for your space.
The second thing to consider is occupant comfort. Most likely this time of year you’re not experiencing anywhere near design heating conditions (luckily, it’s just not that cold out in March!). This means your units should be able to handle some increased ventilation and still heat to a reasonable temperature, however, as with nearly everything, your mileage may vary. Different systems are designed for different operating conditions. Some dedicated outdoor air systems (DOAS) merely temper the air and provide it at or near room temperature. The majority of heating units I’ve seen have design discharge air temperatures of somewhere between 90F and 120F. This temperature rise between your mixed air temperature (mixing return and outdoor air) and the discharge air temperature is directly related to the heating capacity of your coil. A coil may be able to raise the temperature of this air by 40F, for example. When you increase the amount of outdoor air (and it is colder outside than inside) your mixed air temperature will be lower. Ideally, you could calculate the perfect mix so that this new mixed air temperature is never lower than it would be during design heating conditions for your facility. Note: If you have boilers and a hot water loop one easy way to increase the heating capacity of all your heating coils is to temporarily disable the outdoor air reset control – hotter water means more heating capacity for your coils.
Photo credit: https://ww1.prweb.com/prfiles/2011/01/31/8104368/Air-Handling-Unit.png
Finally, it may be prudent to consider the increased energy usage that will be associated with increasing ventilation. Personally, I think public health is the more immediate concern. In the weeks and months ahead when this public health crisis is over, remember to return to normal operations. I’ve been in many facilities where maintenance personnel have overridden controls for a variety of reasons, and never remembered to turn them back.
Also of note, I’ve seen a number of units that have their outdoor air dampers completely closed in an effort to save energy. It is not wise to do that, especially during this pandemic.
Ben Tashjian, P.E., LEED AP
Managing Engineer
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