Armstrong World Industries 302, 243, 242, 122, 123, 203, 182 A Word About the “Switch Closed” Value

UNIT COMPONENTS Magic-Pak: HW/HWC

5-26

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.

SRM-HW/HWC

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