Controls - Pressure Switches - All Models - A.O. Smith Cyclone Xi Service Handbook

CONTROLS – PRESSURE SWITCHES – ALL MODELS
All BTH models are provided with four pressure
switches. These switches are essential to the
safe and proper operation of the unit. The
switches are wired to the central control board
(CCB) individually and each is monitored
individually. The (CCB) is set up to shut the unit
down whenever there is a failure of any of the
switches and declare a fault for each individual
switch. It is important to understand the purpose
of each switch.
Low Gas Pressure
Sw itch
BLOWER PROVING SWITCH
The Blower Proving Switch is provided on the
heater to verify that the fan is operating. It is a
positive pressure switch whose electrical contacts
are normally open. The blower proving switch
electrical contacts will close on a rise in pressure
as the blower increases the pressure in the burner.
This switch is connected to the burner tap by a
piece of Tygon (soft plastic) tubing. This tubing
must be connected in order for the switch to close
the electrical contacts. The controller requires that
the electrical contacts on this pressure switch be
open before it will allow the blower to come on. The
control will declare a fault on the UIM if either
condition occurs.
A.O. Smith – Technical Training Department
CYCLONE Xi COMMERCIAL GAS WATER HEATER
BTH 120 thru 500 SERVICE HANDBOOK
Pressure
Sw itches
Servicing should only be performed by a Qualified Service Agent
BLOCKED OUTLET PROVING SWITCH
The blocked outlet proving switch electrical contacts
are normally closed. The blocked outlet proving
switch electrical contacts will open on a rise in
pressure. Check to see if the condensate is
allowed to flow freely from the exhaust elbow and
for obstructions in the exhaust venting and exhaust
vent terminal. Also check that the equivalent feet of
vent pipe for the specific model has not been
exceeded. Check the vent length tables on page
10
BLOCKED INLET PROVING SWITCH
The Blocked Inlet Proving switch electrical contacts
are normally closed.
switch electrical contacts will open when an
increase in negative pressure (vacuum) occurs
in the intake vent pipe. The switch is connected to
the pressure tap on the PVC flange connected to
the inlet of the blower. When this switch prevents
the unit from igniting, most likely the intake is
blocked by some means. Verify that the air intake
pipe and air intake vent terminal are free of
obstructions that may prevent air from entering the
unit. Also check that the equivalent feet of vent pipe
for the specific model has not been exceeded.
Check the vent length tables on page 10
LOW GAS PRESSURE SWITCH
The Low Gas Pressure Switch electrical contacts
are normally open. The low gas pressure switch
electrical contacts will close on a rise pressure.
The contacts will open when the pressure falls
below the fixed set point. If this happens during a
heating cycle the burner will be shut down and a
fault will be declared on the UIM.
32
The blocked inlet proving
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316586-000