Radiant Panels; Unit Heaters; General - Honeywell AUTOMATIC CONTROL Engineering Manual

RADIANT PANELS

A radiant panel is a surface that transfers 50 percent or more
of its temperature to other surfaces by radiation. Radiant panels
may be used for heating or cooling of individual spaces, or
may be used in conjunction with central fan systems. The panel
can be in a floor, wall, or ceiling, and the surface temperature
can be maintained by electrical heating elements or by
circulating water or steam.
Control for radiant panel heating or cooling can be difficult.
Conventional space control with a thermostat may not be
appropriate because it is difficult, if not impossible, to locate
the thermostat to sense the radiant heat. Because radiant heat
does not heat the air, the thermostat must either see the radiant
heat or sense the space temperature change caused by the
radiation warming the furniture and occupants in the space.
The time lag in the second instance is too long for accurate
control. Figure 13 shows a hot water radiant floor heating panel
with an outdoor reset control system.
OUTDOOR AIR
TEMPERATURE
SENSOR
THREE-WAY
MIXING
CONTROLLER
VALVE
BOILER
Fig. 13. Radiant Floor Heat System
with Outdoor Reset Control.
The space sensor, reset from the outdoor temperature, controls
the valve. As hot water circulates through the panel, heat radiates
to warm objects in the space and space temperature rises. The
space sensor senses the increase in space temperature and
signals the controller to reposition the valve. The outdoor air
sensor provides a reset schedule to raise the controller setpoint
as the outdoor temperature decreases. To prevent tiles from
softening and concrete from cracking, radiant floor panel surface
temperatures should not exceed 85F. Where hot water
temperatures may exceed 85F, the high-limit sensor resets
circulating water temperature.
Wall panels and ceiling panels can be controlled directly by
a room thermostat. Surface temperatures should not exceed
100F for wall panels and 120F for ceiling panels. (The actual
hot water temperature may be higher.)
Radiant heat panels that use electric resistance heating
elements are controlled by a two-position thermostat. As the
space temperature drops below the thermostat setpoint and
differential, the thermostat closes a switch to allow current flow
to heat the elements.
SPACE
TEMPERATURE
SENSOR
HIGH-LIMIT
SENSOR
PUMP
RADIANT PANEL
C3019
INDIVIDUAL ROOM CONTROL APPLICATIONS
When a radiant panel is used for cooling, the temperature of
the water circulating through the panel must be at least 1°F
above the dew point temperature of the space to prevent
condensation on the panel.
A radiant panel may be used for both heating and cooling, as
shown in Figure 14. A heating/cooling panel uses a four-pipe
system regulated by two-way valves on both supply and return.
A call for heat at the thermostat closes the chilled water supply
and return ports and modulates hot water supply and return. A
call for cooling closes the hot water supply and return ports
and modulates chilled water supply and return.
CHILLED
WATER
SUPPLY
HOT WATER
SUPPLY
CHILLED
WATER
RETURN
Fig. 14. Heating/Cooling Radiant Panel.
Constant-volume, constant-temperature central fan systems
can be combined with radiant-panel heating or cooling to satisfy
ventilation requirements. Occupancy schedules determine fan
operation. Room thermostats control the radiant panels to
maintain space temperature. Load anticipation is necessary
because of the high thermal inertia of this system. In general,
the thermostat setting should not be changed because of thermal
lag and the possibility of overshoot. Thermal lag in some radiant
floors can be several hours or more.

UNIT HEATERS

GENERAL

Unit heaters provide space heating for large open areas such
as building entrances, garages, workshops, warehouses, and
factories. Unit heaters are typically hung from the ceiling,
although cabinet versions are available. A fan forces air across
a coil containing hot water, steam, a warm-air heat exchanger,
or electric resistance elements. Warm-air units may be gas or
oil fired.
413 ENGINEERING MANULA OF AUTOMATIC CONTROL
THERMOSTAT
RADIANT PANEL
HOT WATER
RETURN
C3020