HVAC = Heating Ventilating and Air Conditioning

Air handler = main fan that pushes air through coils and ducts

Coil = radiator-type device that exchanges energy with air, heating or cooling it.

Convector = a tube with metal fins within a cabinet, not unlike a baseboard heater, which transfers heat from hot water or steam to the air of the room.

Damper = a mechanism to control the flow of air through a duct. Volume control dampers are used to balance the system so that the proper amount of air is provided to each diffuser (outlet). Fire dampers and smoke dampers are designed to shut of the HVAC system at fire walls so that smoke and fire can't get through the wall by way of the ductwork.

Duct = pipe through which air is blown, round or rectangular, of steel, fiberglass, aluminum, or other materials.

Diffuser = outlet devices designed to spread the air flow in controlled directions.

Fan-coil unit = a cabinet, located along an outside wall or above a ceiling, which consists of a small fan, heating (and possibly cooling) coils, and a control system. Fan coil units can be heat only (two pipe) or heating and cooling (four pipe). They may or many not also bring in outside air. If they don't bring in outside air, then the room must have other means of ventilation such as operable windows.

Outside air = fresh air that is drawn in from outside the building so that rooms don't get stuffy.

Plenum = large volume such as the area over a ceiling through which air is supplied or removed instead of through ducts.

Radiator = a metal casting within which hot water or steam circulates, heating the room. Now usually replaced by convectors.

Return air = air that has been drawn from within the building to be mixed with outside air, heated or cooled, and recirculated.

Relief air = air that must be exhausted to make way for the fresh outside air.

Variable air volume, or VAV = and HVAC system that controls the temperature in part by varying the amount of air delivered to the space.

Zone = an area which can be heated and or cooled to its own temperature.

HVAC systems control the thermal environment of a building. Each area controlled by a thermostat is called a zone. There are many ways to give each zone its own thermal control (see below).

Heating systems can use many different ways to move energy. Most commonly, air is heated or cooled, then circulated through the building. Energy can also be moved as steam, hot water, electricity, gas, or other means.

Controls are the systems that control whether heat or cooling or ventilation are provided. In the last decade, direct digital control (DDC) has become more common. DDC systems allow for sophisticated set-back programs that can result in major energy savings, especially if tied to occupancy sensors or carbon dioxide sensors. However, they are very sophisticated and require special expertise to maintain.

Economizers provide heating and cooling of the building with outside air whenever possible. They are required by code on most air handling systems.

Night cooling is a strategy to pre-cool the building during the coolest night hours to prevent heat buildup during the day. This is most easily achieved with DDC controls, but can also be done with simple time clocks and switches if staff is willing to set up the system in the spring and turn it off in the fall.

Passive systems heat and cool without fuels, such as solar heat from the sun or passive cooling through pre-cooling of building mass at night.

Although many older schools don’t have HVAC systems and are heated with radiators and cooled and ventilated with operable windows, most schools built in the last forty years use HVAC systems. In a conventional HVAC system, outside air is brought in, mixed with return air, filtered, passed through a heating coil and possibly a cooling coil to bring it to the delivery temperature (and possibly to adjust the humidity), and distributed to the various zones. Each zone typically gets local control either by having its own air premixed at the air handler (a multi-zone system) or by have local control of a small heater or coil that brings the temperature of the air up to its final delivery temperature (terminal reheat system) or by adjusting the volume of the air delivered to each zone (variable air volume, or VAV).

Ducts made of steel, aluminum, or fiberglass carry the air from the air handler to the outlets, called diffusers. Along the way, they pass through volume control dampers, which are used to control the volume of air passing through any given duct, and sometimes through fire dampers, which close automatically in the event of smoke or fire.

Fans, ducts, and diffusers can be the source of noise. Excessive air speed, poor ductwork design, and improper diffuser selection can all contribute to noise problems. On the other hand, HVAC systems run continuously and can provide a covering background noise that increases acoustical privacy. In fact, ductwork can be modified in small ways to add more background noise such as by adding a small amount of gravel to the duct near a diffuser.

Temperature control in each zone is done by a thermostat located either in the space or in the return air stream. The former is preferred in nearly all circumstances. Temperature in DDC control systems can also be from a computer that has sensors in each room which "vote" to determine the temperature setting.

