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

For commercial buildings
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RADIANT PANELS

A radiant panel is a surf ace tha t tr ansfers 50 per cent or mor e
of its temper atur e to other surf aces by radia tion. Radiant panels
ma y be used f or hea ting or cooling of indi vidual spaces, or
may be used in conjunction with centr al fan systems. The panel
can be in a f loor, wall, or ceiling , and the surf ace temper atur e
can be maintained by electrical heating elements or by
circulating water or steam.
Contr ol for r adiant panel hea ting or cooling can be dif ficult.
Conventional space contr ol with a ther mosta t ma y not be
appr opr iate because it is dif ficult, if not impossib le, to loca te
the thermostat to sense the radiant heat. Because radiant heat
does not hea t the air , the ther mosta t must either see the r adiant
heat or sense the space temperature change caused by the
r adia tion war ming the fur nitur e and occupants in the space .
The time lag in the second instance is too long for accurate
contr ol. Figur e 13 shows a hot w ater r adiant f loor hea ting 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 fr om the outdoor temper atur e, contr ols
the valve. As hot water cir culates thr ough the panel, heat r adia tes
to war m objects in the space and space temper atur e rises. The
space sensor senses the increase in space temperature and
signals the contr oller to r eposition the v alve. The outdoor air
sensor pr ovides a r eset schedule to r aise the contr oller setpoint
as the outdoor temper atur e decr eases. To pr event tiles fr om
softening and concr ete fr om cr acking, radiant f loor panel surf ace
temper atur es should not e xceed 29.5 C. Wher e hot w ater
temper atur es may exceed 29.5 C, the high-limit sensor r esets
circulating water temper atur e.
Wall panels and ceiling panels can be contr olled dir ectly by
a r oom ther mosta t. Surf ace temper atur es should not e xceed
38 C for w all panels and 50 C for ceiling panels. (T he actual
hot water temper atur e may be higher .)
Radiant heat panels that use electric resistance heating
elements ar e contr olled b y a two-position ther mosta t. As the
space temper atur e dr ops belo w the ther mosta t setpoint and
differential, the ther mosta t closes a switch to allo w cur rent flow
to heat the elements.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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 w ater cir cula ting thr ough the panel m ust be a t least
0.5 kelvins a bove the de w point temper atur e of the space to
pr event condensa tion on the panel.
A radiant panel may be used for both heating and cooling, as
shown in F igur e 14. A hea ting/cooling panel uses a f our -pipe
system r egulated b y two-way valves on both suppl y and r etur n.
A call for hea t at the ther mosta t closes the chilled water suppl y
and r etur n por ts and modula tes hot w ater suppl y and r etur n. A
call for cooling c loses the hot w ater suppl y and r etur n por ts
and modula tes chilled water suppl y and r etur n.
CHILLED
WATER
SUPPLY
HOT WATER
SUPPLY
CHILLED
WATER
RETURN
Fig. 14. Heating/Cooling Radiant Panel.
Constant-v olume, constant-temper atur e centr al fan systems
can be combined with radiant-panel heating or cooling to satisfy
ventila tion r equir ements. Occupanc y schedules deter mine 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 o vershoot. Ther mal la g in some r adiant
floors can be se veral hour s or mor e.

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.
409
THERMOSTAT
RADIANT PANEL
HOT WATER
RETURN
C3020

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