HVAC SYSTEMS

                                

     In modern buildings the heating, ventilation, and air conditioning 

(HVAC) system is designed and installed to maintain a comfortable environment

for the occupants.  This includes maintaining comfortable temperatures and

relative humidity levels uniformly throughout the office space, providing

perceptible air movement without drafts,  eliminating stale air, and providing

adequate quantities of outside air in order to prevent the buildup of air

pollutants.  

     In a typical HVAC system, the air is provided by a supply fan or air

handling unit (AHU)  which receives air from from outside the building, and

air recirculated from the office space. The outside and return air are mixed

in the mixing chambert, filtered, heated or cooled, and delivered to occupied

areas through sheet metal ducts. These sheet metal ducts discharge the air

into the office space, usually through supply air diffusers located on the

drop ceiling.   





MIXING OUTSIDE AND RECIRCULATED AIR 



     The outside air is drawn through a duct which is usually located on the

roof of the building, and the return air is delivered to the supply fan by the

return or exhaust fan.  The return air fan exhausts air from the office

through return air vents into the space above the drop ceiling (called the

return air plenum). This return air then goes through a return duct to the fan

room.  The return air can then be exhausted to the outside and/or

recirculated.  

     The return air moving through the plenum above the drop ceiling often

comes in contact with pollutant sources, including fibrous glass insulation on

the ducts or in sprayed on fire proofing,  asbestos insulation on pipes or in

fire proofing, and chemicals off-gassing from the ceiling tiles. (See the CWA

fact sheet "Office Indoor Air Quality".)

     The amounts of return air and outside air mixed together is controlled

by dampers in the return and outside air ducts. Each  damper can move to any

position from fully closed to fully open. The return air, exhaust air, and

outside air dampers all work together so that when the outside air damper is

fully open, the return air damper is closed and the exhaust air damper is

open.  On the other hand, when the outside air damper is closed, the return

air damper is fully open and the exhaust damper is closed. In this mode the

ventilation system would be entirely recirculating office air.  Indoor air

quality problems usually occur when the outside air dampers are not open

enough to provide adequate amounts of outside air.  The American Society of

Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) normally

recommends that a ventilation system should deliver at least 15 to 20 cubic

feet per minute (cfm) of outside air per occupant.





AIR CONDITIONING (COOLING)



     During the summer months, air is passed through water-filled coils which

look like the radiator in your car. This is called a cooling coil. The water

flowing through the coil is around 40 ° F.  The ventilation air does not

contact the water, only the cold metal surface of the coil.  This is enough to

cool the air to 60 ° F before it is sent to the offices.  The cold water is

produced in the building by refrigeration units called chillers, and pumped

through pipes to the cooling coils in each of the fans. When the water exits

the cooling coil, it is much warmer and it is piped back to the chiller to be

re-cooled. There is also a separate condenser water system which removes heat

from the chiller units at a cooling tower which is located outdoors (usually

on the roof). This heated condenser water can be a good environment for the

growth of  Legionella pneumophila  bacteria, which can cause Legionnaire's

disease. 



     In the summer time ,the outside air is often cooled from 80 ° F to a

supply temperature of 60° F. The cooling of the air causes water to condense

inside the fan unit on the cooling coil. The same thing happens on your window

air conditioner except the water drips from the unit to the outside. In the

office air handler or fan unit, this water is collected in a drip pan beneath

the cooling coil and drained away through a floor drain. If water does not

drain properly from the cooling coil, slime will build up. This slime contains

high amounts of bacteria and molds which can be released into the ventilation

system.  







HEATING

 

     During the winter months, the cooling system is shut down and the

heating system is turned on. The building boiler is used to produce steam or

hot water which is pumped to the heating coil located in each of the fan

units.  The steam heats the cold outside air  to a supply temperature of 75 F. 

A thermostat in the supply air duct measures the temperature and this

information is used to adjust the amount of steam entering the heating coil.

Most buildings also have a perimeter heating system for outside offices which

includes radiators or hot water baseboard heaters located along the outside

walls of the building. 

     In most air handling units the system must be set either on heating or

cooling mode, but not on both.  Probes inside the ducts measure the

temperatures of the outside air, mixed air and supply air; the system controls

use this information to adjust the outside and return air dampers in order to

achieve a supply  temperature close to that desired.  The cooling or heating

coils are intended to give a final adjustment to the supply air temperature.





WHY DO HVAC SYSTEMS RECIRCULATE AIR ?



     It always takes energy to run a refrigeration unit or a boiler.

Consequently, the more outside air a ventilation system takes in, the more

steam or chilled water needed to either heat it or cool it.  Under extreme

weather conditions the coils may not be able to adequately adjust the air

temperature and as a result the outside air dampers often run at a minimum

opening. There is also a cost incentive to operate the outside air dampers in

a close or minimum position in order to save energy and money.  When the

outside air dampers are at minimum or closed, air quality problems can soon

develop.  You can often visually determine whether the outside air dampers are

opened or closed.  If they are closed, it is usually indicative of an indoor

air quality problem.  





VARIABLE VOLUME VENTILATION SYSTEMS



     Modern ventilation systems can vary the amounts of outside air supplied

by the fan unit and also vary the amount of air delivered through a given

supply air diffuser. This is called a variable air volume (VAV) system and is

intended to save energy and maintain comfortable temperatures in many

individual zones or areas. There are variable air volume dampers at the end of

each supply duct branch which control the volume of air delivered to a zone. 

The VAV dampers are controlled by a thermostat located in the zone. During the

summer when the thermostat calls for cooling the VAV dampers open air flow to

the maximum level, and when heating is needed the VAV  dampers close.  Indoor

air quality problems occur when the VAV dampers close so that no or

insufficient air is provided  to the zone.  The return and supply air dampers

work in concert with the VAV controls in order to vary the volume of supply

air to the ventilation system at any given time. The VAV systems are all

controlled by computers.  

     There are other types of ventilation systems that are variations on the

above design.  These ventilation systems play an important role in maintaining

good indoor air quality in an office building.  It is extremely important that

people  investigating indoor air quality problems have an understanding of the

design and operation of the ventilation system.  





FILTERS



     The outside and return air has dust in it which must be removed before

the air goes through the coils.  If not, dust will accumulate in the coils and

clog them. This causes a decrease in air flow and decreases the ability of the

coil to heat or cool the air.  Old filters can become clogged with dust also

decreasing air flow. The dust caked on the filters or in the coils become a

source of pollutants including bacteria and molds. Proper maintenance of the

ventilation system requires replacement of filters on a regular basis and

cleaning of the coils.   

     There are different types of filters which have a wide range of

efficiency at removing dust.  High efficiency  filters (those with a dust spot

efficiency rating of 85% or higher) can remove pollen, mold spores, bacteria

or other dusts which may cause problems for people. Medium efficiency filters

remove most dust and have a dust spot rating between 30 to 40%. Low efficiency

filters will only remove large dust particles and are only intended to protect

the coils. They do not remove mold spores or pollen.  HVAC systems should use

medium efficiency filters as a minimum.  







     For further information, contact your local union or the CWA District 1

NYS Health and Safety Project at (212) 509-6994.





     This fact sheet was written by Edward A. Olmstead, CIH.  It is published

by the CWA District 1 NYS Health and Safety Project with partial funding from

New York State Department of Labor Hazard Abatement Board Contract #C007987.

The opinions expressed are not necessarily those of the NYS Department of

Labor.  1998.





NYS Health and Safety Project

CWA District 1 

80 Pine Street, 37th floor

New York, NY 10005

(212) 509-6994