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Ventilation & Infiltration Control
All human activity requires the use of oxygen, and results in the buildup of the carbon dioxide that is exhaled. The greater the level of activity, the more oxygen is used and the more carbon dioxide that must be removed from the space. Proper ventilation must provide adequate fresh air and also prevent the accumulation of carbon dioxide. Insufficient ventilation can exacerbate comfort within the building.

Maintenance tasks
Many simple jobs that are a normal part of building maintenance are often not done promptly. One such item is replacing cracked or broken windows. This is not only a safety precaution but also minimizes the infiltration of outside air into a space. A second project is to adjust exterior door closers to close the door swiftly and completely. A small permanent crack around a large door can lose enough energy in a year to pay for complete weather-stripping. Also, doorway use permitting, lock it as much of the time as possible, directing traffic to another door or scheduling use at particular times. This can be an energy saver because every time the door is opened energy is lost. The same principles apply to windows: every time a window is opened during the heating or cooling season energy is lost. A properly designed HVAC system provides adequate fresh air to the occupants without having to resort to opening windows. Also, the air conditioning system may have an economizer cycle that increases outside air circulation up to 100% of supply air when ambient temperature and humidity permit. Opening windows is redundant for buildings equipped with such energy saving equipment. If a building has operable windows, though, it is inevitable they will be used. It helps to post signs instructing occupants to keep the windows closed. On upper floors it may be a safety hazard and tenants can be cautioned against the high insurance rates they have to pay if they wish to open windows. Occupants should become more vigilant of the openings in the building envelope thereafter and strive to educate everyone to be more energy conscious.
Cover cooling only AC outlets in the winter
Window air conditioning units are often used for cooling only. In such situations the outside air vent should be covered in the winter to prevent infiltration through the unit. Otherwise the window unit becomes a full opening to the outside. This applies to many air handling units in large buildings as well. They get outside air through louvered wall openings with dampers that are supposed to close airtight when sensing controls do not call for any fresh air. These louvers are gasketed and should close completely when the louver operator is in the closed position. This should be verified periodically to limit unnecessary outside air intake. At the same time, the louvers should be tested to be sure they close fully when the pneumatic or electronic controls so require. There are many other openings through the building envelope that are in seasonal or occasional use. These include exhaust vents and fans, dryer vents, shop vacuum exhausts, kitchen vent hoods and laboratory fume hoods. These devices are often installed to vent through a permanent opening. Modern devices have backdraft dampers that close automatically to prevent reverse flow through the unit, from the conditioned space outward. Many manufacturers market retrofit kits that can be installed to incorporate a damper into existing units. These can be money savers, especially for exhaust and vent penetrations.
Operate exhaust fans only when necessary
Kitchen vent hoods are a common example of over exhausting. Many large kitchens operate their exhausts all day long. They are constantly evacuating conditioned air from the space. The hoods are actually only needed when cooking is producing heat and smoke that needs to be exhausted. Commercial kitchen hoods typically vent huge quantities of air, so any reduction in their use has an immediate impact on utility bills. Proper adjustment of the fan speed can also lead to a lowering of energy use. The air flow can be measured and the fan pulleys or sheaves adjusted to provide the ideal flow rate. Most food service exhaust hoods have manufacturer's guidelines, or the applicable codes can be consulted to determine the proper volume rates.
Designate a smoking area
The presence of smoke in a conditioned space places an extra burden on the HVAC system. This air cannot be recirculated and returned to the space after cooling it down a few degrees, as is the usual case; but must be exhausted. Then fresh air must be cooled to make up the deficit. Normally the outside air is at a higher temperature than the return air so it is more difficult to cool it. If the moisture content of the fresh air is higher - the usual circumstance - the expense of treating the air increases greatly. Designation of a smoking area, preferably in a space isolated from adjacent conditioned zones by doors or corridors, will ameliorate this circumstance. The volume of air affected is minimized, the quantity of air exhausted is reduced and energy is saved in space conditioning costs.
Proper adjustment of unit controls
Large air handlers have many controls that monitor and regulate operation. These devices include thermostats, damper controls, static pressure sensors, enthalpy sensors and smoke detectors. All are located in a rigorous environment: high levels of vibration, unusual temperature swings and lots of dust, grease and grime. It is only natural that delicate electronic and pneumatic devices drift out of calibration in such conditions. Following the manufacturer's recommended maintenance schedule will uncover problems before they cause major unit dysfunction. This can avoid operation of the unit for months at an inefficient level.

For systems that temper the outside air at a separate, dedicated unit - then introduce it into the individual spaces - the system should be designed to provide air at a higher than space return air temperature. When mixing occurs in the return air plenum - if the fan does not cycle off - free dehumidification and cooling will occur.
Seal openings in the building envelope
There are many small jobs that can collectively have a significant impact on the exfiltration and infiltration of outside air. Several are common maintenance projects, such as caulking around exterior window and door frames and weather-stripping operable sash windows. Other penetrations through the building shell are not so obvious. These include convenience outlets and switches in exterior walls, and junction boxes and other appurtenances in ceilings adjacent to attic space. It is also important to tend to all penetrations in walls that front storage or mechanical rooms and other unconditioned spaces. Don't forget plumbing penetrations.
Caulk around roof curbs
There are many roof penetrations in a complex building, for roof mounted AC equipment and exhaust fans. These devices are usually mounted on curbs which are sealed into the roofing material. With time the pitch can harden and the seal deteriorates. Often the seal can be watertight, but not airtight.

Roof mounted air handlers sometimes have short duct runs across the roof. The joints and other connections should be inspected regularly and repaired when leakage occurs. Leaks not only upset the air balance of the unit but allow water to enter the ductwork where it can damage the insulation.
Seal duct runs through unconditioned spaces
Leaking ductwork can lose 20% or more of the conditioned air in a supply duct run. This loss is duplicated on the return air side, where a large volume of untreated air is drawn into the duct. Instead of returning cool air to the unit from the conditioned space, the infiltration of air into the duct makes the unit work harder to cool air. The opposite takes place on the supply side. The net effect is that the ceiling space is being air conditioned.

Leaking ductwork in a ceiling space between floors is not as bad as leaking duct in the roof ceiling space or in an unconditioned zone with an exterior or roof exposure. The average temperature of the former may be five or ten degrees different from the adjacent conditioned spaces. The temperature difference will be significantly higher in a roof space, so there will be a greater impact on the equipment efficiency. As a result, it is important to first devote limited resources to repairing leaking duct in the roof ceiling space, then in unconditioned spaces, then in the balance of ceiling zones.