Pilot Lights, But Burners Will Not Come On - Bosch 520-HN -L Manual

– With water flowing and heater sparking, apply flame
from a match or lighter to the rear of the pilot burner.
Avoid applying flame to flame sensor.
If main burners only light, repeat procedure and watch for
brief flame from pilot burner:
2. If pilot does not light:
– Measure voltage between pilot electrovalve wire
connection and ground when heater is sparking (Do not
disconnect wire); Voltage must be at least 1.1VDC. If
less, contact Bosch Tech Support.
– Clean or replace pilot orifice. See Bulletin CT-22.
– If heater has never fired, check for correct gas type on
rating label on right side of cover. If incorrect, exchange
heater for correct gas type. It is not practical to convert
a heater to a different gas type.
– If heater has never fired or gas lines have recently been
serviced, there may be air in the gas line. Turn water on,
wait until sparking has stopped, turn water off and back
on again. Repeat until pilot lights or have gas technician
bleed the gas line.
– Verify that gas shut off valves are open. Gas must be
supplied to heater.
– Measure gas pressure at inlet tap with manometer. See
service bulletin CT-04.
– If no gas is present, Maxitrol regulator supplied with
heater may be upside down or locked. Reinstall
correctly or unlock regulator (see Bulletin CT-14).
Measure gas pressure on gas line before Maxitrol. If
over 14", Maxitrol should lock up to prevent high gas
pressure from damaging heater.
– Check for tight and clean wire connections to pilot and
burner electrovalves. Remove connector and clean
with pencil eraser if dirty or corroded.
– Check pilot electrovalve function: Turn off gas supply to
heater, remove wire connection from electrovalve,
connect positive terminal from a single cell battery
(1.2-1.5VDC) to electrovalve terminal and a jumper
wire from negative terminal of battery to ground. When
connection is complete, there should be an audible
"click" from the electrovalve. If no click is heard,
electrovalve may be faulty.
3. If pilot and burners light:
– Check for drafts around pilot burner. See Bulletin CT-
03 but apply smoke to pilot burner without heater
operating. If smoke is blown away from pilot burner,
find cause of draft and correct.
– If an AQ4 powervent is installed:
– I. Check for presence of time delay in powervent. If
there is no time delay, replace with newer style AQ4
powervent with time delay.
520 HN
– II. If powervent has time delay, check for correct setting
(2 seconds).
– III. Check for adequate minimum vent length. Refer to
AQ4 Installation Manual.
7.4

Pilot lights, but burners will not come on

1.Verify that gas type indicated in the rating sticker located on
the cover's right hand side, coincides with the gas type you are
using. NG is a natural gas unit and LP is for liquid propane (See
chapter 2.2).
2. Pilot flame should be blue in color and completely engulfing
the flame sensor. If not, the pilot orifice may be dirty or
clogged. See chapter 5.3 on how to clean the pilot and orifice.
3.If sparking does not stop when pilot is lit, the flame sensor
may not be recognizing pilot flame. Verify that wire connection
clip on wire from the bottom of the flame sensor is secure. If
flame sensor is fully engulfed by the pilot flame, it may be dirty.
Clean entire surface area of flame sensor with a pencil eraser or
fine steel wool.
4. Confirm that wire connection to the burner electrovalve is
secure (see components diagram for location of burner
electrovalve). Inspect terminals for corrosion. To clean
terminals, remove the spade connectors and clean with a
pencil eraser.
5. Ignition unit, burner electrovalve or flame sensor maybe
faulty. Measure voltage from the spade connection of the
burner electrovalve to ground with wire connected (see
components diagram for location of burner electrovalve). After
the pilot lights, voltage should read at least 1 VDC. If voltage is
proper, the burner electrovalve may be faulty. If voltage is not
proper, ignition unit may be faulty.
Troubleshooting | 27
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