10 AQ4 Manual
4 Maintenance
The vent system must be inspected at regular 3-month
intervals. Points of inspection are as follows:
1. Screened opening of the vent hood should be free
from foreign material and cleaned as necessary.
2. Check all vent system connections for leakage
and reseal where needed with high temperature
silicone sealant. If any vent pipe component shows
signs of deterioration, replace immediately.
3. Inspect for proper closure of the back draft
apper accessory. If the apper does not properly
close, there is a risk of freezing air coming across
the heat exchanger which may cause the pipes
inside to freeze and burst. This condition is not
covered by the water heater warranty. See Fig.
11A for a properly closed back-draft apper.
4. Conduct safety interlock test and operation of
backdraft apper once a year (Chapter 3.1.3).
Maintenance
Fig. 11B Improperly closed backdraft apper
Fig. 11A Properly closed backdraft apper
5 Troubleshooting
5.1 Introduction
Many of the questions customers ask regarding
operation of this unit can be answered by following
the troubleshooting steps outlined below. Visit
our website at www.boschhotwater.com for more
detailed troubleshooting. For best results, perform
each step before proceeding to the next.
5.2 Power vent motor runs all the time or
intermittently without water ow
1. The power vent motor is ow switch activated.
If the internal paddle inside the ow switch is
jammed, it may constantly send a signal to turn on
the fan. Shut off water supply to heater and drain.
Remove ow switch from water heater piping by
removing clips on both ends. Flush water through
both ends of ow switch to free up internal paddle
and clear debris. After ushing, shake ow switch
up and down. The internal paddle should be heard
moving freely back and forth.
2. Large amounts of water demand can create
pressure drops throughout the plumbing. This
may allow for movement of the power vent ow
switch and activate the fan. Activated sprinkler
systems or ushed toilets are common causes for
this symptom. Consult a plumbing professional
to correct unintentional water ow through the
heater.
3. In buildings with excessively high water pressure,
a condition known as “water hammer” may exist.
This condition is caused by a combination of high
water pressure and the abrupt closing of a faucet
valve or internal washer/toilet valve. This instant
change in ow results in an instant change in the
internal pressures in the pipe, which then sends
a shock wave through the pipe. This physical
shock will shake, rattle or cause a loud bang in the
plumbing which is referred to as “water hammer.”
Such a shock wave may cause movement of the
power vent ow switch which will activate the fan.
Consult a plumbing professional for the use of a
pressure reducing valve, expansion tank or water
hammer arrestor to combat this condition.
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