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)