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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
W1 W
2
Y
R
G
B
DIAGNOSTIC INDICATOR
FLASHING SLOW NORMAL, NO CALL FOR HEATFLASHING FAST NORMAL CALL FOR HEATCONTINUOUS ON REPLACE CONTROLCONTINUOUS OFF CHECK POWER2 FLASHES SYSTEM LOCKOUT (NO FLAME)3 FLASHES PRESSURE SWITCH PROBLEM4 FLASHES THERMAL PROTECTION DEVICE OPEN5 FLASHES FLAME SENSED WITH GAS VALVE OFFREFER TO SERVICE INSTRUCTIONS FOR MORE INFORMATION
FP PS2 MVH
HLI TH GND
GNDPS1
MVLMV
COMTR
HLO
WHITE-RODGERS Emerson Electric Co.
R
WR MODEL 50A51 - 405 D340021P01REPLACE WITHCNT 1308
Error Flash Codes(See p. 9)
W1 W
2
Y
R
G
B
FP PS2 MVH
HLI TH GND
GNDPS1
MVLMV
COMTR
HLO
WHITE-RODGERS Emerson Electric Co.
R
WR MODEL 50A51 - 405 D340021P01REPLACE WITHCNT 1308
DIAGNOSTIC INDICATOR
FLASHING SLOW NORMAL, NO CALL FOR HEATFLASHING FAST NORMAL CALL FOR HEATCONTINUOUS ON REPLACE CONTROLCONTINUOUS OFF CHECK POWER2 FLASHES SYSTEM LOCKOUT (NO FLAME)3 FLASHES PRESSURE SWITCH PROBLEM4 FLASHES THERMAL PROTECTION DEVICE OPEN5 FLASHES FLAME SENSED WITH GAS VALVE OFFREFER 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 SwitchSingle Stage
INTEGRATED FURNACE CONTROL
6
3
9
12
4
5
10
7
11
1
2
8
Y W G R C
FLAMESENSOR
TCO
TCO-BFUSE LINK
FUSE LINKPRESSURE
SWITCH
31,
2GAS VALVE
TR
TH
MV
MV
PS
HLI
HLO
FP
GND
Pressure SwitchTwo Stage
INTEGRATED FURNACE CONTROL
2
7
11
8
12
6
10
9
1
3
5
4
YW GR
FLAMESENSOR
TCO
TCO-BFUSE LINK
FUSE LINKPRESSURE SWITCH
1ST STAGE
BW1 2 CO
M
CL
HI
LO
PRESSURE SWITCH2ND STAGE
3
1
2
GAS VALVE
THERMOSTAT TWINNING
FP
HLO
HLI
PSI
GND
MVL
MVCOM
MVH
PS2
GND
TH
TR
Inducer Motor
IFC
9
1
3
5
4
YW GR
FLAMESENSOR
TCO
TCO-BFUSE LINK
FUSE LINKPRESSURE SWITCH
1ST STAGE
BW1 2 CO
MC
L
HI
LO
THERMOSTAT TWINNING
FP
HLO
HLI
PSI
GND
1
2
4
5
3
IGN N
IND N
IND LO
IND HI
IGN
IGNITOR
IND.
MTR.
IFC
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”)
What can cause the integrated furnace control toindicate a pressure switch error?
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”)
What can cause the integrated furnace control toindicate a pressure switch error?
Pressure Switch Error Diagnostics
• Vent Length
• Draft/Vacuum Measurements
• Electrical Checks
• Factors Affecting Draft
Determining Total Vent LengthsSee p. 9 of Installer Guide 18-CD19D6-5
MAXIMUM VENT LENGTH:MAXIMUM TOTAL EQUIVALENT FEETFOR VENT AND INLET AIR (See Notes)
MODEL2” PIPE
& FITTINGS2-1/2” PIPE& FITTINGS
3” PIPE& FITTINGS
*UX040C924
*UX060C936
*UX080C942
*UX100C948
*UX100C960
*UX120C960
6060603030
N/A
8080808080
15
100100100100100
60NOTES: 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 EXCEEDED3. One 90 o elbow is equivalent to 10 ‘ of 3 “ pipe 71/2’ of 21/2” pipe, & 5’ of 2” pipe. Two 45o elbows equal one 90o elbow.4. The termination tee or bend must be included in the total number of elbows. The BAYVENT100A termination is equal to 5 equivalent feet of pipe. The BAYVENT200A 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
INDUCEDDRAFTBLOWER
INLET PIPE
BURNERBOX PRIMARY HEAT
EXCHANGER
FLUE COLLECTOR BOX
TRANSFERTUBE RECOUP
CELL
PRESSURE DIFFERENTIALSWITCH
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 CurrentFlame
Current
MS PowerPoint 4.0 Presentation
Flame Rectification Ref p.25 of 40” Furnace “Book”
• Remember Ohm’s Law
E = I X R
Basic Electrical Circuit
+ -SOURCE
CO
ND
UC
TO
R
LOAD
CO
NT
RO
L
Applied AC voltage
+
-
+ +
-
Alternating Current Sine Wave
Flame Rectification circuit
What devices are used for electrodes?
Flame rodBurner
• Voltage Source: Ignition Control• “Load”: flame sensing circuit inside the control• Conductors: wires, and electrodes already
identified.• “Switch”?• The flame completes the circuit.
Flame Rectification Circuit
Flame Sense Circuit
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
+0-
5a
Current flow when flame rod is positive
+
_
FP
GND
Current flow when flame rod is negative
+
_FP
GND
+0-
1a
a
Effect = 4a pulsating DC
-
+ +
-Applied AC voltage
a
Effective Flame Signal
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?
Flame Rectification Circuit
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
0
5
10
15
20
25
DC CURRENT
AC CURRENT
VALVE DROPOUT AC MICROAMPS
DC MICROAMPS
VALVE DROPOUT IS IN AC MICROAMPS
DC Current vs. AC Current
DCCURRENT
ACCURRENT
ACDROPOUT
0.5 3.94 10.391.0 4.37 17.281.5 4.88 20.162.0 5.45 20.302.5 6.20 24.373.0 6.95 24.203.5 8.29 24.22
50A50 1 STG CONTROL
DC Current vs AC Current2 Stage IFC
DC CURRENT AC CURRENT AC DROPOUT0.5 2.08 9.150.6 2.19 9.930.7 2.38 10.860.8 2.5 12.020.9 2.61 12.05
1 2.74 13.421.1 2.88 13.42
1.22 3.07 13.51.32 3.19 15.081.42 3.35 15.11.54 3.56 15.141.6 3.68 15.181.7 3.81 15.211.8 3.98 17.41.9 4.17 17.4
2 4.36 17.42.1 4.58 17.45
2.28 4.81 17.492.44 5.13 17.522.63 5.51 17.62.86 5.98 20.393.1 6.51 20.443.4 7.22 20.48
AC Dropout Current
0
5
10
15
20
25
1 3 5 7 9 11 13 15 17 19 21 23AC Microamps
DC CURRENT
AC CURRENT
AC DROPOUT
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)