State Industries SHE50-100PE - Manual (Page 30)

28 Servicing should only be performed by a Qualified Service Agent
pressure switches
The water heaters covered in this manual are factory equipped with three pressure switches. Figure 4 on
page 9 shows the location of the three pressures switches on top of the water heater. This section of the
manual covers pressure switch construction, operation and the test procedures used to diagnose operational
problems associated with pressure switches.
construction & operation
Pressure switches activate in response to changes in pressure they sense through a plastic sensing tube
connected to sensing ports on the water heater, see Sensing Tubes on page 30.
A diaphragm divides the body of a pressure switch into two chambers. The chamber the sensing port is
attached to is sealed. The other chamber is vented to the atmosphere which allows the diaphragm to move
up and down. There is linkage that attaches the switch contacts to the diaphragm so that when the diaphragm
moves the contacts are activated, see Figure 30 below.
Activate means to change from the normal state. If the contacts are normally open, activating the contacts
will cause them to close. If the contacts are normally closed, activating the contacts will cause them to open.
Vent Port
Sensing Port Flexible Diaphragm
Wiring Terminals Switch Contacts
Normal State
(open switch contacts)
Activated State
(closed switch contacts)
Internal Linkage
Normal State Activated State
Blower Prover
Pressure Switch
Normally open contacts,
close on a rise in pressure .
Blocked Exhaust
Pressure Switch
Normally closed contacts,
open on a rise in pressure .
Blocked Intake Air
Pressure Switch
Normally closed contacts,
open on a fall in pressure .
CONSTRUCTION
TYPES AND APPLICATION
Figure 30