520 Indoor / 520 Outdoor Service Manual
Ver. 1.00
1
CONFIDENTIAL
520 Direct Vent Indoor
and Outdoor Models
On-Demand Water Heater
Service Manual
Water Heater
Model 520 Direct Vent Indoor
Water Heater
Model 520 Outdoor
A.O. Smith Water Products Company
500 Tennessee Waltz Parkway
Ashland City, TN 37015
Toll Free: 1
-
877
-
737
-
2840
520 Indoor / 520 Outdoor Service Manual
Ver. 1.00
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CONFIDENTIAL
Table of Contents
1.
Specifications………………………………………...…...………….……………………...…………………………
3
2.
Exterior view……………………...…….……………………………………………………………………………….
4
3.
Interior view…………………………….………………………………………………………………………………..
6
4.
List of main components in the interior view…….………………………………………………….……
8
5.
Schematic diagram…………………………………………………………………………………………………....
10
6.
Wiring diagram………………………………………………………………………………………………………….
11
7.
Wiring diagram checkpoints for diagnosis………………………..………………….………………….…
12
8.
8.
(A) Resistance values of the temperature thermistors……..…………………………………………
(B) Resistance values of the temperature Exhaust thermistors…………………………………..
14
15
9.
Operational flow chart…………………………………………………………………………………………….…
16
10.
Component specifications………………...…………………………………………………………………….…
17
11.
Fault Analysis & Specifications…………………………………………………………………………………. 41
12.
Controls and settings……………………………………………………………………………………………….…
54
13.
Components diagrams………………………………………………………………..................................
89
14.
Parts list……………………………………………………………………………………………………………………..
92
15.
Revisions……………………………………………………...…………………………………………………………...
97
520 Indoor / 520 Outdoor Service Manual
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CONFIDENTIAL
1. Specifications of the 520 Direct Vent Indoor* and Outdoor
Model 520 Direct Vent Indoor 520 Outdoor
Dimensions H25.6"×W18.5"×D12.4"
Weight 73 lbs. 71 lbs.
Combustion
INPUT
BTU/h Max 199,000
Min 13,000
Combustion System
Power vent
Installation Direct-vent Outdoor
Fan motor PWM turbo fan
Manifold
Pressure**
Max
LP 4.9” WC
Natural 3.6” WC LP 4.6” WC
Natural 2.8” WC
Min
LP 1.0” WC
Natural 0.7” WC LP 0.9” WC
Natural 0.6” WC
Water control
Flow rate 0.5 GPM to 9.0 GPM
Available set
temperatures 100°F to 185°F
default: 120°F
Temperature on
the unit settings 100°F, 105°F, 110°F, 115°F, 120°F, 125°F, 135°F, 140°F,
145°F, 150°F, 155°F, 160°F, 165°F, 170°F, 175°F, 185°F
Bypass valve Yes
Thermistors 4 thermistors
(In, Out, Mixing, Exhaust) 3 thermistors
(In, Out, Mixing)
Operation
Remote controller
model 9007603005
PCB model 520
Indicators Red LED on PCB during operation & 3-digit 7-SEG LED
Power supply
control Surge absorber & Power ON-OFF switch
Features
Freeze protection
Ceramic heaters and Auto-firing system
Self
-
combustion
improvement
Air-Fuel Ratio Detection System
Easy-Link system
Yes (Up to 4 units)
Multiple-system N/A
* The 520 Direct Vent Indoor means the 520 Indoor in the text.
**The manifold pressure measurement is based on conditions without front cover.
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CONFIDENTIAL
4. List of main components in the interior view
No.
