A.O. Smith T-M50 ASME User manual

T-M50 / T-M50 ASME Service manual
Ver. 1.05
1
CONFIDENTIAL
T-M50 / T-M50 ASME
On-Demand Water Heater
Service manual
T-M50
A.O. Smith Water Products Company
500 Tennessee Waltz Parkway
Ashland City, TN 37015
Toll Free: 1
-
877
-
737
-
2840

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
Table of Contents
1.
Specifications………………………………………...…...………….……………………...…………………………
3
2.
Exterior view……………………………………………………………………………………………………………..
4
3.
Interior view….…………………………………………………………………………………………………………..
5
4.
List of main components in the interior view……………….…………………………………………….
6
5.
Schematic diagram………………………………………………………………………..…………………………..
8
6.
Wiring diagram…………………….…………………………………………………………..……………………….
9
7.
Wiring diagram checkpoints for diagnosis……………………………………………………………….…
10
8.
Resistance values of the temperature thermistors……………..………………………………………
12
9.
Operational flow chart..……………………………………………………………………………………………..
13
10.
Component specifications………………………………………………………………………………….………
14
11.
Fault Analysis & Specifications……………………………………………………………........................
36
12.
Controls and settings………………………………………………………………………………………………….
47
13.
Components diagrams………………………………………………………………..................................
81
14.
Parts list……………………………………………………………………………………………………………………..
85
15.
Revisions…...................................…………………………...………………………………………………….
91

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
1. Specifications
Unit Model T-M50 / T-M50 ASME
Unit dimensions H25.3"×W24.8"×D11.8"
Weight 102 lbs.
Max 380,000
INPUT
BTU/h
Min 15,000
Combustion System
Power vent
Installation Indoor, Outdoor, Direct vent
Fan motor PWM Turbo fan
Max LP 3.7”WC
Natural 2.7”WC
Combustion
Manifold
Pressure*
Min LP 0.9”WC
Natural 0.7”WC
Flow rate 0.5 to 14.5 GPM
Available set temp.
100°F to 185°F
default set 120°F
Temperature
dipswitch setting
100°F, 115°F, 120°F, 135°F, 145°F, 155°F, 165°F, 185°F
Bypass valve Yes
Water control
Thermistors 3 thermistors (In, Out, Mixing)
Available remote
controller TM-RE30
PCB Model Central board: MC50 (Part # EM306) /
Right and left computer board:T-M50 (Part # EM307)
Indicators Red LED on PCB during operation & 7-Seg LED
Operation
Power supply
control GFI (Excluding surge absorber)
Freeze protection
Ceramic heaters, Auto-FM system and Auto-firing system
Self-combustion
improvement Air Fuel Ratio detection system
Easy-link system
Yes
Future
Multi-unit system
Yes
*The manifold pressure measurement is base on conditions without front cover.

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
5" (1 27mm)
24-7/ 8" (632 .9mm)
25-1 /4" (64 2m m)
7-3/8 " (18 6m m) GAS
1-5/ 8" (40mm) C OLD
12-1 /4 " (311 .5mm)
4-7/8" (12 2.9mm)
1/2" (11.9m m)
5-3/8" (137.3 mm) HO T
4-3/ 8" (11 0.8mm) GA S
7-5/8" (194.3 mm) C OLD
4" ( 100mm) 1 20 VAC
6-3/ 4" (171 .3mm) 12 0 VAC
1/2" (12.5 mm)
2-1/ 2" (62.1m m)
5-7/ 8" (15 0mm ) HOT
15" (381mm )
8" ( 203mm)
29-7/8 " (7 59 mm)
GAS
COLD
HOT
7-3/8 " (18 6m m)
4" ( 100mm)
4-3/8" (110.8 mm)
6-3/8" (17 1.3mm)
1-5/8" (40mm)
5-7/ 8" (15 0m m)
6-3/4 " (137.3mm)
7-5/8" (19 4.3mm)
4-7/ 8" (123.7mm)
8" ( 203.2m m)
2. Exterior view
Side view Front view
Top view Bottom view

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
3. Interior view
1
4 4
5
5
2
2
6
7
6
7
8
8
9
10
11
12
12
13
14
15
16
16
17
18
18
19
19
2122 23
23
24 25
26
3
27
5
30
20
20
29
31
28
3
15
30

