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C-Heating Control Box Components
Figures 17, 18 and 19.
1-Transformer T3
460 (G) and 575 (J) voltage units use a line voltage to 230V
autotransformer to power the combustion air blower. The
autotransformer is connected directly to line voltage and is
powered at all times. It has an output rating of 0.5A In units
equipped with two heat exchangers, T3 supplies power to
the first stage combustion air blower (B6). T3 is locatd in
the unit control box in units with 12.5 ton and under capac
ity. T3 is located in the heating control box in all other units.
2-Transformer T12 (1853, 2553, 2753, 3003)
All 15 ton and larger units use a line voltage to 24V trans
former to power the heating control circuits. This trans
former is also connected directly to line voltage and is
powered at all times. It has a maximum VA rating of 30VA.
3-Transformer T13
Transformer T13 is used in 460 (G) and 575 (J) voltage
units to stepdown line voltage to 230V to power the sec
ond stage combustion air blower (B15). The autotrans
former is connected directly to line voltage and is powered
at all times. It has an output rating of 0.5A T13 is only used
on the second stage of units equipped with two heat ex
changers.
4-Combustion Air Blower Relay K13
(1st Stage Heat Relay)
Relay K13 is a DPDT relay located inside the heating control
box of 15 ton and larger units. K13 is energized by 1st stage
heating demand from thermostat (pilot relay K77) and is en
ergized throughout the heating demand. When energized,
K13 normally open contacts close to energize combustion
air blower and begin a heating sequence. A centrifugal
prove" switch located in combustion air blower motor closes
as the motor nears full speed to prove" combustion air blow
er operation. When the switch closes, the ignition control and
gas valve are energized to begin a heating sequence.
5-Indoor Blower (Limit) Relay K20
Relay K20 is a SPDT relay also located in the heating con
trol box. Relay K20 is energized when either the primary or
secondary high temperature limits trip. When K20 is ener
gized, a set of N.O. contacts close to energize relay K3 in
the unit (and relay K65 if so equipped). Relay K3 is respon
sible for energizing the indoor blower. When either the pri
mary or secondary limit are reset relay K20 is energized
and indoor blower starts.
6-2nd Stage Heat Relay K19
Relay K19 is also a SPDT relay located in the heating con
trol box of fifteen ton and larger units. Relay K19 is ener
gized by 2nd stage heating demand (pilot relay K49) and is
energized throughout 2nd stage heating demand. When
the thermostat calls for second stage heating, heat delay
relay DL3 is energized.When time has elapsed, DL3
closes to energize K19.
Relay K19 functions differently depending on whether the
unit is equipped with single or dual heat exchangers. In
single heat exchanger models, K19 normally open con
tacts close to energize the 2nd stage operator of the gas
valve. When 2nd stage demand stops relay K19 is de-en
ergized, time delay DL3 resets and the gas valve 2nd
stage operator closes.
In dual heat exchanger models, K19 normally open con
tacts close to energize the second stage combustion air
blower (B15). Once B15 is energized, the second stage
heat exchanger begins a normal ignition sequence (identi
cal to first stage ignition sequence).
7-1st Stage Heat Blower Relay K56
Relay K56 is used only in dual heat exchanger models.
K56 is a SPST 24V relay connected in parallel with the first
stage operator of the first stage gas valve (GV1-W1). The
contacts of K56 are connected in parallel with the contacts
of relay K57 so that if either gas valve is energized, the in
door blower will automatically be energized. When K561
closes, blower delay relay K25 is energized and the indoor
blower is powered after a time delay.
8-2nd Stage Heat Blower Relay K57
Relay K57 is used only in dual heat exchanger models.
K57 is a SPST 24V relay connected in parallel with the first
stage operator of the second stage gas valve (GV3-W1).
The contacts of K57 are connected in parallel with the con
tacts of relay K56 so that if either gas valve is energized,
the indoor blower will automatically be energized. When
K571 closes, blower delay relay K25 is energized and the
indoor blower is powered after a time delay.
9-2nd Stage Gas Heat Delay DL3
DL3 is a SPST time delay located in the heating control
box. Upon receiving increased heating demand from relay
K19, DL3 initiates a 180 second time delay before closing
its N.O. contacts. The purpose of DL3 is to prevent fre
quent cycling of 2nd stage heat. When DL3 contacts close,
2nd stage operator of gas valve is energized and 2nd
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