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51M39 SERIES 5/2
Removing and Installing Thermostat
The thermostat hinges on tabs on the top of the subbase;
no tool is needed to remove the thermostat from the sub-
base. Pivot the bottom of the thermostat outward (releas-
ing the snaps), then lift up to remove.
To install it, first position the top tilted toward the wall brack-
et and align it until you feel the tabs and slots engage; then,
while the top is in place, pivot the bottom toward the wall
until the thermostat snaps into place.
Default Thermostat Settings
Default thermostat settings are in table 2 and the default
program is shown in table 3.
Table 2
Default Thermostat Settings
Mode Heat (Perm. Hold)
Heat Setpoint (Perm Hold) 70°F (or 21°C)
Cool Setpoint (Perm Hold) 78°F (or 26°C)
Fan Auto
Filter Reminder OFF
Maintenance Reminder OFF
Equipment Protection Timers Reset Back to Zero
Autochangeover OFF
Table 3
Default Program Settings (Time & Temp.)
Programs Time Temp.− Heat Temp.− Cool
All days − A 6:00am 70_ F / 21°C 78_F / 26°C
All days − B 8:00am 62_ F / 17°C 85_F / 29°C
All days − C 6:00pm 70_ F / 21°C 78_F / 26°C
All days − D 10:00pm 62_F / 17°C 82_F / 28°C
Thermostat Output Table
Table 4 depicts the 51M39 thermostat output states for var-
ious input conditions. The following notes described terms
used in the table.
Table 4
Thermostat Outputs
Condition W1 W2 Y1 Y2 G
Gas Heat, Auto Fan
Heat Demand, SMALL X
Heat Demand, LARGE X X
Cool Demand, SMALL X X
Cool Demand, LARGE X X X
No Demand
Gas Heat, Continuous Fan
Heat Demand, SMALL X X
Heat Demand, LARGE X X X
Cool Demand, SMALL X X
Cool Demand, LARGE X X X
No Demand X
Electric Heat, Auto Fan
Heat Demand, SMALL X X
Heat Demand, LARGE X X X
Cool Demand, SMALL X X
Cool Demand, LARGE X X X
No Demand
Electric Heat, Continuous Fan
Heat Demand, SMALL X X
Heat Demand, LARGE X X X
Cool Demand, SMALL X X
Cool Demand, LARGE X X X
No Demand X
NOTE − X = output is activated with 24VAC.
NOTE − The temperature ranges expressed in the follow-
ing definitions of SMALL/LARGE" demands are for guid-
ance only; actual temperatures may vary.
With a SMALL heat demand, temperature is:
below setpoint –0.5F but above setpoint –1.5F
AND 30-minute upstage timer HAS NOT expired.
With a LARGE heat demand, temperature is:
less than setpoint –1.5F
OR 30-minute upstage timer HAS expired.
With a SMALL cool demand, temperature is:
above setpoint +0.5F but below setpoint +1.5F
AND 30-minute upstage timer HAS NOT expired.
With a LARGE cool demand, temperature is:
above setpoint +1.5F
OR 30-minute upstage timer HAS expired.
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