Takagi T-H3J-DV Service Handbook page 15

2.3.1 Push the button in the
middle of it. The switch was
reset if you heard and felt
the button click.
2.3.2 Turn the power back on
and test the heater. If the
burner lights, the cause of
the high limit trip must be
determined. Continue with
the troubleshooting process.
2.3.3 If the high limit switch trips
again, the water heater may
need to be descaled. See
"Descaling the Unit" on page
38.
Also, be sure to check the
temperature of the incoming
water supply. If the unit is
supplied with pre-heated
water, such as from a solar
system, the incoming water
temperature may be too
high.
If descaling and limiting the
incoming water temperature
and did not solve the issue,
replace the high limit switch.
(See page 74 for the
appropriate part number.)
2.4 If the high limit switch is
operating properly and you
don't hear the "clunking"
noises of the gas valves,
then check the overheat
cutoff fuse (OHCF) for
continuity. If there is no
continuity, or for instructions
on how to test the OHCF for
continuity, refer to "Checking
the Overheat Cutoff Fuse
(OHCF)" on page 35.
3. Check for blockages in venting, such
as bird nests, animals, or trash. An air
blockage of the exhaust, burner, or
heat exchanger will over-pressurize
On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models
the burner chamber and force the
diaphragm in the proportional valve
to close. This will prevent gas flow
through the gas valve.
4. Make sure that the clear plastic
tube connecting the proportional
gas valve to the combustion
chamber is attached. See Figure 4
and Figure 5. If necessary, see also
Figure 41 (p. 49) or Figure 42 (p.
50).
5. Check for a spark inside the
burner chamber between the
spark electrode and the burner.
The spark may be viewed through
the sight glass located next to the
igniter and flame sensor. A strong
blue spark will remain steady and
in place, while a weak spark will
jump around.
5.1 If no spark or a weak spark is
observed, check the purple
igniter wire connections at
both the igniter and at the
computer board. Unplug
and reconnect them to clean
the electrical connections.
5.2 Check the igniter rod to
make sure the black wire is
properly attached and that
the rod doesn't move. If
the rod moves, replace the
igniter rod, part number
100074222. You will also
need a rod holder gasket,
part number 100074219.
5.3 When the igniter is sparking,
check the voltage across the
purple wires coming to the
igniter. Normal voltage is
108-132 VAC. If voltage is
outside of this range, then
replace the computer board.
(See item 701 on page 78
for the correct part number.)
5.4 If the previous steps still
don't result in a spark being
generated, replace the igniter
(part number 100074640).
6. If flame comes on for only 1-2
seconds before going out, verify
that the green IN USE LED on
the built-in controller or remote
controller did not turn on. If the
LED stayed off, then inspect the
flame sensor. Clean it if necessary.
Replace it if any damage (chips or
cracks in the ceramic) is seen or
if the metal rod can move freely
in the ceramic insulator. See
"Cleaning the Rod Assembly (Flame
Sensor and Air/ Fuel Ratio Rod)" on
page 36.
Unplug both ends of the wiring
harness from the igniter assembly
and control board, then reconnect
them to ensure good connection.
Also, verify that the green wire that
is part of the circuit is attached
to one of the screw connections
around the manifold.
7. Verify proper operation of the
main gas valve (MV). To do so,
turn the power off, then check the
resistance of the MV. A normal
resistance is 1.35-1.65 kΩ. If the
resistance is out of range, replace
the gas valve/manifold assembly.
8. Verify proper operation of the gas
solenoid valves. To do so, turn
the power off, then check the
resistance of the valves. A normal
resistance reading for SV1 and SV2
is 1.35-1.65 kΩ. Normal resistance
for SV3 is 2.07-2.53 kΩ. If the
resistance is out of range, replace
the gas valve.
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