A.O. Smith Water Heater 520 Direct Vent Indoor User manual

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
2
CONFIDENTIAL
Table of Contents
1.
Specifications………………………………………...…...………….……………………...…………………………
3
2.
Exteriorview……………………...…….……………………………………………………………………………….
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 valuesof the temperature thermistors……..…………………………………………
(B) Resistance values of the temperature Exhaust thermistors…………………………………..
14
15
9.
Operationalflowchart…………………………………………………………………………………………….…
16
10.
Componentspecifications………………...…………………………………………………………………….…
17
11.
FaultAnalysis & Specifications…………………………………………………………………………………. 41
12.
Controls and settings……………………………………………………………………………………………….…
54
13.
Components diagrams………………………………………………………………..................................
89
14.
Parts list……………………………………………………………………………………………………………………..
92
15.
Revisions……………………………………………………...…………………………………………………………...
97

520 Indoor / 520 Outdoor Service Manual
Ver. 1.00
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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 PWMturbo 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 GPMto 9.0 GPM
Available set
temperatures 100°F to 185°F default:120°F
Temperatureon
the unitsettings 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 LEDon PCB duringoperation &3-digit7-SEGLED
Power supply
control Surge absorber & Power ON-OFF switch
Features
Freezeprotection
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
* The520 Direct Vent Indoor means the520 Indoor in the text.
**The manifold pressure measurement isbased on conditions without front cover.

520 Indoor / 520 Outdoor Service Manual
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CONFIDENTIAL
2. Exterior view
Top view Bottom view
Side view
Front view
520 Indoor
18-1/2" (470mm)
6-1/4" (162mm)
3-1/2" (90mm)
12-1/2" (320mm)
1/2" (13mm)
GAS 6" (154mm)
HOT 7-1/4" (182mm)
COLD 8-5/8" (221mm) HOT 5-5/8" (142mm)
120VAC 8-1/2" (216mm)
10-1/2" (267mm)
6- 1/ 4" (159mm)
2-3/4" (69.1mm) INTAKE
2-3/4" (71.8mm)EXHAUST
12-1/2" (316mm)
25-1/2" (649mm)
120VAC 2-1/4" (58mm) 4-5/8" (117mm) GAS
Male 4" (101mm) 4" (102mm) Female
3/4" (19mm)
Condensate drain port
Condensate drain port
29-1/2" (751mm)
1-1/4" (29mm) COLD
6-5/8" (168mm)
18-1/2" (470mm)
7-1/4" (184mm)
6-3/8" (162mm)
GAS 4-5/8" (117mm)
120VAC 2-1/4" (58.2mm)
HOT 5-1/2" (142mm)
GAS 6" (154mm)
HOT 7-1/4" (182mm)
120VAC 8-1/2" (216mm)
COLD 8-5/8" (221mm)
COLD 1-1/4" (29.2mm)
10-1/2"(267mm)
6-1/4"(159mm)
Condensate drain port

520 Indoor / 520 Outdoor Service Manual
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CONFIDENTIAL
Side view Frontview
Bottom view
Top view
520 Outdoor
1/2" (12.5mm)
GAS 6" (154mm)
HOT 7-1/4" (182mm)
COLD 8-5/8" (221mm)
12-1/2" (316mm)
120VAC 8-1/2" (216mm)
10-1/2" (267mm)
Condensate drain port
18-1/2" (470mm)
3-1/2" (90mm)
12-1/2" (320mm)
4-1/2" (115mm)
4-1/2" (115mm)
4-5/8" (117mm) GAS120VAC 2-1/4" (58.2mm)
HOT 5-5/8" (142mm) 6-1/4" (159mm)
Condensate drain port
29-1/2" (751mm)
1-1/4" (29.2mm) COLD
25-1/2" (650mm)
1-5/5"(42.6mm)
8"(203mm)
1-5/8"(42mm)
18-1/2" (470mm)
GAS 4-5/8" (117mm)
120VAC 2-1/4" (58.2mm)
HOT 5-1/2" (142mm)
GAS 6" (154mm)
HOT 7-1/4" (182mm)
120VAC 8-1/2" (216mm)
COLD 8-5/8" (221mm)
COLD 1-1/4" (29.2mm)
10-1/2"(267mm)
6-1/4"(159mm)
Condensate drain port

520 Indoor / 520 Outdoor Service Manual
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CONFIDENTIAL
3. Interior view
520 Indoor
20
32
16
10
18
24
11
15
3
19
37
38
17
33
58
27
6
7
21
23
30
31
9
1
13
25
2
22
4
12
14
28
29 26
26
35
35
34
36

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520 Outdoor
1
3
4
5
627
7
8
9
2
11
13
15
16
17
19
20
21
22
23 24
25
29
28
10
12
33 37
38
26
18
35
34
35
26
14
36

520 Indoor / 520 Outdoor Service Manual
Ver. 1.00
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CONFIDENTIAL
4. List of main components in the interior view
No.
Description Item# in components
diagram AOS Part# Common parts
Forothermodels
1 Manifold assemblywith
gas valve assembly 118 319143-046 (LP model)
319143-047 (NA model)
2 Computerboard 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
Overheatcut-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

520 Indoor / 520 Outdoor Service Manual
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CONFIDENTIAL
No.
Description Item# in components
diagram AOS Part# Common parts
Forothermodels
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
Heater101 405 319143-068 910, 710
27
Outlet heater 415 319143-078 110,310, 510,710
28
Overheat cutoff fuse
for combustion chamber
008 319143-016 710
29
Secondary heat
exchanger
drain plug 458 319143-120
30
Fan motorfor 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

520 Indoor / 520 Outdoor Service Manual
Ver. 1.00
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CONFIDENTIAL
5. Schematic diagram
Thediagram below refersto both the 520 Indoor and 520 Outdoor.
The 520 Outdoor doesn’t have the fan motor for exhaust, exhaust thermistor and hi-limit switch
forexhaust.
1. When a hot water tap is opened,cold water enters the 520 Indoor or520 Outdoor.
2. The water flow sensor detects this waterflowand sends this information tocomputer.
3. The computer initiatesfan motorand sends signal to igniter to create ignition spark.
4. The main, proportional, and solenoid gas valves open to allow gas input.
5. The gas ignitesand flames appear inside the burner chamber.
6. Water circulates through the heatexchanger and isheated up to the set temperature.
7. Using thermistors to measure temperatures, the computer modulates the gas and water valves to
ensureproper output water temperatures.
8. When the tap is closed, the520 Indoor or 520 Outdoor shutsdown.
*520 Indoor controls the fan motor for exhaust and detects the exhaust temperature by using
the exhaust thermistor and hi-limit switch for exhaustduring procedures of #3 to #8.
COLD
HOT GAS
Condensate drain
Flow sensor
Output thermisotor
Inlet thermisotor
Bypasss vavle
Water control valve
Drain pan
Igniter rod
Air-fuel ratio rod
Hi-limit switch
Intake port Primary heat exchanger
Exhaust
AC120V
PCB
ON-OFF-switch
for power supply
Transformer
Surge box
Main gas vavle
Gas proprotional valve
Gas solenoid valve 1
Freeze protection thermostat
Gas solenoid valve 2
Heater
Fan motor
for combustion
Igniter
Gas solenoid valve 3
Flame rod
Mixing thermistor
Secondary heat exchanger
Fan motor
for exhaust
Hi-limit switch
for exhaust
Exhaust thermistor