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CONFIDENTIAL
4. List of main components in the interior view
No. Description
Items# in
components
diagram
Takagi
Part #
Common parts
For other models
1 Case assembly 001 EM305
2 Burner assembly (Burners) 101 EM445
3 Igniter 102 EKN74
T-K3, T-K3-Pro, T-K3-SP/OS,
T-H2-DV/OS, T-KJr2-IN/OS,
T-K4-IN/OS, T-D2-IN/OS,
T-M32
EM302
(LP model)
4 Manifold assembly with gas
valve assembly 114
EM303
(NA model)
5 Fan motor 416 EKK25
T-K3, T-K3-Pro, T-K3-SP,
T-H2-DV/OS, T-K4-IN/OS,
T-D2-IN/OS,T-M32
6 Flame rod 103 EKK0E
T-K3, T-K3-Pro, T-K3-SP/OS,
T-H2-DV/OS, T-K4-IN/OS,
T-D2-IN/OS, T-M32
7 AFR rod 103 EKK0E
T-K3, T-K3-Pro, T-K3-SP/OS,
T-H2-DV/OS, T-K4-IN/OS,
T-D2-IN/OS, T-M32
8 Igniter rod 104 EKK0F
T-K3, T-K3-Pro, T-K3-SP/OS,
T-H2-DV/OS, T-K4-IN/OS,
T-D2-IN/OS, T-M32
9 Gas inlet 124 EM284
401 EM308
(Regular model)
10 Heat exchanger
444 EM323
(ASME model)
11 Exhaust combining box 128 EM331

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
No. Description
Items# in
components
diagram
Takagi
Part #
Common parts
For other models
12 Water control valve 418 EKH32 T-H2-DV/OS, T-M32
13 Water inlet 434 EM295
14 Water outlet 439 EM309
15 Inlet thermistor 424 EKK38 T-K3, T-K3-Pro, T-H2-DV/OS,
T-D2-IN/OS, T-M32
16 Output thermistor 405 EKK2T T-K3, T-K3-Pro, T-H2-DV/OS,
T-D2-IN/OS, T-M32
17 Mixing thermistor 430 EKK1A T-K3, T-K3-Pro, T-D2-IN/OS
18 Hi-limit switch 403 EKN34
T-K3, T-K3-Pro, T-K3-SP/OS,
T-H2-DV/OS, T-D2-IN/OS,
T-M32
19 Overheat cut-off fuse 402 EK333
T-K3, T-K3-Pro, T-K3-SP/OS,
T-KJr2-IN/OS, T-K4-IN/OS,
T-D2-IN/OS
20 Heater101 426 EX002 T-D2-IN/OS, T-M32
21 Inlet drain plug (water filter) 437 EX006 T-H2-DV/OS, T-M32
22 Outlet drain plug 442 EKK2E
T-K3, T-K3-Pro, T-K3-SP/OS,
T-KJr2-IN/OS, T-K4-IN/OS,
T-D2-IN/OS
23 Left / Right computer board 701 EM306
24 Central computer board 702 EM307
25 Transformer 718 EM296 T-H2-DV/OS
26 GFl 720 EM207 T-M32
27 Freeze protection thermostat
140 EM286
28 Heater 502 425 EX001
29 Surge box 722 EM385 T-M32
30 Manifold port N/A N/A
31 Gas inlet port N/A N/A

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CONFIDENTIAL
5. Schematic diagram
1. When a hot water tap is opened, cold water enters the T-M50.
2. The water flow sensor detects this water flow and sends this information to computer.
3. The computer initiates fan motor and sends signal to igniter to create ignition spark.
4. The main, proportional, and solenoid gas valves open to allow gas input.
5. The gas ignites and flames appear inside the burner chamber.
6. Water circulates through the heat exchanger and is heated up to the set temperature.
7. Using thermistors to measure temperatures, the computer modulates the gas and water valves to
ensure proper output water temperatures.
8. When the tap is closed, the T-M50 shuts down.
Heat
Exchanger
Output
Thermistor
Water
control
valve
Flow sensor
Mixing
Thermistor
Exhaust Output
Thermistor Heat Exchanger
Water control
vale
Flow sensor
Inlet
Thermistor
Computer board
Burner
Gas valve
Fan motor
Fan motor
Hot water
outlet
Cold water
inlet
Gas
Inlet
Thermistor
120 VAC

