42 | Maintenance and service
8. Empty condensate trap.
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
A water softener is required if the water hardness exceeds 6 grains/gal
(103 mg/l) calcium carbonate. Damage to the water heater resulting
from hard water/scale deposits will not be covered under warranty.
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. 69.)
▶ 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
▶ 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
6 720 805 440 (2014/11)
▶ Reconnect electrical supply to unit, open water shutoff valves, and
return the unit to service.
Condensing heat exchanger unit
The condensing heat exchanger unit must be checked once a year by a
qualified and trained technician. If repairs are needed, the repairs
should be done by a certified Bosch technician. Commercial or
recirculating applications may require more frequent condensing heat
Fig. 70 Condensing heat exchanger unit
Condensing heat exchanger unit maintenance;
▶ Shut off power. Disconnect power cord.
▶ Dismantle all inlet, outlet pipes and the condensate tube from the
condensing heat exchanger unit.
▶ Remove the condensing heat exchanger unit from the appliance by
pulling it towards you.
▶ Examine the fin tubes visible through the back ports of the heat
exchanger. If there are significant deposits, flush out with warm
water. If obstruction remains or damage to the fins is visible,
dismantle the heat exchanger for further examination and repair.
▶ Dismantle all parts for inspection and cleaning.
NOTE: A condensing heat exchanger gasket and o-ring
kit is necessary for this step.