Outdoor Air Damper Use in Hot and Humid Climates

Control air infiltration and moisture condensation.

THIS column provides information about the use of outdoor air dampers in hot and humid climates and during hot and humid seasons in temperate climates in commercial buildings.

Terms used in this column:

RH = Relative humidity; the amount of water vapor in the air compared to the amount of water vapor the air could hold at saturation. For example, when RH = 50%, the air contains half of the water it could hold when it is saturated with water vapor. In most buildings, RH should be maintained between 30% and 60%.

DP = Dew point temperature; the temperature at which air will be "saturated" (RH = 100%), given the amount of water vapor in the air.

AH = Air handling. The term is often used with "air handling unit" (AHU), "air handling equipment," (AH equipment) and so forth. HVAC ("heating, ventilating and air conditioning") is a parallel term.

What are outdoor air (OA) dampers and how are they used? A properly designed, installed, operated, and maintained OA damper will preclude the entry of water (rain, snow) into the AH system and will stop the flow of air into the system when it is closed. When open, it allows the AH equipment to supply a known amount of fresh outdoor air (OA) to the building to dilute air contaminants that are generated by the building, its furnishings, and its occupants. Damper-controlled OA also provides for space pressurization (to help prevent infiltration of air at the building exterior wall or "envelope") and makeup air for building exhaust fans (e.g., kitchen, bathroom, mechanical room, hoods).

The actual OA flowrate is determined by the damper opening, the pressure difference between the mixed air plenum and the outside air plenum, and other factors. The determination of appropriate design and actual OA flowrates is beyond the scope of this column. (See, for example, my book, IAQ and HVAC Workbook, for ways to estimate proper OA flowrates.)

The OA damper is normally an integral part of the AH system and its controls. In automatically controlled AH systems, the OA damper will open and close depending on a number of factors: the outside air temperature, humidity, and dew point temperature; the number of people in the building; the age of the building and its AH equipment; cost of air conditioning; and so forth.

In some older buildings, the OA damper may be manually operated.

For AH systems in hot and humid conditions and without return air fans, the OA damper should normally remain open only when the supply fan operates. The OA damper should open quickly when the fan turns on, to prevent excessive negative duct pressurization. In some systems, the fan on-off switch can be wired to open the outside air damper before the fan is started.

What happens if too much hot and humid air infiltrates the building or the AH equipment? If hot and humid air infiltrates the building or AH equipment (e.g., through the wall or through an open OA damper when the AH equipment is off) water will condense on building and equipment surfaces, possibly resulting in the growth of mold and bacteria. This, in turn, can cause odors, occupant complaints, adverse reactions, building material degradation, and so forth.

How is the OA damper normally controlled in hot and humid climates? The opening and closing of the OA damper can be controlled in several different ways, ranging from fully automated systems to manual systems.

Fully automated systems will open and close the damper depending on OA conditions. For example, when the air is cool and dry (e.g., about 55-70F and RH is less than about 60%), automated systems may open the damper completely to allow 100% fresh air to enter the building. Outside these conditions, the system may close the damper to a setting that provides the minimum amount of air required for dilution. This automated process is called an "economizer" system.

In less automated systems, the damper may simply be opened by the AH equipment to some "set point" during building occupancy and then closed during non-occupied times.

In manual systems, someone is required to open the damper when the building is occupied and then close the damper when the building is unoccupied.

What is the significance of the dew point (DP) temperature? The dew point temperature is the temperature at which air will be "saturated" (RH = 100%), given the amount of water vapor in the air. For example, air at 70F and 50% RH has a DP temperature = 50F. If that air is cooled to 50F, it will reach a saturated condition, and if the air is cooled below 50F, water will begin to condense out. (The science of the relationship between air temperature and water vapor is called "psychrometrics.")

At DP temperatures above 55F, water vapor control must normally be provided by the AH system, e.g., adjusting the amount of OA allowed into the system and building, dehumidifying the air, and so forth. Fully automated AH systems will provide adequate OA damper control if maintained properly.

Here are two conservative rules-of-thumb for semi- or non-automated AH systems: (1) For buildings maintained at some temperature less than ambient (say, 78F) during unoccupied times, and when the outside DP temperature is above 55F, the OA damper should generally be closed when the building is unoccupied. (2) In manual damper control systems, someone should open the damper when the building is occupied (to some pre-arranged set point) and then close the damper when the building is unoccupied and the OA dew point temperature exceeds 55F.

Where can I find the DP temperature for my location? You can get the actual DP temperature from your local weather station or by going to the NOAA website, www.noaa.gov.

The above figure shows approximate average DP temperatures for the United States during the hottest months, i.e., July and August. It is common practice to simply use such charts to estimate the average DP temperature for the cooling season and operate the OA damper accordingly. So, for example, during the hot and humid cooling season, any building located in an area where the DP temperature exceeds 55F might well consider arranging for the OA damper to be closed during unoccupied times.

Okay, so how should we specify, maintain and operate our OA Dampers? Dampers should be gasketed, rubber-sealed, and designed for use as OA dampers. They should be certified by the Air Movement and Control Association (AMCA). They should be maintained in good working condition at all times. They should be properly connected to the AH system and set in the right position. They should be checked at least monthly during the cooling season and quarterly in other seasons. If there are manual OA dampers in addition to automated dampers, they should be properly balanced.

What else should I know to help avoid moisture condensation problems related to OA dampers? When the OA damper is closed, building exhaust fans (e.g., bathrooms, kitchens, mechanical rooms) should not be operated because unconditioned, hot, and humid replacement air leaking into the building is likely to result in water condensation somewhere in the AH system or building. For example, if the outdoor air damper is left open, hot and humid replacement air may enter through the OA damper and water may condense on surfaces inside the AH equipment. Even with exhaust fans off, if the OA damper is left open, hot humid air is likely to infiltrate the air handling system, resulting in water condensation inside the AH equipment.

Like the OA dampers, all ductwork and the AH equipment must be "airtight," insulated as required, and maintained in good working order to avoid moisture problems in hot and humid conditions. Supplementary dehumidification (which is usually cost-effective) can greatly enhance moisture control inside such buildings.

This column appeared in the March 2006 issue of Occupational Health & Safety.

This article originally appeared in the March 2006 issue of Occupational Health & Safety.

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