Electrical Diagnostic Points
Wire Color
Voltage
Resistance
Connector No. Pin No.’s
Thermal Fuse / Overheat Switch
Red - Red
11 - 13 VDC
below 1 ohm
F6 H1
F6 ~ H12
Flame Rod
Place one lead of the meter to the flame rod and the other to ground. With the unit running, 5-150
VAC should be read. Set the meter to the
μ
amp scale and series the meter in line with the flame
rod. Proper flame circuit should read 1
μ
amp or greater. If not, then remove the flame rod and
check for carbon and damage.
Thermistors
Check all thermistors by inserting meter leads into each end of the thermistor plug. Set the meter to
the 20 K ohm scale and read resistance. Applying heat to the thermistor bulb should decrease the
resistance. Applying ice to the thermistor bulb should increase the resistance. Typical resistance
values are: 11.4-14 K ohm for 59°F; 6.4-7.8 K ohm for 86°F; 3.6-4.5 K ohm for 113°F; 2.2-2.7 K ohm
for 140°F; 0.6-0.8 K ohm for 221°F
Outgoing Water Thermistor
4
~
3
5
F
e
v
o
b
a
e
e
s
e
ti
h
W
-
e
ti
h
W
Heat Exchanger Temperature Thermistor
1
1
~
3
4
F
e
v
o
b
a
e
e
s
e
ti
h
W
-
k
n
i
P
2
1
~
3
3
F
e
v
o
b
a
e
e
s
e
ti
h
W
-
e
g
n
a
r
O
Surge Protector
Black - White
108 - 132 VAC
N/A
D2
1 ~ 3
Blue - Brown
108 - 132 VAC
N/A
D1
1 ~ 3
With the power off, check the continuity through the surge protector. Check by placing one meter
lead on the top pin #1 and bottom pin #3. Check by placing one meter lead on the top pin #3 and
bottom pin #1. If there is continuity across both sets of points, then the surge protector is good.
Controller
Terminals B1
10 - 13 VDC
1.5 - 3.0 K ohms
B
1 ~ 3
Frost Protection
There are electrical heating elements mounted at different points to protect the water heater from
freezing.
heaters located on the hot water outlet line
180 - 207 ohms
heater located on heat exchanger piping
156 - 180 ohms
heater located on water flow sensor
24 - 28 ohms
Amp fuses
There are two inline 3 amp glass fuses. Remove the fuse and check continuity through it. If there is
continuity then the fuse is good.
Intake Air Thermistor
On-Demand Gas Tankless Water Heater Service Manual
14
305e/o, 305i, 505e/o, 505i