Design of heating systems needs to be matched to abilities of the maintenance staff. There is no reason to install a state-of-the-art VAV system with complex controls and have it maintained by a technician who barely knows which end of the screwdriver to hold. If you buy a sophisticated system you will need to train or hire sophisticated technicians.

Schools and small commercial buildings are sometimes built with systems that are designed for lowest first cost, such as packaged roof-top heating and cooling units. This can result in long-term problems, especially if corners are cut in the sizing and installation of the ductwork itself, as ductwork is very expensive to replace. Or, in some cases, ductwork is omitted entirely such as in a plenum return system. These systems can’t be balanced properly, and occupants invariably complain about hot and cold areas. Sometimes ductwork is too small in an attempt to save costs. Smaller ductwork makes the air travel faster, creating more noise, using much more fan energy, and possibly wearing out the fan sooner.

Classooms are sometimes heated and possibly cooled with fan-coil units located on an outside wall. These give each room its own control, and are fairly inexpensive to install and to operate. However, each unit must be maintained at least once or possibly twice per year, so many fan-coil units can be more expensive to maintain than one or two centralized systems. These also require a central boiler to heat the water that circulates and, if cooling is provided, a central chiller. Even so, this can be a very cost effective, flexible, and expandable type of system, and tends to have fewer complaints than central air systems, perhaps because excessive cost reduction can't make the system ineffective.

A variation on this approach is to install a single high-quality residential furnace in each classroom. These units are inexpensive, efficient, easy to replace, and, with proper maintenance, can last twenty or thirty years. They should be installed in an easily accessible location (such as a closet) and be equipped with sophisticated controls to maximize energy savings. This approach works best if combined with passive cooling strategies as opposed to full-bore air conditioning.

HVAC systems require regular maintenance in order to last for their expected lives. At a minimum, filters should be changed and belts and bearings checked at least every six months, but the exact maintenance schedule depends on the individual piece of equipment. The operating manuals that were supplied with the equipment should provide full maintenance requirements.

Balancing and troubleshooting HVAC systems sometimes appears to be a black art. However, a well-designed and well-built system is usually not difficult to keep working well. Operational troubleshooting often looks at faulty controls, inadvertently closed dampers, dirty coils, bad fan bearings, worn belts, and other common problems.

Solving Problems

Most problems with climate control in buildings relates to people either being too hot or too cold. Start with a little research. Either have people record conditions manually, or get a small data logger called a Hobo which will record temperature automatically. Are all rooms too cold or too hot at the same time? Is there a pattern of time of day or time of year?

Here is a list of common symptoms and common solutions:

Fans are noisty (bump bump bump): Fan needs balancing. Belt needs replacing. Bearing needs replacing.

System is noisy (whoooooosh, sssssssss, hmmmmmm): ducts are too small. Dampers are closed down too tight.

Some areas are very cold or very hot: Control system isn't working right. Fire damper is closed and not re-set.

Rooms on the west or south side get too hot: The climate in the northern latitudes tends to make certain orientations too hot in late spring, summer, and early fall. Shading of these sides of the building with deciduous trees or vines is probably the most cost effective solution.

In a newer building, in rooms with high ceilings the occupants are always cold: In a buildng with a VAV system, air can stratify when air flows are low, such as in the heating season. The hot air stays near the ceiling, and the occupants near the floor get quite cold. Rather than add heat, it is often more effective and less expensive to add a ceiling fan.

Black stains on the ceiling near air diffusers: Two factors cause this problem. One is excessive air velocity, often caused by excessively small ducts (to save construction money). This is then exacerbated by low-quality filters that aren't changed often enough (to save operating money).

Indoor air quality, or IAQ, is intrinsically linked to ventilation, and therefore to HVAC systems. IAQ is agrowing area of complaint in school buildings, and a number of simple steps can prevent IAQ problems or help trace down and eliminate the sources. Follow this link to Indoor Air Quality for more information.

There are many strategies for decreasing the energy requirements of buildings, and the heating and ventilating system is a major player in that effort. Any significant HVAC design effort should be informed by this information in order to reduce long-term costs and to reduce the global impact of our buildings. Follow this link to Energy Issues for more information.

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