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WARM AIR
RETURN AIR
SUPPLY
RETURN
Fig. 8. Convection Heater.
REFLECTOR
INFRARED
SOURCE
RADIANT HEAT
Fig. 9. Infrared Heater.
In mild climates, heat can be provided by a coil in the central
air handling system or by a heat pump. Heat pumps have the
advantage of switching between heating and cooling modes as
required. Rooftop units provide packaged heating and cooling.
Heating in a rooftop unit is usually by a gas- or oil-fired furnace
or an electric heat coil. Steam and hot water coils are available
as well. Perimeter heat is often required in colder climates,
particularly under large windows.
A heat pump uses standard refrigeration components and a
reversing valve to provide both heating and cooling within the
same unit. In the heating mode, the flow of refrigerant through
the coils is reversed to deliver heat from a heat source to the
conditioned space. When a heat pump is used to exchange heat
from the interior of a building to the perimeter, no additional
heat source is needed.
A heat-recovery system is often used in buildings where a
significant quantity of outdoor air is used. Several types of heat-
recovery systems are available including heat pumps, runaround
systems, rotary heat exchangers, and heat pipes.
In a runaround system, coils are installed in the outdoor air
supply duct and the exhaust air duct. A pump circulates the
medium (water or glycol) between the coils so that medium heated
by the exhaust air preheats the outdoor air entering the system.
A rotary heat exchanger is a large wheel filled with metal
mesh. One half of the wheel is in the outdoor air intake and the
other half, in the exhaust air duct. As the wheel rotates, the
metal mesh absorbs heat from the exhaust air and dissipates it
in the intake air.
A heat pipe is a long, sealed, finned tube charged with a
refrigerant. The tube is tilted slightly with one end in the outdoor
air intake and the other end in the exhaust air. In a heating
ENGINEERING MANUAL OF AUTOMATIC CONTROL
FINNED TUBE
TO OTHER
FLOOR
HEATING UNITS
FROM OTHER
HEATING UNITS
C2705
C2706
application, the refrigerant vaporizes at the lower end in the
warm exhaust air, and the vapor rises toward the higher end in
the cool outdoor air, where it gives up the heat of vaporization
and condenses. A wick carries the liquid refrigerant back to the
warm end, where the cycle repeats. A heat pipe requires no
energy input. For cooling, the process is reversed by tilting the
pipe the other way.
Controls may be pneumatic, electric, electronic, digital, or a
combination. Satisfactory control can be achieved using
independent control loops on each system. Maximum operating
efficiency and comfort levels can be achieved with a control
system which adjusts the central system operation to the
demands of the zones. Such a system can save enough in
operating costs to pay for itself in a short time.
Controls for the air handling system and zones are specifically
designed for a building by the architect, engineer, or team who
designs the building. The controls are usually installed at the job
site. Terminal unit controls are typically factory installed. Boilers,
heat pumps, and rooftop units are usually sold with a factory-
installed control package specifically designed for that unit.
COOLING
GENERAL
Both sensible and latent heat contribute to the cooling load
of a building. Heat gain is sensible when heat is added to the
conditioned space. Heat gain is latent when moisture is added
to the space (e.g., by vapor emitted by occupants and other
sources). To maintain a constant humidity ratio in the space,
water vapor must be removed at a rate equal to its rate of addition
into the space.
Conduction is the process by which heat moves between
adjoining spaces with unequal space temperatures. Heat may
move through exterior walls and the roof, or through floors,
walls, or ceilings. Solar radiation heats surfaces which then
transfer the heat to the surrounding air. Internal heat gain is
generated by occupants, lighting, and equipment. Warm air
entering a building by infiltration and through ventilation also
contributes to heat gain.
Building orientation, interior and exterior shading, the angle
of the sun, and prevailing winds affect the amount of solar heat
gain, which can be a major source of heat. Solar heat received
through windows causes immediate heat gain. Areas with large
windows may experience more solar gain in winter than in
summer. Building surfaces absorb solar energy, become heated,
and transfer the heat to interior air. The amount of change in
temperature through each layer of a composite surface depends
on the resistance to heat flow and thickness of each material.
Occupants, lighting, equipment, and outdoor air ventilation
and infiltration requirements contribute to internal heat gain.
For example, an adult sitting at a desk produces about 117 watts.
Incandescent lighting produces more heat than fluorescent
lighting. Copiers, computers, and other office machines also
contribute significantly to internal heat gain.
11
CONTROL FUNDAMENTALS
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