Description Item# in
components
diagram AOS Part # Common parts
For other models
1 Manifold assembly with
gas valve assembly 118 319143-046 (LP model)
319143-047 (NA model)
2 Computer board 701 319143-126
3 Igniter 123 319143-052 110, 310, 510, 910, 710
4 Transformer 702 319143-127 910
5 Water inlet 411 319143-074 710
6 Water outlet 435 319143-098 710
7 Mixing thermistor 418 319143-081 710
8 Inlet thermistor 422 319143-085 510, 910, 710
9 Output thermistor 433 319143-096 510, 910, 710
10
Overheat cut-off fuse 404 319143-067
11
Burner assembly
(Burners) 101 319143-030
12
Hi-limit switch 432 319143-095 510, 910, 710
13
Fan motor 114 319143-043 310 Indoor, 510 Indoor,
910, 710
14
Freeze protection
thermostat
704
319143
-
129
(
520
Indoor
)
719
319143
-
144
(
520
Outdoor
)
15
Flow sensor 429 319143-092 910, 710
16
Primary heat exchanger
assembly
401
319143
-
064
(
510
Indoor
)
456
319143
-
118
(
510
Outdoor
)
17
Gas inlet 121 319143-050 110, 310, 510
18
Pipe heater 426 319143-089
19
Water control valve 423 319143-086 910, 710
20
Secondary heat
exchanger 446 319143-108
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CONFIDENTIAL
No.
Description Item# in
components
diagram AOS Part # Common parts
For other models
21
Outlet drain plug 416 319143-079 710
22
AFR rod 106 319143-035 310, 510, 910, 710
23
Flame rod 106 319143-035 310, 510, 910, 710
24
Igniter rod 108 319143-037 310, 510, 910, 710
25
Surge box 715 319143-140
26
Heater 101 405 319143-068 910, 710
27
Outlet heater 415 319143-078 110, 310, 510, 710
28
Over heat cut off fuse
for combustion chamber
008 319143-016 710
29
Secondary heat
exchanger
drain plug 458 319143-120
30
Fan motor for exhaust 124 319143-053
31
Hi-limit switch for
exhaust 442 319143-104
32
Exhaust thermistor
assembly 706 319143-131
33
Inlet drain plug (water
filter) 409 319143-072 910, 710
34
Flat heater 453 319143-115
35
3 array heater 454 319143-116
36
Case assembly 001
319143
-
009
(
510
Indoor
)
015
319143
-
023
(
510
Outdoor
)
37
Manifold port N/A N/A
38
Gas inlet port N/A N/A
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CONFIDENTIAL
5. Schematic diagram
The diagram below refers to both the 520 Indoor and 520 Outdoor.
The 520 Outdoor doesn’t have the fan motor for exhaust, exhaust thermistor and hi-limit switch
for exhaust.
1. When a hot water tap is opened, cold water enters the 520 Indoor or 520 Outdoor.
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 520 Indoor or 520 Outdoor shuts down.
*520 Indoor controls the fan motor for exhaust and detects the exhaust temperature by using
the exhaust thermistor and hi-limit switch for exhaust during procedures of #3 to #8.
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CONFIDENTIAL
7. Wiring diagram check points for diagnosis
The table below applies to both the 520 Indoor and 520 Outdoor.
Check-
point
Part and Description Color of wires Normal range
A, A1
100V Power supply White – Black (A)
Brown – Brown (A1) 90 to 110 VAC
A2 120V Power supply Black - White 108 to 132 VAC
B1 Heater Black - Black 90 to 110 VAC
B2 Igniter Purple - Purple 90 to 110 VAC
C Gas valves
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
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 1.0 Ω
C2 Overheat cutoff fuse Blue - Blue Less than 1 VDC
and
less than 1.0 Ω
D1 Easy-link connectors Red – Red
Blue - Blue 15 VDC
(during Easy-link operation)
D2 White - White
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CONFIDENTIAL
Check-
point
Part and Description Color of wires Normal range
E1 Mixing thermistor Black - Black See table on p. 14 E2 Output thermistor Black - Black
E3 Inlet thermistor Black - Black
F Remote controller * 11 to 25 VDC
G1 Fan motor Red – Blue
Yellow – Blue
Orange - Blue
110 to 160 VDC
13 to 17 VDC
2 to 6.5 VDC
G2 Fan motor for exhaust
H1 Gas proportional valve White - red
1 to 15 VDC
(during operation)
and
20 to 40 Ω
H2 Flow sensor Red - Black 4 to 5.5 VDC
White(+) – Black(GND) 1 to 4 VDC
(1,200 pulse / min)
I
Air-fuel ratio rod Yellow - AFR rod
(Between AFR rod and
the computer board)
More than 1 μA
(during operation)
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
13.0 to 16.0 VDC
ON: 12.5 to 16.0 VDC
OFF: 0 to 1 VDC
1 VDC Less ( 0° position)
K1 Exhaust thermistor White - White See table on p. 15
K2 Hi-limit switch for exhaust White - White Less than 1 VDC
and
less than 1.0 Ω
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CONFIDENTIAL
8. (A) Resistance values of the temperature thermistors
The 520 Indoor and 520 Outdoor use the same 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
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CONFIDENTIAL
8. (B) Resistance values of the temperature Exhaust thermistor
Only the 520 Indoor is equipped with the exhaust thermistor.