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
6. Wiring diagram
BK:BLACK
P:PURPLE
LB:LIGHT BLUE
BL:BLUE
G;GREEN
Y:YELLOW
O:ORANGE
BR:BROWN
Heater
Thermostat
Inlet
thermistor
Output
thermistor
Mixing
thermistor
BK
BK
BK
GFI
Tra n s-
for m er
12 3 4 5 6
O F F
BR
BR
120 VAC
10A
G
Ground
W
BK BR
Ground
Air-fuel
ratio rod
Ground
Flame
rod
1 2 3 4 5 6
OFF
1 2 3 4 5 6
OFF
MV
SV1
SV2
SV3
Remote
controller
Parent
W
W
IG
Ground
Ground
Inlet
thermistor
Output
thermistor
Mixing
thermistor
BK
BK
BK
12 3 4 5 6
O F F
Air-fuel
ratio rod
Ground
Flame
rod
MV
SV1
SV2
SV3
FM
Igniter
rod
IG
Heater
Heater
Heater
Heater
Heater
7
7
8
8
RYGWBK
BL
ROYGW
BK
BL
RYGW
BK
BL
ROYGW
BK
BL
Water
Control
valve
Water
Control
valve
B K
B K
B K
B K
BK
BK
BK
BK
Y
G
O
O
G
Y
BK B K
W
W
BL
BK
BK
RR
BL
BL
GG
R
BL
Y
O
W
P
PP
PP
PP
P
G
WR B K
W
R
WR B K R
W
BL
LB
G
O
R
W
BK
Y
WBK WBK
BL
LB
G
O
R
BK
W
BK
W
W
BK
BK
W
W
W
W
W
W
W
Gas
Porpor-
tional
Valve
Flow
Sensor
WR BK
W
R
WR BK R
W
Hi-
limit
BL
O.H.C.F
BL
BL BL
BL
FM
R
BL
Y
O
W
BL
Hi-
limit
BL
O.H.C.F
BL
BL B L
BL
BL
Flow
Sensor
Left side unit Right side unit
Surge
box
BK
W
MAX button
MIN button
Increase button
Decrease button
Burning lamp
Communication
lamp (right)
Communication
lamp (left)
Multi-system
on line lamp
Lower bank of
dipswitches
Upper bank of
dipswitches
7 Seg LED
Pump test run
Error call
button
Burning lamp
FM
port
Pump
port
Button to check
unit number
Priority SW
Alarm
contact
Draft switch
Dipswitches
MAX button
MIN button
Increase button
Decrease button
2
1
Dipswitches
Gas
Porpor-
tional
Valve
Igniter
rod
I
F
D1
D2
H1
H1
H2
J
J
E1
E1
E3
E3
E2
E2
CC
A
A
A A
C2
C2
C1
C1
BB
I
N
M
K
A3
A1
A2
L
L
L
L
L
L
L
G G
TM-RE30
H2

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
7. Wiring diagram check points for diagnosis
Check-
point
Parts and Description
Color of wires Normal Range
A
A1 100 V Power supply White –Black (A) Brown –Brown (A1) 90 to 110 VAC
A2,A3
120 V Power supply Black - White 108 to 132 VAC
B Igniter Purple –Purple 90 to 110 VAC
Light blue - blue at COM (MV)
78 to 100 VDC (during operation)
/ 0.9 to 1.3 kΩ
Green - blue at COM (SV1) 78 to 100 VDC (during operation)
/ 1.3 to 1.9 kΩ
Orange - blue at COM (SV2) 78 to 100 VDC (during operation)
/ 1.3 to 1.9 kΩ
C Gas valves
Red - blue at COM (SV3) 78 to 100 VDC (during operation)
/ 0.9 to 1.7 kΩ
C1 Hi-Limit switch Blue - Blue
Less than 1 VDC
and
Less than 1 Ω
C2 Overheat cutoff fuse Blue - Blue
Less than 1 VDC
and
Less than 1 Ω
D1 White - White 15 VDC
D2
Easy-link and
Multi-unit link
connectors
Black –Blue
Black –Red
15 VDC
(during Easy-link operation)
E1 Mixing thermistor Black - Black
E2 Output thermistor Black - Black
E3 Inlet thermistor Black - Black
See table on p.12
F Remote controller Black - White 11 to 25 VDC
Red - Blue 110 to 160 VDC
Yellow - Blue 13 to 17 VDC
G Fan motor
Orange - Blue 2 to 6.5 VDC

