Bryant 312AAV - Manual (Page 27)

Place LHT in ON position. Jumper R to W/W1 and W2 to
check high-gas-heat temperature rise. To check low-gas-
heat temperature rise, remove jumper to W2. Determine air
temperature rise for both high and low fire. Do not exceed
temperature rise ranges specified on unit rating plate for
high-and low-fire.
The furnace must operate within the temperature rise ranges
specified on the furnace rating plate. Determine the air
temperature as follows:
a. Place thermometers in return and supply ducts as close to
furnace as possible. Be sure thermometers do not see
heat exchangers so that radiant heat does not affect
readings. This practice is particularly important with
straight-run ducts.
b. When thermometer readings stabilize, subtract return-air
temperature from supply-air temperature to determine
air temperature rise.
NOTE: If the temperature rise is outside this range, first check:
(1.) Gas input for low-and high-fire opeation.
(2.) Derate for altitude if applicable.
(3.) Return and supply ducts for excessive restrictions
causing static pressures greater than 0.50–in. wc.
c. Adjust air temperature rise by adjusting blower speed.
Increase blower speed to reduce temperature rise. De-
crease blower speed to increase temperature rise. Fro
high fire, speed selection can be med-high, med
(5–speed blowers only), or med-low (factory setting).
For low fire, speed selection can be low (factory setting),
med-low, or med (5–speed blowers only).
WARNING: Disconnect 115-v electrical power before
changing speed tap. Failure to follow this warning could
result in personal injury.
d. To change motor speed selection for high heat, remove
blower motor lead from control HI-HEAT terminal. (See
Fig. 22 and 23.) Select desired blower motor speed lead
from 1 of the other terminals and relocate it to the
HI-HEAT terminal. (See Table 11 for lead color identi-
fication). Reconnect original lead to SPARE terminal.
Follow this procedure for proper selection of COOL and
LO-HEAT speed selection.
CAUTION: Recheck temperature rise. It must be within
limits specified on the rating plate. Recommended opera-
tion is at the midpoint of rise range or above.
8. Set thermostat heat anticipator.
a. When using a nonelectric thermostat, the thermostat
heat-anticipation must be set to match the amp draw of
the electrical components in the R-W/W1 circuit. Accu-
rate amp draw readings can be obtained at the wires
normally connected to thermostat subbase terminals, R
and W/W1.
Fig. 16 illustrates an easy method of obtaining actual
amp draw. The amp reading should be taken after blower
motot has started and furnace is operating in low-heat.
To operate furnace in low-heat, first move LHT to ON
position, then connect ammeter wires as shown in Fig.
37. The thermostat anticipator should NOT be in th
circuit while measuring current. If thermostat has no
subbase, the thermostat must be disconnected from R
and W/W1 wires during current measurement. Return
LHT to OFF after completing the reading. See thermo-
stat manufacturer’s instructions for adjusting heat-
anticipator.
b. When using an elctronic thermostat, set cycle rate for 3
cycles per hr.
V. CHECK SAFETY CONTROLS
The flame sensor, gas valve, and pressure switch were all checked
in the Start-up procedure section as part of normal operation.
1. Check Main Limit Switches
This control shuts off combustion control system and energizes
air-circulating blower motor, if furnace overheats.
Recommended method of checking this limit control is to gradu-
ally block off return air after furnace has been operating for a
period of at least 5 minutes. As soon as limit control has shut off
burners, return-air opening should be unblocked to permit normal
air circulation. By using this method to check limit control, it can
be established that limit is functioning properly and will operate if
there is a restricted return-air supply or motor failure. If limit
control does not function during this test, cause must be deter-
mined and corrected.
TABLE 11–SPEED SELECTION
COLOR SPEED AS SHIPPED
White Common BLW
Black High COOL
Yellow Med-High SPARE
Orange† Med SPARE
Blue Med-Low HI-HEAT
Red Low* LO HEAT
* Continuous-blower speed-as shipped default
† Available on 5–speed blowers only.
TABLE 12–GAS RATE (CU FT/HR)
SECONDS
FOR 1
REVOLUTION
SIZE OF TEST DIAL SECONDS
FOR 1
REVOLUTION
SIZE OF TEST DIAL
1
Cu Ft
2
Cu Ft
5
Cu Ft
1
Cu Ft
2
Cu Ft
5
Cu Ft
10 360 720 1800 50 72 144 360
11 327 655 1636 51 71 141 355
12 300 600 1500 52 69 138 346
13 277 555 1385 53 68 136 340
14 257 514 1286 54 67 133 333
15 240 480 1200 55 65 131 327
16 225 450 1125 56 64 129 321
17 212 424 1059 57 63 126 316
18 200 400 1000 58 62 124 310
19 189 379 947 59 61 122 305
20 180 360 900 60 60 120 300
21 171 343 857 62 58 116 290
22 164 327 818 64 56 112 281
23 157 313 783 66 54 109 273
24 150 300 750 68 53 106 265
25 144 288 720 70 51 103 257
26 138 277 692 72 50 100 250
27 133 267 667 74 48 97 243
28 129 257 643 76 47 95 237
29 124 248 621 78 46 92 231
30 120 240 600 80 45 90 225
31 116 232 581 82 44 88 220
32 113 225 563 84 43 86 214
33 109 218 545 86 42 84 209
34 106 212 529 88 41 82 205
35 103 206 514 90 40 80 200
36 100 200 500 92 39 78 196
37 97 195 486 94 38 76 192
38 95 189 474 96 38 75 188
39 92 185 462 98 37 74 184
40 90 180 450 100 36 72 180
41 88 176 439 102 35 71 178
42 86 172 429 104 35 69 173
43 84 167 419 106 34 68 170
44 82 164 409 108 33 67 167
45 80 160 400 110 33 65 164
46 78 157 391 112 32 64 161
47 76 153 383 116 31 62 155
48 75 150 375 120 30 60 150
49 73 147 367
—27—