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 kΩ30.04 24.12 19.50 15.87 13.00 10.71 8.87 7.39 6.19 5.21
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 kΩ4.41 3.74 3.19 2.73 2.35 2.03 1.76 1.53 1.33 1.16 1.02
Temperature
Temperature(°F) vs. Resistance(kΩ)
Temperature(° ) vs. Resistance(kΩ)
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
20 40 60 80 100 120 140 160 180 200 220
Temperature / ºF
Resisitance / k
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
0 10 20 30 40 50 60 70 80 90 100
Temperature / ºC
Resisitance / k
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CONFIDENTIAL
10. Component specifications
10-1.
Burners……………………..………………………………….………………………………....……….
18
10-2.
Gas manifold………….………………………………………………..…………………………….....
19
10-3.
Fan motor……………………………………………………………………………………….……...…
20
10-4.
Fan motor for exhaust……………………………………………………………………………….
21
10-5.
Gas valve assembly………………..……………………….………………………………..……….
22
10-6.
Flame rod………………………………………………………….……………………………………….
23
10-7.
AFR rod……………………………………………………………………………………………….…….
24
10-8.
Heat exchanger………………………………………………………………………………..………..
25
10-9.
Secondary heat exchanger…………………………………………………………………………
26
10-10.
Flow sensor……………………………………..……………………………………………………..…
27
10-11.
Water control valve………………………………………………….………………………………..
28
10-12.
Thermistors……….………………………..…………………………………………..……………..…
30
10-13.
Exhaust thermistor…………………………………………………………………………….……...
31
10-14.
Hi-limit switch…………………..………………………………………………….......................
32
10-15.
Hi-limit switch for exhaust………………………………………………………………………...
33
10-16.
Overheat cutoff fuse…..………………………………………………………….....................
34
10-17.
Freeze protection heaters…………….……………………………………………………………
35
10-18.
Computer board……………….………………………………………………………………….……
36
10-19.
Transformer. ………...…………………………………………………………………………………..
37
10-20.
Igniter………………………………………………………………………………………………………..
38
10-21.
Freeze protection thermostat………………………………………………………….………..
39
10-22.
Surge box……………………………………………………………………………………………….... 40
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CONFIDENTIAL
10-1. Burners
Unit Part # #101 AOS Part # 319143-030
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 events
1. Unable to initialize/sustain combustion
2. Dust or soot deposit on the burner surface.
3. Cracks on 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 Visual inspection: excessive dust deposit on the burner surface and/or
unstable flame conditions during operation.
Color/Number of wires N/A
The burner for the 520 Indoor and 520 Outdoor has the “46V” mark as shown below.
The burner damper is not
included in 319143-030.
Burner damper (#111 / 319143-040)
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CONFIDENTIAL
10-2. Gas manifold
Unit Part # Included
in #118 AOS Part # 319143-046 (LP)
319143-047 (NG)
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 events
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
319143-046 (for LP model)
and 319143-047 (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 for the LP
has the “46V” mark, and the
gas manifold for the NA has
the “59J” mark on the
manifold plate.
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CONFIDENTIAL
10-3. Fan motor
Unit Part # #114 AOS Part # 319143-043
Checkpoint G1
Function To provide combustion air into the combustion chamber and to exhaust flue
gas.
Failure events
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.
Color/Number of wires
Red-Blue 110 to 160 VDC (Input)
Yellow-Blue 13 to 17 VDC (Input)
Orange-Blue 2.0 to 6.5 VDC (Input)
White-Blue Verify the fan motor speed using the Diagnostics Mode of
the 520 models computer board or the 9007603005
temperature remote controller. See Section 12-1 for details.
PWN turbo fan motor
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