T-M50 / T-M50 ASME Service manual
Ver. 1.05
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CONFIDENTIAL
Check-
point
Parts and Description Color of wires Normal Range
H1 Gas proportional valve White - red
1 to 15 VDC(during operation)
and
20 to 40 Ω
Red - Black 4 to 5.5 VDC
H2 Flow sensor
White(+) –Black(GND)
1 to 4 VDC (1,200Pulse/min)
Air-fuel ratio rod
Yellow –AFR rod
(Between AFR rod and
the computer board)
More than 0.5 μA
(during operation)
I
Flame rod
Orange –Flame rod
(Between flame rod and
the computer board)
More than 1 μA (during operation)
J Water control valve
Blue –Brown
Orange –Brown
Red –Brown
13 to 16 VDC
ON: 12.5 to 16 VDC
OFF: 0 to 1 VDC
1 VDC Less ( 0° position)
K Pump connector White - Black Less than 1.3 Ω
L Heater Black –Black 90 to 110 VAC
M FM port White - Black Less than 1.3 Ω
N Alarm contact White - Black Less than 1.3 Ω

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
8. Resistance values of the temperature thermistors
Resistance values at different temperatures
ºF 32 41 50 59 68 77 86 95 104 113
ºC 0 5 10 15 20 25 30 35 40 45
Resistance kΩ23.76 19.08 15.43 12.56 10.28 8.47 7.02 5.85 4.90 4.12
Temperature ºF 122 131 140 149 158 167 176 185 194 203 212
ºC 50 55 60 65 70 75 80 85 90 95 100
Resistance kΩ3.49 2.96 2.53 2.16 1.86 1.60 1.39 1.21 1.05 0.92 0.81
Temperature / ºF vs Resistance / kΩ
Temperature / ℃vs Resistance / kΩ
Temperature
0.00
5.00
10.00
15.00
20.00
25.00
20 40 60 80 100 120 140 160 180 200 220
Temperature / ºF
Resisitance / kΩ
0.00
5.00
10.00
15.00
20.00
25.00
0 10 20 30 40 50 60 70 80 90 100
Temperature / ºC
Resisitance / kΩ

