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1. Automatic thermal cutout
2. Heat limiters
3. Air flow switch
The standard electric heater section construction Typical options for the electric heater
includes: section are:
Maximum KW per
contactor and per fuse
block (at least one
contactor is required per
step of heating):
Notes:
1. You must order the split shipment of the fan
and coil sections to get electric heat on unit
size 03 through unit size 18.
2. All electric heat controls are built into the heater
control box.
3. Specify two pole break contactors or three pole
break contactors. Two pole contactors only
de-energize, while three pole contactors
disconnect.
4. Be aware of requirements for heater circuits
rated more than 48 amps. According to
N.E.C. Code and U.L. you must subdivide
the heater elements of such circuits. N.E.C.
Code requires that the sub-circuit fuses be
furnished by the heater manufacturer.
1. Fiberglass insulation in cabinet
2. At least one magnetic contactor per step of
heating (two or three pole break)
3. Three phase fuse blocks and fuses
4. Transformer with primary fusing
5. Manual reset thermal cutout in control circuit
Commonly Used Control Systems:
A. For small loads (up to 20 KW),consider a
multi-stage thermostat.
B. For medium loads (up to 200 KW), consider a
modulating thermostat that operates a step
controller.
C. For fine control of large loads (above 75 KW),
consider a combination step control and SCR
vernier system. This scheme has 80% of heater
capacity controlled by conventional step controller,
with the other 20% on a solid state SCR controller.
D. For 100% proportional control of any size,
consider a total SCR control.
E. Heatcraft does not supply :
multi-stage thermostats
modulating thermostats
But you will want to order contactors and fuse
blocks from Heatcraft that are appropriate for
for the control scheme specified in the field.
Accessories- Electric Heat
Maximum KW
Component 208 V 240 V 460 V 480 V
Low amps contactor 9.0 10.4 19.9 20.8
High amps contactor 17.3 19.3 38.2 39.9
Fuse block 17.3 19.9 38.2 39.9
Electric Heater Selection Example
A. How many KW is needed to increase temperature of 17000 CFM by 27 °F for Air Handler Model Size 34?
17000 X 27 is: 153 KW IS REQUIRED
3000
B. How many steps required at 10°F rise per step?
300 X 153 is: 2.7 STEPS ROUNDS UP TO 3 STEPS
17000
C. How many steps required at 460 volts with low amps contactors?
153 is: 7.7 STEPS ROUNDS UP TO 8 STEPS
19.9
D. How many steps required at 460 volts with high amps contactors?
153 is: 4.0 STEPS
38.2
E. How many fuse blocks required?
153 is: 4.0 FUSE BLOCKS
38.2
°F. Order the 153 KW to get the 27 degree rise.
Throw out the 3 steps (figured at 10°F. rise in part B), because it is smaller than the two values figured
per contactor size (parts D & E). Decide whether you want high amps or low amps contactors, if you
want low amps contactors, round the 7.7 to 8 and order 8 of the low amps contactors (you now have a
possible 8 steps of control). If four steps of control are satisfactory, order the high amps contactors. In
either case, the application requires four fuse blocks (part E).