Instructions 7218-2 Page 2
B3. If minimum gas pressure at regulator inlet is not at least 2 osi more than maximum air pressure at the burner, install a bleeder in air pressure impulse line as shown in Figure 2. Amount of bleed is set so maximum impulse pressure is 2 osi less than gas inlet pressure (see Bleeder bulletin). Use this arrangement only when combustion chamber pressure is atmospheric.
Control valve
air and gas mix in venturi
air
low pressure gas
Mixer
built-in Limiting ori fi ce
Sealed-in Premix burner
Figure 4.
Control valve
Bleeder
drip leg
Air/Gas ratio regulator
air
lock open
Figure 2.
LIGHTUP and ADJUSTMENT D1. Light the burner. Start blower per instructions. All gas lines to 7218 Regulators must be purged. Light pilot per instructions. Open gas shutoff valve. If burner dœs not light in a few seconds, close nearest upstream gas shutoff valve to prevent accumulation of unburned gas. Purge chamber for a few minutes; then open limiting orifice gas valve a few more turns and reopen gas shutoff valve.
low pressure gas
10D min.
2 osi drop
4D min.
spring adjustment
Repeat this procedure until main flame lights.
B4. If gas and air pressures are not known, make a "trial and error" setting of bleeder by lighting burner and bringing it to high fire. Then reduce impulse to regulator by using bleed until you find a bleeder setting that produces an effect on the flame. This is the correct bleeder setting for all firing rates. Readjust limiting orifice gas valve. (If inlet gas pressure is not reasonably constant, set bleeder for lowest expected inlet pressure.) B5. If combustion chamber pressure is not atmospheric and if gas pressure is less than air pressure, use a 3-pipe bleeder as shown in Figure 3. This arrangement allows fluctuations in combustion chamber pressure to be neutralized by applying them to both sides of regulator controlling diaphragm .
D2. Adjust high fire air/gas ratio. Slowly turn air valve to high fire position while adjusting limiting orifice gas valve as needed to maintain desired air/gas ratio. Replace limiting orifice valve cover. D3. Adjust low fire air/gas ratio only after step D2 is completed. Turn air valve to low fire position. Remove gas diaphragm cover plug on the bottom of the regulator. Use a screwdriver to turn spring adjusting plug (clockwise for more gas, counterclock-wise for less gas) to obtain desired low fire air/gas ratio. Replace gas diaphragm cover plug. D4. On-Off Control. Set regulator to closed position by follow- ing the procedure in step D3, turning spring adjusting plug counter-clockwise until spring compression is relaxed. The 7218 Regulator is not intended to be used for on-off control. To obtain enhanced shutoff capabilities, a 7216 Regulator is recom- mended. The 7216 uses an extension spring to achieve good on-off control with zero impulse pressure. The 7216, however, is not intended to be a tight shutoff device. For extended periods at no flow conditions, an approved shutoff valve should be used to prevent fuel leakage into furnace. D5. Dual-Fuel ™ Burners. A Dual-Fuel burner operated on gas with atomizing air left on may require gas to match the atomizing air, plus a varying amount to match main air at any firing rate.
To check regulator operation, use 1⁄8 " pipe taps provided on body for inlet and outlet pressure readings.
INSTALLATION, for PREMIX BURNERS For an open type premix burner, regulator vent can be left open or piped outdoors. For a sealed-in premix burner, vent should be cross- connected to combustion chamber to correct regulator action for difference between furnace and atmospheric pressure. When one regulator is used for each burner, connect vent through burner mounting plate (Figure 4) or to combustion chamber.
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