3 FM quick reads on outside air
1. Maintaining the Quantity of Outside Air for Good IAQ
As indoor air is often more polluted than outside air, maintaining a proper mix of outside air in your facility is key to flushing out toxins and preserving good indoor air quality. If your HVAC system cannot deliver sufficient outside air or properly condition the outside air, consider retrofitting your system.
Depending on the type and age of your HVAC system, there are many modifications you may be able to make that will improve its performance. The following retrofit options may be useful. You could add separate outside air fans to each air handling unit, use a make-up air unit or use energy recovery equipment.
Another operations step you can take to protect indoor air quality is making sure that outside air dampers are set and controlled properly.
Do not rely on fixed outdoor air dampers or settings last established at the time of construction or renovation.
Periodically check that the outdoor air dampers can open and that linkages, controls and related mechanisms are functioning. For constant air volume systems, check outdoor air flow and reset the damper position at least every few years or as occupancy changes.
Fixed outdoor air damper positions cannot ensure sufficient ventilation nor meet the ASHRAE Standard 62 Ventilation Rate Procedure as it applies to variable air volume (VAV) systems. Consider air flow monitoring that is integrated into your HVAC system to maintain the desired VAV ventilation air flow.
2. Demand Control Ventilation Systems Can Save Energy, Achieve Occupant Satisfaction
Today's tip from Building Operating Management comes from Angela Cremeans and R. Stephen Spinazzola of RTKL: In facilities with wide swings in occupancy, demand ventilation systems — which strike a balance between occupant satisfaction, energy efficiency and cost savings — are worth a close look.
Demand ventilation systems — also known as dedicated outdoor air systems — are 100 percent outdoor air systems that include filtration along with heating and cooling coils. Normally, demand ventilation systems have some sort of heat recovery and are variable in volume, modulating down to 25 to 50 percent air flow. The systems provide humidity control for the entire building because they dehumidify outside air down to as little as 30 percent humidity, allowing the base building system to be dedicated strictly to cooling, not dehumidification.
Recent development of economical and accurate carbon dioxide sensors makes it feasible to use them as an integral part of a demand ventilation system. Model codes now allow the amount of outdoor air to be reduced below a constant 15 CFM per person.
Of course, there are issues that should be evaluated when deciding to proceed with a demand ventilation system.
The first question is whether local code allows outside air levels to be reduced. If the answer is no, determine whether the code official is willing to look at alternative schemes as part of a comprehensive design solution.
Depending on the base mechanical system, there may be some added first costs to implement a demand ventilation system. A demand ventilation system requires a parallel duct system to get outside air to individual spaces. The biggest savings in operating costs are in conference rooms and other assembly-type spaces.
Demand ventilation should not be considered unless the facility manager plans to do a full commissioning of mechanical systems prior to occupancy. Commissioning will ensure systems are performing as intended. Also, care should be taken to make sure the air balance is designed so that the minimum outside air doesn't drop below what is required to maintain positive building pressure.
This has been a Building Operating Management Tip of the Day. Thanks for listening.
3. Get Air Where It Needs to Go for Good IAQ
Bringing in sufficient amounts of outside air can be an important component in maintaining good indoor air quality at your facility. But just because you are bringing sufficient amounts of outdoor air into the building doesn't mean the outdoor air is actually being efficiently delivered to occupants.
In order to ensure that outdoor air is getting where it needs to, it's important to optimize the operation of your ductwork by removing air flow resistances.
Review your ductwork, and make sure that proper duct sizes are being used, smooth duct transitions are in place, proper tuning vanes are installed and excessive flex duct connections are removed.
Through a comprehensive audit of your duct system, you can identify opportunities to remove air flow resistances that can not only improve the air quality of the building, but that also save energy.
While you're at it, make sure air supply diffusers serving the occupied spaces are not obstructed. Obstructed supply air diffusers can ruin proper air balance. When this occurs, some areas receive too much supply air, while others receive too little. Also, improper air pressure relationships between the building and the outdoors, as well as between key areas within the building, can develop. This facilitates pollutant transfer.
If occupants are complaining of drafts, several remedial options exist:
- occupant location can be changed
- diffuser location can be moved
- different diffuser design can be used.
Some of these options require that the air system be rebalanced.
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