Furnace Troubleshooting Combustion Service Objectives Good furnace troubleshooting

Furnace Troubleshooting

Combustion Service

Objectives : Good furnace troubleshooting requires enhanced skills from the servicer • Service Tips • Induced draft furnace combustion process • Combustion system service checks • System effects on draft • Flame detection - principles and service checks • Flame sensing variables

Service Tip • Use of diagnostic LED – 40” Furnace “book” (p. 9)

Service Tip • Use of diagnostic LED (40” Furnace “book” (p. 9) DON’T TOUCH THAT DIAL !!

Error Flash Codes D 340021 P 01 REPLACE WITH CNT 1308 R FP PS 2 MVH HLI TH GND TR MV COM MVL PS 1 HLO GND DIAGNOSTIC INDICATOR FLASHING SLOW NORMAL, NO CALL FOR HEAT FLASHING FAST NORMAL CALL FOR HEAT CONTINUOUS ON REPLACE CONTROL CONTINUOUS OFF CHECK POWER 2 FLASHES SYSTEM LOCKOUT (NO FLAME) 3 FLASHES PRESSURE SWITCH PROBLEM 4 FLASHES THERMAL PROTECTION DEVICE OPEN 5 FLASHES FLAME SENSED WITH GAS VALVE OFF REFER TO SERVICE INSTRUCTIONS FOR MORE INFORMATION G B MODEL 50 A 51 - 405 Y R WR Emerson Electric Co. W 1 W 2 WHITE-RODGERS

Error Flash Codes (See p. 9) WHITE-RODGERS WR MODEL 50 A 51 - 405 DIAGNOSTIC INDICATOR Emerson Electric Co. D 340021 P 01 REPLACE WITH CNT 1308 FP PS 2 MVH HLI TH GND MVL PS 1 HLO GND Y R MV COM W 1 W 2 TR G B R FLASHING SLOW NORMAL, NO CALL FOR HEAT FLASHING FAST NORMAL CALL FOR HEAT CONTINUOUS ON REPLACE CONTROL CONTINUOUS OFF CHECK POWER 2 FLASHES SYSTEM LOCKOUT (NO FLAME) 3 FLASHES PRESSURE SWITCH PROBLEM 4 FLASHES THERMAL PROTECTION DEVICE OPEN 5 FLASHES FLAME SENSED WITH GAS VALVE OFF REFER TO SERVICE INSTRUCTIONS FOR MORE INFORMATION

Pressure Switch Error • What can cause the Integrated Furnace Control to indicate a pressure switch error? (Flowchart on p. 51 of 40” Furnace “Book”)

Pressure Switch Single Stage INTEGRATED FURNACE CONTROL GAS VALVE 2 1, 3 6 TR 3 TH 9 MV 12 MV 4 FUSE LINK TCO PRESSURE SWITCH 5 10 PS 7 HLI 11 TCO-B FLAME SENSOR 1 HLO 2 FP 8 GND Y W G R C

Pressure Switch Two Stage INTEGRATED FURNACE CONTROL PRESSURE SWITCH 2 ND STAGE GAS VALVE 3 2 TR 7 TH 11 GND 8 PS 2 12 MVH 10 MVL FUSE LINK TCO PRESSURE SWITCH 1 ST STAGE TCO-B FLAME SENSOR 9 GND 1 PSI 3 HLI 5 HLO 4 FP W 1 W 2 Y R G THERMOSTAT B LO 1 HI 6 COM CL MVCOM 2 TWINNING

Inducer Motor 1 IGN 2 IND HI 3 IND LO MTR. IGNITOR FUSE LINK PRESSURE SWITCH 1 ST STAGE TCO 4 IND N 5 IGN N 9 GND 1 PSI IFC 3 HLI TCO-B IFC 5 HLO 4 FLAME SENSOR FP W 1 W 2 Y R G B THERMOSTAT COM CL HI LO IND. TWINNING

Pressure Switch Error What can cause the integrated furnace control to indicate a pressure switch error? • Switch CLOSED when it should be OPEN – 24 VAC at “PS” too early (inducer not “energized) (See Flowchart on p. 51 of 40” Furnace “Book”)