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
9. Operational flow chart
Power supply ON
Operatio n SW ON
FAV&BV open
Pre-c he ck
NORMAL?
Faucet opens
Water Fl ow
Above 0.5GPM?
FM speed
(Below
1,000rpm?)
Both fan moter
start operation
Flame r od
detects Below
DC0.1 5 A?
Hi-li mi t
Norma l?
OHCF
Normal?
Left MV ON
Left SV 2 ON
Left GPV ON
Flame rod
detects Above
DC1.0 A?
Burning lamp ON
Left Igniter OF F
Proportional control
BTU
change o ver
point?
Left SV1 SV3 ON
Bel ow water
heat capability ?
Left FAV OPEN
Safet y devices
s ensed
abnor mal condit ion
Left FAV CLOSE
Left SV1 SV3 OFF
False D ipswitch
setting s
InTH failure
PCB f ai lure
MV circ uit failure
031 blin k
321 blin k
701 blin k
510 blin k
*611 bli nk
Both fa n motor
stop op eration
721 b linkFalse fl ame
Left Igniter OFF
Reignition
Success?
Left
OuTH OPEN
or SHORT
Left
Fan motor
fault
Left SV2 OFF
Left GPV OFF
Left SV 1 SV3 OF F
Fan m otor
stop operation
Faucet c loses
Flow sensor OFF
Remov e cause
Opeartio n lamp OFF
Operation stop
YES
NO
YES
NO
NO
YES
YES
NO
(20sec later)
YES
NO
NO
YES
YES
NO
YES
NO
(5sec later)
(3sec later)
YES
YES
NO
YES
NO
(20sec later)
YES
NO
NO
NO
YES
FM speed
(Above
1,000rpm?)
Bel ow
Max f low rate
rest riction?
111 b link
121 b link
*611 blink
311 blink
OuTH failure 311 blink
MiTH failure 331 blink
Left
Fla me
blows o ut
Left
OHCF melted
(abov e 430 F)
Left
AFR rod
failure
Left
Abnormal
burning
121 blink121 blink991 blink391 blink
BV modu lates
Flow&temp
321 blink
Left
InTH OPEN
or SHORT
331 blink
Left
MiTH OPEN
or SHORT
SV ci rc uit fail ure 551 blin k
661 blin k
Fault of Flow
adjustment function
in Water control
Valve
Fault of Bypass
function in Water
control Valve
651 blink
B
Only one unit
should work?
NO
YE S
Bo th com bu st i on sec t io ns
(r ight a nd l e ft ) wi l l
st art op er at i on .
Left Igniter ON
Left Fa lse dete ction
Left Ign it ion fail ure
Left Fl ame blows up
FAV ; Flow Adjustm ent Valv e
B V ; Bypass Valve
F M ; Fan Motor
OHCF; Over Heat Cu t-off Fu se
SV1 ; Solenoid Val ve 1 (Se e the sect ion 6)
SV2 ; Solenoid Val ve 2 (Se e the sect ion 6)
SV3 ; Solenoid Val ve 3 (Se e the sect ion 6)
M V ; Ma in gas Val ve
GPV ; Ga s Proporti onal Valve
InTH; In le t thermi stor
OuTH; Ou tp ut therm istor
MiTH; Mi xi ng therm istor
PCB ; Pr in t Circui t board
(Compute r board)
AFR ; Ai r Fuel Rat io rod
In this chart,
The primary unit is
Left combustion section.
The secondary unit is
Right combustion section.
Primary combustion section
change from Left to Right
No error code
in Right unit
YE S
B
Error code
in Left unit
YE S
NO
NO
Comp o n e nt s in t hi s c hart
swit c h fr om Lef t c om bust i on
sect i o n t o Righ t c om bust i on
sect i o n .
*W hen th e " 611" e rr o r
co de a pp e ar s, bot h
co mbus ti o n sectio ns
(r ight a n d/ or lef t)
wi ll s to p o perati on .
Left MV OFF

T-M50 / T-M50 ASME Service manual
Ver. 1.05
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CONFIDENTIAL
10. Component specifications
10-1.
Burners……………………..…………………………………………………..…………………....………
15
10-2.
Gas manifold…………….………………………………………………………..…………………….....
16
10-3.
Fan motor………………………………………………………………………...………………………….
17
10-4.
Gas valve assembly……………………………………….……………………………………………..
18
10-5.
Flame rod………………………………………………………….…………………………………………
19
10-6.
AFR rod………………………………………………………………………………………………………..
20
10-7.
Heat exchanger…………………………………………………………….....………………………….
21
10-8.
Flow sensor………………………………………………………………………………………………….
22
10-9.
Water Control valve………………………………………...…………………………………………..
23
10-10.
Thermistors………………………………..……………………………………..………………………..
25
10-11.
Hi-limit switch………………………….……………………………………………………………..……
26
10-12.
Overheat cutoff fuse..………………………………………………………………………………..…
27
10-13.
Freeze protection heaters….……….………………………………………………………………..
28
10-14.
Computer board for Burner control……………………………………………………….…….
29
10-15.
Computer board for Multiple controls……………………………….…………..…………...
30
10-16.
Transformer…………………………………………………………………………….……...…………..
31
10-17.
GFI………………………………………………………………………………………………….…………...
32
10-18.
Igniter……………………………………………………………………………….............................
33
10-19.
Freeze protection thermostat……………………………………………………....................
34
10-20.
Surge box……………………………………………………………………………………………………. 35

