Crown Boiler PHNTM080 - Page 120

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Air Proving Switch (APS) – This pressure switch is normally open and is connected to the outlet tapping on the gas valve as
shown in Figure 14.5. During pre-purge, when the gas valve is closed, the IAS is monitoring the pressure drop across gas-air
mixer(venturiforthe80-120,swirlplateonthe150and180).Greaterairowthroughthemixercausesthispressurereading
to increase. The APS is calibrated to close when this pressure rises to 1.05 +/- 0.05” wc, which is a pressure at pre-purge
correspondingtoadequateairow.Afterthegasvalveopens,thispressurewilldropveryclosetozero,regardlessoftheactual
airowratethroughtheboiler.Forthisreason,theAPSisignoredafterpre-purge.Intheeventthattheburnerrescontinuously
foronehour,theboilercontrolwillrecycletheburnertoverifythattheAPSstillclosesandthatadequateairowisstillpresent.
If the APS opens, look for the following:
 • Blockageintheventorairintakesystem.
 • Disconnected,looseorblockedAPStube.
Flow Switch–Thepaddletypeowswitchiscalibratedtocloseatawaterowrateofapproximately3.2GPM.Thisswitchis
intended to do two things:
 • Provethatthereiswaterintheboiler(nowatermeansnoowatthisswitch)
 • Provethatthereisenoughowforthesupplyandreturntemperaturesensorstoaccuratelymeasurethewater 
temperatures entering and exiting the heat exchanger.
Theminimumowraterequiredforallmodelsissignicantlyabove3.2GPM(Table9.1).Theboilercontrolveriesthatthere
isadequateowthroughtheboilerbymonitoringthetemperaturerisethroughtheboiler;theowswitchmerelyassuresthatthe
measuredriseisaccurate.Theboilercontrolstartstolimittheringratewhenthetemperatureriseexceeds53Fandtheboiler
goes into a soft lockout if the rise reaches 63F.
Iftheowswitchdoesnotclose,lookforthefollowing:
 • Nowaterintheboiler
 • Trappedairintheboilerorsystem-makesurethatasteadystreamofwaterexitsthemanualairventshowninFigure
11.1 when it is opened.
 • Valveclosedinboilerlooppiping
 • FouledYstrainerinboilerlooppiping
FIGURE 14.4: CONTROLS LOCATION
COMBUSTION FAN
PUMP FUSE
24V FUSE
THERMAL FUSE
SENSOR
AIR
SWITCH
PRESSURE
SUMP
SWITCH
PROVING
SWITCH
FLOW
SENSOR
SUPPLY
SENSOR
RETURN
FLUE
TEMPERATURE
GAS VALVE