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Winterizing For Seasonal Use; Mineral Scale Build-Up; Adjusting Co2 - Bosch 940 ESO Manual

940 eso - natural gas / 940 eso - liquefied petroleum (lp) gas
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30 | Maintenance and service
Pressure Relief
• Manually open the pressure relief valve to ensure proper operation.
Inlet Water Filter
• Verify the inlet filter screen is clean and undamaged. The inlet water
filter is located on the bottom of the appliance, to the right of the cold
water inlet fitting. (See Fig.26, page 20). Close installer supplied
water shutoff and remove wireform spring clip from filter. Remove
filter, clean and or replace if damaged.
• In areas where the water supply has a high mineral content, the heat
exchanger should be flushed with a descaling solution. Scale build up
will shorten the life of the water heater and damage resulting from
scale is not covered under warranty. Refer to section11.3 for
detailed instructions on descaling the heat exchanger.
Fin Coils
• Inspect heat exchanger fin coils for soot build-up or blockage. To
access fincoil, consult service bulletin at If
there is evidence of soot build-up or blockage, the heat exchanger
should be removed by a professional and cleaned thoroughly.

Winterizing for seasonal use

The water heater must not be installed in a location where it may be
exposed to freezing temperatures. If the heater must be left in a space
which is likely to experience freezing temperatures, all water must be
drained from the heater. If precautions are not taken, resulting damage
will not be covered under the warranty. NOTE: Use of agents such as
anti-freeze is not permitted and voids the warranty, as they may cause
damage to the water heater's internal components.
1. Press ON/OFF switch on the water heater to turn OFF the heater and
unplug power supply cord. The display should be blank.
2. Shut off gas supply to heater.
3. Shut off the water supply to the water heater using installer supplied
shutoff valve.
4. Open hot water taps to drain and relieve pressure from the plumbing
system. If water continues to flow after 5 minutes, a crossover of the
hot and cold water pipes is present and must be corrected before
5. Open service valves (if available) or disconnect inlet and outlet water
pipes from the water heater. Place a small bucket underneath the
water heater to catch residual water remaining inside the water
6. Using an air compressor, blow short bursts of air (100psi max)
through the inlet water connection until there is no water present
coming through the outlet water connection of the heater.
7. Reconnect water fittings and return heater to service when danger of
freezing has passed.

Mineral scale build-up

Periodic descaling may be necessary in areas with high mineral content
in the water. Scale buildup in the heat exchanger may result in lower flow
rates, error codes of A7 and E9 and boiling sounds in the heat
Descaling using a pump
▶ Disconnect electrical supply from the water heater.
▶ Shut off the water supply to the water heater using (installer
supplied) shutoff valve.
▶ Open hot water taps to drain and relieve pressure from the plumbing
▶ Drain water from the unit's heat exchanger by disconnecting inlet and
outlet water connections from the heater.
▶ Connect a line (A) from the outlet of the circulating pump (installer
supplied) to the inlet water fitting on the water heater (see fig. 55.)
6 720 644 956 (2016/05)
▶ Using another line (B), connect the water outlet fitting on the water
heater. Route the other end of this line into a descaling reservoir.
▶ Using a 3rd line (C) from the descaling reservoir, connect to the inlet
side of circulating pump. Install a filter on the end of the line in the
descaling reservoir.
▶ Make sure all connections are "water tight.".
▶ Fill tank with descaling solution so both lines inside are submersed.
We recommend a straight white vinegar solution. If using a
commercial descalant, refer to manufacturer's instructions for
proper dilution ratio.
▶ Operate the circulating pump.
▶ Make sure there are no leaks and the solution is flowing from the
descaling reservoir through the heater and returning to the reservoir.
▶ Run solution through the heater until the solution returning to the
descaling reservoir comes out clear. (Changing to a fresh solution
may be necessary during this process).
▶ Disconnect all lines and drain all solution from heat exchanger.
Properly discard of solution.
▶ Position a container below the hot water outlet and connect cold
water supply. Open cold water supply shutoff valve and flush heat
exchanger with clean water.
▶ Shut cold water shutoff valve and reconnect hot water supply to the
water heater.
▶ Reconnect electrical supply to unit, open water shutoff valves, and
return the unit to service.
Fig. 55

Adjusting CO2

The CO2 can only be adjusted by a certified gas technician with a
calibrated CO2 analyzer.
adjustment is required in Natural Gas
installations where energy content is less than 900
BTU/cuft, and in installations with repeated
unresolved EA and EC errors (ref. to page 36
"Problem solving").
▶ One factor that may affect CO
pressure. Please see Chapter 8.6 for the procedure
to measure gas pressure and record your findings
levels is improper gas
940 ESO



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