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
10-1. Burners
Unit Part ##101 AOS Part #EM445 Checkpoint N/A
Function
There are 2 types of burners in the water heater: the gas-rich burner
stabilizes the flames within the combustion chamber and the air-rich burner
produces more heat in the combustion chamber. The burners facilitate the
air/gas mixture necessary to produce the proper heat during the combustion
reaction.
Failure event
1. Unable to initialize/sustain combustion.
2. Dust or soot deposit on the burner surface.
3. Cracks of the burners.
4. Gas leakage from the burners.
Effects on the water
heater if burners fails
1. Unexpected combustion.
2. Unstable flame conditions and/or flame loss.
3. Ignition failure.
4. Back-firing.
Error codes when the
burners fails 101 111 121 991
Diagnostic V isual inspection: excessive dust deposit on the burner surface and/or
unstable flame conditions during operation.
Color/Number of wires N/A
The burner damper is not
included in EM445.
Burner damper (#105 / EKK1P)
The burner for the T-M50 has the “26Y”mark as shown below.
7-1/ 2" ( 190mm)5- 1/ 4" (132mm)
"26Y" mark
26Y
9" (22 9mm)
7-7/8" (201mm )
1-3/4 " (44mm) 11V
"11V " m ark

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
10-2. Gas manifold
Unit Part #Included
in #114 AOS Part #
EM302 (LP model)
EM303 (NA model) Checkpoint
N/A
Function
1. The manifold distributes gas from the gas valves to the burners. The
manifold has two types of the nozzles: one type for gas-rich burners (16
nozzles) and the other for air-rich burners (15 nozzles)
2. There are 3 zones within the manifold, to ensure efficient combustion
operation.
Failure event
1. Dust deposit on the manifold.
2. Gas leakage from a failed manifold.
3. Ignition failure.
4. Imperfect combustion.
Effects on the water
heater if the manifold
fails
1. The burners cannot receive proper gas flow from the manifold, which can
cause poor combustion in the combustion chamber. In this case, the AFR rod
will detect an improper flame condition and computer will take safety
measures.
2. Gas leakage from the manifold.
Error codes when the
manifold fails 101 111 121 991
Diagnostic
1. Visual inspection: Excessive dust deposit around the nozzles or cracks on
the manifold.
2. Check voltages and resistance: proper range of values shown below.
Color/Number of wires Blue-Red 78 to 100 VDC / 0.9 to 1.7 kΩ
Nozzles for gas-rich burners
Nozzles for air-rich burners EM302 (for LP model) and
EM303 (for NA model) is an
assembly of the gas
manifold with the gas valve
assembly. For safety
reasons, these assemblies
should be installed in the
unit as a whole set.
The gas manifold has the
“26Y”mark on the manifold
plate.
9-1/ 8" (232mm )
6-7/8" (175mm)
26Y
"26Y" mar k

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
10-3. Fan motor
Unit Part ##416 AOS Part #EKK25 Checkpoint G
Function To provide combustion air into the combustion chamber and to exhaust flue gas.
Failure event
1. Fan speed failure, causing abnormal sounds with or without combustion during
operation.
2. Unexpected activation caused by the connectors of the fan motor getting wet.
3. Disconnects from the bottom of the combustion chamber.
Effects on the water
heater if fan motor fails
1. The water heater does not function properly
2. Failure to ignite or abnormal ignition
3. Unstable combustion conditions
Error codes when the fan
motor fails 101 111 121 611 991
Diagnostic
1. Visual inspection: connection/breakage of wires or dust (causing electrical
shortage)
2. Voltage check: proper range of voltages shown below.
Red-Blue 110 to 160 VDC (Input)
Yellow-Blue 13 to 17 VDC (Input)
Orange-Blue
2.0 to 6.5 VDC (Input)
Color/Number of wires
White-Blue Verify the fan motor speed using the Diagnostics Mode of the
temperature remote controller. See Section 12-1 for details.
Fan damper (#138 / EK270)
1-3/4"
( 44mm)
PWN turbo fan motor
6-1/2" (165.5mm)
3-3/4"(95.15mm)
Openin g 2-1 /8"( 54mm)
The fan damper
is not included in
EKK25.

