User Guide

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B. Troubleshooting when the Display is Blank
UsetheowchartinFigure14.5tolocatetheproblemwhenthedisplayisblankorisnotreadable.
C. Operation and Troubleshooting of Boiler Limits, Switches and Sensors
For the location of the controls described below, see Figure 14.6.
Thermal Fuse–Thethermalfuseisdesignedtopreventboileroperationintheeventthattheheatexchangerisdamagedby
excessiveuegastemperatures.Itissettoopenat358Fandisaoneshotnon-replaceabledevice.Ifthisfuseopens,theheat
exchangermustbereplaced.Thethermalfuseisessentiallyaback-uptotheuegassensor,whichisdesignedtopreventboiler
operationiftheuegastemperatureexceeds210F.Note,however,thatitispossibleforthethermalfusetoopeneventhoughthe
uetemperaturelimitisworkingproperly.Ifthethermalfuseissuspectedofhavingopened,unplugthewireheadingtoitand
check continuity directly across the thermal fuse. If no continuity is present, the heat exchanger needs to be replaced. If continuity
is present, the fault lies elsewhere.
Sump Pressure Switch (SPS)–Thispressureswitchisnormallyclosedandmonitorsthedifferenceinpressuresbetweenthe
boilercabinetandthesump(thesumppressureisveryclosetothepressureattheboiler’sventconnection).Thisswitchispiped
as shown in Figure 14.7. The SPS is calibrated to open at 3.15 +/- 0.10 inches water column. A blockage in vent system that
causes the vent pressure to exceed the SPS set point will cause the burner to shut down and recycle. Likewise, a blockage in the
air intake system that causes the vacuum in the cabinet to fall below -3.15 inches will also cause the boiler to shut down and
recycle. If this switch opens, look for a blockage in the vent or air intake system.
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 14.7. During pre-purge, when the gas valve is closed, the APS 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twohours,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Hold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
WARNING
Asphyxiation Hazard. Fire Hazard. Do not attempt to jump or replace the thermal fuse. Doing so may
cause a breach of the heat exchanger casing resulting in property damage, personal injury or loss of
life.