Lennox CB29M-21 Unit Information Download Page 23

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III–OPTIONAL ECB29 ELECTRIC HEAT

A–Matchups and Ratings

Tables 2 through 11 show all approved CB29M and
CB30M to ECB29 matchups and electrical ratings.

B–Electric Heat Components

ECB29 parts arrangement is shown in figures 5 through
10. All electric heat sections consist of components
mounted to the electric heat vestibule panel and electric
heating elements exposed directly to the airstream.
208/230V electric heat sections may be equipped with
circuit breakers. The circuit breakers are designated by
CB in the model number. 460 and 575 volt electric heat
sections are equipped with fuses. The electric heat
section is connected to the unit via jack J2, which plugs
into plug P2 of the unit.

1–Electric Heat Sequencer Relays

(K32, K33, K34, K35, and K116) (208/230 volt only)

Relays K32, K33, K34, K35 and K116 are N.O. se-
quencer relays with a resistive element for a coil
and a bi-metal disk which actuates the contacts.
The relays are located on the electric heat vestibule
panel and are energized by a 24V heating demand
(W1, W2, and W3) via jack/plug 2. When energized,
the internal resistance heats the bi-metal disk causing
the contacts to close. When the relay is de-energized,
the disk cools and the contacts open. The relays
energize different stages of heat, as well as the
blower. The blower is always first on and last off.

2–Heat Blower Relay (K36) (460 & 575 volt only)

Contactor K36 is a Three Pole Double Throw (3PDT)
N.O. contactor located on the electric heat vestibule
panel. The contactor is equipped with a 24VAC coil
and is energized by a 24V heating demand (W1, W2,
and W3) via jack/plug 2. The contactor energizes the
blower, as well as the electric heat relay (K15). The
blower is always first on and last off.

3–Electric Heat Contactor (K15) (460 & 575 volt only)

Contactor K15 is a Three Pole Double Break (3PDB)
N.O. contactor located on the electric heat vestibule
panel. The contactor is equipped with a 24VAC coil
and is energized by a 24V heating demand (W1, W2,
and W3) via jack/plug 2. The contactor energizes the
heating elements.

4–Primary(S15) & Secondary(S20) Temperature Limits

Both the primary (S15) and secondary (S20) limits are
located on the electric heat vestibule panel and are
exposed directly to the airstream through an opening
in the panel. The high temperature limits are SPST
N.C. limits with the primary limit being an auto-reset
limit and the secondary limit being a “one-time” limit.
One-time limits need to be replaced when opened.
The limits are factory set and are not adjustable.

208/230 Volt Electric Heat Sections

Each stage of the 208/230 electric heat is pro-
tected by a primary (S15) and secondary (S20)
high temperature limit. Both S15 and S20 are located
in the same housing. Each stage use the same style of
limits. Both the primary and secondary limits are
wired in series with a heat element. When either S15
or S20 opens, the corresponding heat element is
de-energized. All other heating elements remain
energized. The primary high temperature limit opens
at 150

F + 5

F (65.5

C + 2.8

C) on a temperature rise

and automatically resets at 110

F + 9

F (43.3

C +

5.0

C) on a temperature fall. The secondary high

temperature limit opens at 333

F + 10

F (167.2

+ 5.6

C) on a temperature rise. If the secondary

limit opens it will need to be replaced.

460 and 575 Volt Electric Heat Sections

The 460 and 575 volt electric heat sections are
protected by three separate high temperature
limits: one primary (S15) and two secondary
(S20) limits. The primary limit is wired in series
with the contactor coil (K15); while the secondary
limits are wired in series with the electric heating
elements, after the contacts (K15-1). When S15
opens, all heating elements are de-energized. If either
S20 switch opens, the heating output is cut in half. If
both S20 switches open, all heating elements are de-
energized. The primary high temperature limit opens
at 150