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CONFIDENTIAL
10-4. Gas valve assembly
Unit Part #Included
in #114 AOS Part #
EM302 (LP model)
EM303 (NA model ) Checkpoint
C,H1
Opens and closes the gas pathways of the water heater (main and solenoid
gas valves)
Function
Modulates the gas flow from the gas inlet (proportional gas valve)
Failure event
1. Gas leak from the valves.
2. Unable to open/close (main and solenoid gas valves)
3. Unable to modulate the gas flow (proportional gas valve)
Effects on the water
heater if valves fails
1. Gas leak from the unit.
2. Excess carbon monoxide emissions. / No flames.
3. Lack of water temperature control.
Error codes when the
valves fails 111 121 510 701
Diagnostic
1. Visual inspection: connection/breakage of wires.
2. Listen for "clunk" sounds from the gas valves opening right after fan motor
initiates.
3. Check voltages and resistance of coils; proper range of values shown
below.
Blue - Light blue (MV) 78 to 100 VDC (during operation) / 0.9 to 1.3 kΩ
Blue - Green (SV1) 78 to 100 VDC (during operation) / 1.3 to 1.9 kΩ
Blue - Orange (SV2) 78 to 100 VDC (during operation) / 1.3 to 1.9 kΩ
Blue - Red (SV3) 78 to 100 VDC(during operation) / 0.9 to 1.7 kΩ
Color/Number of wires
White - Red
(Proportional valve) 1.0 to 15 VDC (during operation) / 20 to 40 Ω
3-5/8" (91.5 m m)
1-3/8" (34mm )
2-5/8" ( 67mm )
Proportional valve
Main gas valve
Gas
valve
4-1/4" (108mm) 7/8" (22.2mm)
4-5/8" (119mm)
Solenoid valve
Proportional Valve
EM302 (for LP model) and EM303 (for NA model) is an
assembly of the gas manifold with gas valve assembly.
For safety reasons, these assemblies should be installed
in the unit as a whole set. The diagram below shows the
gas valve assembly.

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10-5. Flame rod
Unit Part ##103 AOS Part #EKK0E Checkpoint I
Function To detect flames while the water heater is in operation.
Failure event 1. Unable to detect flames when flames actually do occur
2. Detecting a false flame when no flames actually occur
Effects on the water
heater if flame rod fails
1. The water heater stops operating. The "111" and/or "121" error code(s)
will display.
2. The water heater will not initiate the ignition process. The "721" error
code will display.
Error codes when the
flame rod fails 111 121 721
Diagnostic 1. Visual inspection: connection / breakage of wires or soot buildup on rod.
2. Check amperes: proper range of values shown below.
Color/Number of wires
Orange(17) - Flame rod
(Between the flame rod and
the computer board)
(During operation)
More than DC 1 μA
The flame rod is assembled with the AFR rod.
AFR Rod
Flame rod
3/ 4" (1 7.5 m m)
1/ 4" (7.5 mm )
1/ 2" (1 2mm )
1- 1 / 8" (29 .3m m )
1-3/4" (44mm)
Leads to computer board
Detecting elements of the flame rod

T-M50 / T-M50 ASME Service manual
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CONFIDENTIAL
10-6. AFR rod
Unit Part ##103 AOS Part #EKK0E Checkpoint I
Function
-Checks flame conditions during combustion.
-When AFR rod detects unexpected flame conditions, the computer of the
water heater makes adjustments in the fan motor speed to compensate.
Failure event 1. Unable to detect flames when flames actually do occur
2. Detecting a false flame when no flames actually occur
Effects on the water
heater if AFR rod fails
1. The water heater will not initiate the ignition process. The "721" error
code will display.
2. The fan motor speed cannot be modulated properly under abnormal flame
conditions, which can result in excessive CO emission.
Error codes when the
AFR rod fails 101 391 991
Diagnostic 1. Visual inspection: connection/braking of wires, soot on it.
2. Check voltages: proper range of values are shown below.
Color/Number of wires
Yellow(8) - AFR rod
(Between the flame rod and
the computer board)
(During operation)
More than DC 0.5 μA
The AFR rod is assembled with the flame rod.
Leads to computer board
Detecting elements (AFR and flame rods)
AFR Rod
Flame rod
3/4" (17.5mm)
1/4 " ( 7.5m m )
1/2 " ( 1 2m m )
1-1 /8 " ( 2 9.3 mm )
1-3/4" (44mm)