Page 18
BURNER BOX PRESSURE
(Negative inches water gauge
−1.0−0.20 −0.4 −0.6 −0.8
2.5
3.0
3.5
2.6
2.7
2.8
2.9
3.1
3.2
3.3
3.4
GAS VALVE OUTPUT
MANIFOLD PRESSURE (positive inches water column)
NORMAL OPERATION (Natural Gas Units)
measured on right side of burner box)
Gray area indicates normal operating range + 10% of manifold pressure
The purpose of this chart is to explain unit operation . Each unit may vary depending
on installation, altitude, intake/exhaust configuration and other factors.
OPERATION AT THIS EXTREME
MAY INDICATE A BLOCKED
OUTLET OR OTHER PROBLEM
OPERATION AT THIS EXTREME
MAY INDICATE A BLOCKED
INLET OR OTHER PROBLEM
FIGURE 21
−1.0−0.20 −0.4 −0.6 −0.8
6.0
6.5
7.0
6.1
6.2
6.3
6.4
6.6
6.7
6.8
6.9
NORMAL OPERATION (L.P. Gas Units)
BURNER BOX PRESSURE
(Negative inches water column
GAS VALVE OUTPUT
MANIFOLD PRESSURE (positive inches water column)
measured on right side of burner box)
OPERATION AT THIS EXTREME
MAY INDICATE A BLOCKED
OUTLET OR OTHER PROBLEM
OPERATION AT THIS EXTREME
MAY INDICATE A BLOCKED
INLET OR OTHER PROBLEM
Gray area indicates normal operating range + 10% of manifold pressure
The purpose of this chart is to explain unit operation . Each unit may vary depending
on installation, altitude, intake/exhaust configuration and other factors.
FIGURE 22
Figures 21 and 22 show how gas valve output changes as
burner box pressure changes. Generally, a lower burner
box pressure produces a leaner gas/air mixture and a
higher burner box pressure produces a richer mixture. A
procedure showing how to check manifold pressure is
shown on page 29.
15−Differential Pressure Switch (S64)
(Combustion Air Prove Switch)
G26 series units are equipped with a differential pressure
switch located on the vestibule panel. The switch is con-
nected to the combustion air blower housing by means of a
flexible silicon hose. A separate hose connects the pres-
sure switch to the burner box and the gas valve regulator.
The switch monitors air pressure in the combustion air
blower housing and burner box.
The switch is a single-pole single-throw normally open
pressure switch electrically connected in series with the
ignition control. The purpose of the switch is to prevent
burner operation if the combustion air blower is not operat-
ing or if sufficient combustion air is not available,
On start-up, the switch senses that the combustion air blow-
er is operating. It closes a circuit to the ignition control when
the difference in pressure across the pressure switch in-
creases above 0.2 in. w.c. The pressure sensed by the
switch is relative to the pressure in the burner box. In order
for the furnace to operate, the larger negative must al-
ways be on the combustion air blower side of the
switch. If the flue or air inlet become obstructed during op-
eration, the switch senses a loss of pressure differential
(drops below 0.20 in. negative w.c.) and opens the circuit to
the ignition control.
FIGURE 23
DIFFERENTIAL PRESSURE SWITCH CIRCUITRY
BURNER
BOX
GAS
VALVE
SENSING
HOSE
COMBUSTION
AIR PRES-
SURE SENS-
ING HOSE
COMBUSTION
AIR BLOWER
BURNER
BOX
SENSING
HOSE
GAS
VALVE
TEE
PRESSURE
SWITCH
SENSING
HOSE
HOSE
BARB
DIFFERENTIAL
PRESSURE
SWITCH
LEFT SIDE OF PRESSURE SWITCH = MORE NEGATIVE
RIGHT SIDE OF PRESSURE SWITCH = LESS NEGATIVE (Closer to Zero)
The switch is factory set and is not adjustable. It is a safety
shut-down control and MUST not be bypassed.
Figure 24 shows the pressure differential required to ob-
tain unit operation. If the switch does not successfully
sense the required differential, the switch cannot close
and the furnace cannot operate.
When measuring the pressure differential, readings
should be taken at the pressure switch.
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