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All WF models are equipped with a pressure sensing device. This device performs the two important functions
shown below:
Prevents main burner operation in the event of inadequate combustion air or a failed combustion
air blower.
Prevents main burner operation in the event the vent terminal should become obstructed.
This pressure device is commonly referred to as a pressure switch. The switch consists of normally open
contacts which close when a specified amount of negative pressure (vacuum) is applied to the mechanical
side of the device. The switch will remain closed as long as the required amount of negative pressure is
present. If that negative pressure or vacuum reduces below the required amount, the switch contacts will
open, thus shutting down the unit. The source of this negative pressure is created within the combustion air
blower housing, and is transferred from the blower housing to the pressure switch through a 3/16" diameter
silicone tube. If any unusual restriction is applied any point downstream of the combustion air blower, the
negative pressure or vacuum within the blower housing will decrease, opening the switch contacts and shutting
down the unit.
All WF series use the same pressure switch. The pressure switch contacts close at negative .62" W.C. and
open at negative .52" W.C. for units installed from 0 – 6,000 feet elevation above sea level. If unit is installed
above 6,001 feet the unit must be converted using a high altitude pressure switch, Bard kit number, 8620-189.
The pressure switch included with this kit is designed for the contacts to close at a negative .52" W.C. and
open at .42" W.C.
PRESSURE MEASUREMENT WITHIN THE SYSTEM
To measure the pressure within the system, a pressure gauge with a range of 0 to 1 inch water column in .02
inch W.C. increments is required.
This gauge is to be connected to the silicone tubing between the pressure switch and the combustion air
blower. Refer to Figure 1 on Page 4.
A small plastic or copper tee and a 3/16" diameter piece of hose is used to connect the gauge between the
combustion air blower and the pressure switch.
With the gauge in place, start the unit and monitor the pressure within the system. The ignition device will be
activated once the pressure exceeds the switch contact close points specified above. As the furnace warms
up, the pressure will drop about .2" W.C.
The pressure within the system should not drop to or below the switch contacts open point shown above. If
the switch contacts open above or below the specified set point ± .05" W.C., the switch must be replaced.
If the pressure within the system drops to the switch open set points, the following items should be checked:
1. Any obstruction in the vent terminal
2. Any excessive lint, dirt or scale on the combustion air blower wheel
3. Any obstructions or moisture inside the 3/16" silicone tubing
WF SERIES
UNDERSTANDING THE PRESSURE CONTROL
Manual
2100-494
Page
3 of 4
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