EngA DJM3 - TEMPERATURE CONTROL; PRE-PURGE PROBLEM

A DJM3
IOM-17 22 of 27 Mar 99 R3
TEMPERATURECONTROL
Modulationofthefiringrateisgraduallymadeuntilthedischargeairtemperatureiswithin1.5°Fof
the“calculated”set‐point.Whentheheatloadislightandtheburneriscyclingbetweenlowfireand
offtherewillbesmalltemperatureswings.
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AIRBALANCING
Installationandairbalancingisoftendoneduringwarmerweatherthanthatexperiencedinthecoldof
winter.Iftheairbalancerdidnotallowforthechangesthatwilloccurinairvolumeincoldweather
thentheunitwillappeartobeshortoftemperaturerise.Asthefanisaconstantvolumedeviceandas
itislocatedbeforetheheatexchanger,airwillexpandasitisheated.Theamountofchangewillbe
about20%increaseinairvolumefrom‐30°Fto+70°F.
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PRE‐PURGEPROBLEM
IfaPre‐Purgeproblemisfound,typicallytheDJM3isreceivingasignalfromthetachometersensor
(HallEffectsensor)thatisnotconsistentwiththecombustionblowerstatus.Ifthereisacallforheat
andthecombustionblowerisnotrunningtheDJM3haslikelydetectedafalseorgrounded
tachometersignal.RemoveallthreewirestoterminalsO,YS,andG.Ifthecombustionblowerfailsto
start,checkthecombustionmotorandormotorwiring.
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Ifthecombustionblowerisrunningandthetachometersignalisabsent,weak,orgrounded.Witha
digitalACvoltmeter,measuretheACvoltspresentonterminals“YStoG”.Whenthecombustion
blowerisrunningthereshouldbe4to6Vacpresent.IftheACvoltageisnotpresent,checkthe
tachometersensortomagnetgap.Itshouldbe1/16to1/8inch.Ifthegapissatisfactory,attemptto
repairbyflippingthemagnetover,thenflippingthetachometersensorover,beforereplacingthe
tachometersensortocorrecttheproblem.
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Note:Nopartofthespeedsensor’ssensingelementshouldbelocatedovertheendofblowermotor
shaft.
BlowerShaft
1/8”Gap
Magnet
Magnet
RetainerRing
HallEffect
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