Pressure Switch Error What can cause the integrated furnace control to indicate a pressure switch error? • Switch OPEN when it should be CLOSED – No 24 VAC at “PS”, inducer circuit “energized” (See Flowchart on p. 51 of 40” Furnace “Book”)

Pressure Switch Error Diagnostics • • Vent Length Draft/Vacuum Measurements Electrical Checks Factors Affecting Draft

Determining Total Vent Lengths See p. 9 of Installer Guide 18 -CD 19 D 6 -5 MAXIMUM VENT LENGTH: MODEL MAXIMUM TOTAL EQUIVALENT FEET FOR VENT AND INLET AIR (See Notes) 2” PIPE 2 -1/2” PIPE 3” PIPE & FITTINGS *UX 040 C 924 60 80 100 *UX 060 C 936 60 80 100 *UX 080 C 942 30 80 100 *UX 100 C 948 30 80 100 *UX 100 C 960 N/A 15 60 *UX 120 C 960 NOTES: First Letter may be “A” or “T” 1. DO NOT MIX PIPE DIAMETERS IN THE SAME LENGTH OF PIPE OUTSIDE THE FURNACE CABINET (except adapters at top of furnace). 2. MAXIMUM PIPE LENGTHS MUST NOT BE EXCEEDED 3. One 90 o elbow is equivalent to 10 ‘ of 3 “ pipe 71/2’ of 21/2” pipe, & 5’ of 2” pipe. Two 45 o elbows equal one 90 o elbow. 4. The termination tee or bend must be included in the total number of elbows. The BAYVENT 100 A termination is equal to 5 equivalent feet of pipe. The BAYVENT 200 A is 0 equivalent feet. 5. Pipe adapters are field supplied.

Combustion Air Pressure Switch FIELD SUPPLIED TUBE & TEE SECTION

Examine the effects of system variations on draft • Vent Length • Blockages • Wind • Gas Input • Airflow

Vacuum / Draft Pathway PRESSURE DIFFERENTIAL SWITCH BURNER BOX INDUCED DRAFT BLOWER PRIMARY HEAT EXCHANGER INLET PIPE TRANSFER TUBE FLUE COLLECTOR BOX RECOUP CELL

Effects of Low System Airflow on Internal Vacuum Hot furnace Means hot flue gases Hot flue gases expand - take up more space Expanding flue gases raise pressure in furnace

Flame Current

Flame Rectification Ref p. 25 of 40” Furnace “Book” • Remember Ohm’s Law E=IXR

Basic Electrical Circuit SOURCE CONTROL CONDUCTOR + LOAD

Alternating Current Sine Wave + + + Applied AC voltage - -

Flame Rectification circuit What devices are used for electrodes? l Flame rod l Burner

Flame Rectification Circuit • Voltage Source: Ignition Control • “Load”: flame sensing circuit inside the control • Conductors: wires, and electrodes already identified. • “Switch”? • The flame completes the circuit.

Flame Sense Circuit l l One electrode is larger than the other causing electron flow to be greater in one direction than in the other. Compare the burner size to the flame rod size.

Flame Sensing - Service • Measuring outputs • Measuring inputs • Circuit integrity checks – Polarity – Grounding method / continuity

+ FP GND _ Current flow when flame rod is positive 5 a + 0 -

_ FP GND + + 0 - Current flow when flame rod is negative 1 a

Effective Flame Signal + + a Effect = 4 a pulsating DC Applied AC voltage - a -

Flame Rectification Circuit Remember Ohm’s Law ? What happens in a circuit if resistance increases and voltage stays the same? What can affect the resistance in the circuit? What else can affect the flame signal?

Increased circuit resistances • • Burner oxidation Non-secure burner Flame “lift off” Loose wire connections Contaminated flame sensor Faulty grounds No grounds Reverse polarity

DC Current vs. AC Current VALVE DROPOUT IS IN AC MICROAMPS

DC Current vs. AC Current 50 A 50 1 STG CONTROL

DC Current vs AC Current 2 Stage IFC

Flame Sensing - Variables • • • Remember Ohms Law E=I x R Watch that resistance Too little DC = too bad Too much AC = too bad Grounded flame sensor Look out for noise!

Question Name something which can cause a “ 2 flash” lockout which has not yet been discussed. Answer: 1) No gas available - check supply 2) Multiple recycles caused by other faults (ie: pressure switch trips) (See p. 10)