Bryant 120 instruction manual Redundant Automatic Gas Control Valve

a.Turn gas valve ON/OFF switch to OFF.

b.Remove manifold pressure tap plug from gas valve.

c.Connect a water column manometer or similar device to manifold pressure tap.

d.Turn gas valve ON/OFF switch to ON.

e.Move setup switch LHT (SW-1) on furnace control to ON position to lock furnace in low-heat operation. (See Fig. 34 and Table 10.)

f.Manually close blower door switch.

g.Jumper R and W/W1 thermostat connections on control to start furnace. (See Fig. 34.)

h.Remove regulator adjustment cap from low-heat gas valve pressure regulators. (See Fig. 54.) Turn low-heat adjusting screw (3/16 in. or smaller flat tipped screwdriver) counterclockwise (out) to decrease input rate or clockwise (in) to increase input rate.

A06667

Fig. 54 - Redundant Automatic Gas Control Valve

NOTE: DO NOT set low-heat manifold pressure less than 1.4-in wc or more than 1.7-in. wc for natural gas. If manifold pressure is outside this range, change main burner orifices.

i.Install low-heat regulator adjustment cap.

j.Leave manometer or similar device connected and proceed to Step 4.

NOTE: If orifice hole appears damaged or it is suspected to have been re-drilled, check orifice hole with a numbered drill bit of correct size. Never re-drill an orifice. A burr-free and squarely aligned orifice hole is essential for proper flame characteristics.

4.Verify natural gas low-heat input rate by clocking meter. NOTE: Gas valve regulator adjustment caps must be in place for proper input to be clocked.

a.Turn off all other gas appliances and pilots served by the meter.

b.Run for 3 minutes in low-heat operation.

c. Measure time (in sec) for gas meter to complete 1 revolution and note reading. The 2 or 5 cubic feet dial provides a more accurate measurement of gas flow.

d.Refer to Table 12 for cubic ft of gas per hr.

e.Multiply gas rate cu ft/hr by heating value (Btuh/cu ft) to obtain input. If clocked rate does not match required input from Step 1, increase manifold pressure to increase input or decrease manifold pressure to decrease input. Repeat steps b through e until correct low heat input is achieved. Re-install low-heat regulator seal cap on gas valve.

5.Set low-heat temperature rise. The furnace must operate within the temperature rise ranges specified on the furnace rating plate. Do not exceed temperature rise ranges specified on unit rating plate for high-and low-heat. Determine the temperature rise as follows:

NOTE: Blower access door must be installed when taking
temperature rise reading. Leaving blower access door off will	312A
result in incorrect temperature measurements.

!WARNING

ELECTRICAL SHOCK HAZARD

Failure to follow this warning could result in personal injury or death.

Disconnect 115-v electrical power before changing speed tap.

BURNER

ORIFICE

A93059

Fig. 55 - Orifice Hole

a. Verify unit is running in low heat per Step 4. Place thermometers in return and supply ducts as close to furnace as possible. Be sure thermometers do not see radiant heat from heat exchangers. Radiant heat affects temperature rise 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 heat operation.

(2.) Derate for altitude if applicable.

(3.) Return and supply ducts for excessive restrictions causing static pressures greater than 0.50-in. wc.

37