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5-26
UNIT COMPONENTS Magic-Pak: HW/HWC
SRM-HW/HWC 2/99
A Word About the “Switch Closed” Value
Switch Closed is the pressure at which the contacts close as the induced
draft blower comes up to speed. It is calculated by adding – 0.1" W.C. to
the sum of the switch’s Open value and its upper tolerance.
For example, for a Armstrong P/N 44819-005 switch:
– 0.1" + (– .60" + – .06") = – 0.76" W.C.
On a system operating correctly, it is likely that the manometer reading
will go to a value that is more than enough to close the switch.
A Word About the “Switch Open” Value
Switch Open is the pressure at which the contacts open as the negative
pressure decreases (pressure fall).
Procedure for Checking Pressure Switch
1. Position the manometer so it can be watched easily during testing.
Balance and zero the manometer according to the manufacturer’s
instructions.
2. Insert a test line with tee between the pressure switch and blower.
One piece of the tubing connects the pressure switch to the fitting at
the back side of the induced draft blower housing. Connect the tee to
the manometer as shown in Figure 5-20.
Note: Always keep the pressure switch in the circuit when measuring.
Switches incorporate internal bleeds to keep moisture out of
the pressure switch line.
3. Using Table 5-4 on page 5-22 and the formula provided above,
calculate the close value for the switch being worked with.
4. Start the furnace to call for heat. Observe the manometer reading
increase (increasing pressure rise) as the induced draft blower
comes up to speed. Ignition should start at a reading that is approxi-
mately the Close value that was earlier determined for the specific
switch in Step 3.
5. With the furnace running in the full heat cycle, observe the pressure
reading. Note this reading and compare it to the Open value shown
for that switch in Table 5-4 on page 5-22.
Using Armstrong P/N 44819-005 pressure switch for an example, the
reading on pressure fall when the contacts open can be between – 0.54"
W.C. and – 0.66" W.C.