White Rodgers 1F85-277 Guía para resolver problemas descargar pdf (Pagina 57)

INTEGRATED

FURNACE CONTROLS

21M51U-843

OPERATION

OPTION SWITCHES

Option switches on the 50M51-843 control are used to deter-

mine the length of the delay-to-fan-off periods. The following

tables show the time periods that will result from the various

switch positions.

When using a single stage thermostat, second stage delay is

based on the setting of switch S1-1, S1-2 shown below.

OPTION SWITCHES S1-3 & S1-4 POSITION

HEAT delay-

to-fan-off:

On “S1,”

set switch #:

Off

90 sec.*

120 sec.

150 sec.

180 sec.

*Factory default setting

OPTION SWITCHES S1-1 & S1-2 POSITIONS

2nd Stage delay for single stage thermostats

Delay Time:

On "S1" set switch #

Off* Off Off

10 min On Off

Auto min Off On

20 min On On

*Factory default setting– two stage thermostat

Average

Calculated Duty

Cycle % Equals

Or less

than

Low to High

Stage Delay

Demand

0 38 12 minutes Light

38 50 10 minutes Light to Average

50 62 7 minutes Average

62 75 5 minutes Average to

Heavy

75 88 3 minutes Heavy Light

88 100 1 minute Heavy

HEAT MODE

In a typical system, a call for ﬁrst stage heat is initiated by closing

the W1 thermostat contacts. The inducer blower is energized

at high speed and the control waits for the low pressure switch

contacts to close. The humidiﬁer (optional) is also energized

at this time. Once the low pressure switch contacts close, a

15-second pre-purge is initiated. Then the inducer changes to

low speed and the 120V ignitor is powered.

At the end of the ignitor warm-up time, the ﬁrst stage of the two

stage manifold gas valve is energized (low ﬁre). Flame must be

detected within 4 seconds. If ﬂame is detected, the 45-second

HEAT delay-to-fan-on period begins. After the delay-to-fan-on

period ends, the 50M51 control will energize the circulator fan

at low heat speed. The electronic air cleaner (optional) will also

energize at this time.

For a two-stage thermostat, a call for second stage heat (W1

and W2) after a call for ﬁrst stage heat will energize the inducer

at high speed and the circulator at high heat speed. The sec-

ond stage pressure switch contacts will close and energize the

second stage gas valve (high ﬁre).

For a single-stage thermostat, when a call for heat occurs

(W1), a 10, 20 minute or auto mode heat staging timer will be

activated (timing is selectable with option switches S1-1 and

S1-2 positions). Following this delay, the second stage heat is

energized as above.

The AUTO model algorithm is a method of energizing the second

stage gas valve based on the recent average of the heating

duty cycle. During a typical heating day, the low to high stage

delay is determined by using the average calculated duty cycle

from the table below.

Once the speciﬁed delay time has expired the second stage

valve will be energized.

See the table below for the different duty cycles.

When the second stage of the thermostat is satisﬁed, the inducer

motor is reduced to low speed and the second stage gas valve

is de-energized.

On the 50M51 control, the circulator will remain at high heat

speed for 30 seconds following the opening of the second stage

gas valve and then is reduced to low heat speed.

When the ﬁrst stage of the thermostat is satisﬁed, the ﬁrst stage

gas valve is de-energized and the HEAT delay-to-fan-off begins

timing. The inducer will postpurge for an additional 15 seconds,

then the inducer and humidiﬁer will turn off. Upon completion

of the HEAT delay-to-fan-off period, the 50M51 circulator is

turned off. The electronic air cleaner on the control is also de-

energized at this time.

If ﬂame is not detected during the trial-for-ignition period or if

the ﬂame is detected/sensed and then lost before completion

of 10 seconds of establishment, the gas valve is de-energized,

the ignitor is turned off, and the control goes into the “retry”

sequence.

The “retry” sequence provides a 60-second wait with the inducer

interpurge following an unsuccessful ignition attempt (ﬂame not

detected). After this wait, the ignition attempt is restarted. Two

retries will be attempted before the control goes into system

lockout.

If ﬂame is established for more than 10 seconds after ignition,

the 50M51 controller will clear the ignition attempt (or retry)

counter. If ﬂame is lost after 10 seconds, the control will restart

the ignition sequence.

A momentary loss of gas supply, ﬂame blowout, or a shorted or

open condition in the ﬂame probe circuit will be sensed within

2.0 seconds. The gas valve will de-energize and the control will

restart the ignition sequence. Recycles will begin and the burner

will operate normally if the gas supply returns, or the fault condi-

tion is corrected, before the last ignition attempt. Otherwise, the

control will go into system lockout.

If the control has gone into system lockout, it may be possible

to reset the control by a momentary power interruption of 10

seconds or longer. Refer to SYSTEM LOCKOUT AND DIAG-

NOSTIC FEATURES.

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