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Bosch C 950 Installation Manual page 20

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20 | Installation instructions
Freeze prevention for PVC, CPVC, and ABS systems
In cold climates, components of a tankless water heater can freeze and
burst from negative draft. A leading cause of negative draft is
combustion appliances in the building not being supplied with sufficient
combustion air. A wood stove or furnace can pull its combustion air from
the water heater's vent pipe, allowing the cold incoming air to freeze the
cold water in the heat exchanger. The best prevention for this risk is two
pipe venting which draws the appliance combustion air from outside.
Listed below is an additional measure designed to further limit backdraft
in conditions where there is a high risk of freeze damage.
Backdraft reducer
The Heat Fab backdraft reducer (9301BFP) is the preferred option for
limiting backdraft (see fig. 22).
Note: The following solution is available only for 3" diameter
Fig. 22 Part # 9301BFP
For this solution to be effective, the internal flapper must be 100%
closed when the water heater is not running. Refer to Figures 24 and25
for preferred installation positions in the vent system.
Installation considerations:
• Install damper per the supplied manufacturer's instructions.
• The damper is only to be used in the exhaust vent piping.
• Ensure directional arrow on damper label faces in the same direction
as exhaust flow.
• If installed horizontally, the axis must be horizontal or slightly pitched
up towards termination to ensure damper closes 100% when heater
is not running.
• To allow accessibility, damper must not be installed in an enclosed
section of vent pipe.
• Do not install damper in unconditioned spaces (e.g. attics)
Condensation can build up while the heater is running which can later
freeze and potentially block the flapper.
Fig. 23 Blackflow reducer (9301BFP) installed
6 720 805 440 (2014/11)
Fig. 24
Fig. 25
Enclosed vent pipe (Do not install damper)
Unconditioned space (Do not install damper)
Preferred damper position for vertical terminations



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