Bosch SM SERIES - ECM Interface Board; Thermostat Connections

Electrical | 13SM Split AH/AV Air Handler
8733826256 (2020/06)SM Split AH/AV Air Handler
Subject to change without prior notice
ECM Interface Board
In addition to providing a connection point for
thermostat wiring, the interface board also
translates thermostat inputs into control
commands for the Electronic Commutated Motor
(ECM) fan motor and provides thermostat signals
to the unit’s UPM board (located in the SM
condensing section).
Fig. 13 ECM Interface Board
[1] Motor Harness Plug
[2] Blower CFM Adjustment
[3] Motor Settings
[4] Dehumidification Indication
[5] Thermostat Contact Inputs
[6] CFM Count Indicator
[7] Thermostat Input Status Indication
[8] Reheat Digital Outputs
[9] Thermostat Outputs
[10] 24 VAC
[11] Dehumidification Method Selector
Thermostat Connections
Thermostat wiring is connected to the 10-pin
screw type terminal block on the lower-center
portion of the ECM Interface Board. (See Fig. 13)
In addition to providing a connecting point for
thermostat wiring, the interface board also
translates thermostat inputs into control
commands for the variable speed programmable
ECM DC fan motor and displays an LED indication
of operating status.
The thermostat connections and their functions
are as follows:
Table 4 Thermostat Connections
If the unit is being connected to a thermostat with
a malfunction light, this connection is made at the
unit alarm output.
On the above series units the thermostat wiring
connections are made at the air handling section.
This will allow the capability to utilize both the
condensate overflow switch and the alarm output.
Connection Function
Y2
Second-Stage Compressor
Operation
Y1
First-Stage Compressor Operation
G
Fan
O
Reversing Valve (energized in
cooling)
W1
Auxiliary Electric Heat
(runs as supplemental heat when
needed along with compressor)
W2
Second-Stage Auxiliary Heat/
Emergency Heat
C
Transformer 24 VAC Common
(extra connection)
C1
Transformer 24 VAC Common
(primary connection)
R
Transformer 24 VAC Hot
HUM
Passive Dehumidification Mode
If the thermostat is provided with a
malfunction light powered off of the
common (C) side of the transformer, a
jumper between “R” and “COM” terminal of
“ALR” contacts must be made.
If the thermostat is provided with a
malfunction light powered off of the hot (R)
side of the transformer, then the
thermostat malfunction light connection
should be connected directly to the (ALR)
contact on the unit’s UPM board.