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Hot Water Temperature Fluctuates At Tap; Noisy Burner/Heater During Operation - Bosch 940 ESO Manual

940 eso - natural gas / 940 eso - liquefied petroleum (lp) gas
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Hot water temperature fluctuates at tap

1. Hot water is very hot out of the water heater, requiring mixing in cold
water in order to attain a usable hot water temperature. The addition
of too much cold will overpower hot water flow from the tankless
water heater.
If this slows the flow through the tankless water heater below its
activation point, it will shut off the burners. The end result is nothing
but cold water coming out of the outlet. Consult for a detailed service bulletin on overcoming temperature
2. Unbalanced pressure in water lines. Any restriction in the water
heater, such as a clogged inlet filter screen, can result in unequal
pressures between the cold and hot water lines. In such cases, when
mixing in the higher pressure cold water at the tap, the lower
pressure hot water can be overpowered. This will shut down the
burners because the hot water flow rate fell below the minimum flow
rate required for activation. Verify inlet filter screen is clean and clear
of debris. See chapter11.1, page 29 for inlet filter cleaning
3. Heater deactivated by temperature balancing valves. If the outlet
water temperature is set too high, the heater can produce
temperatures that are too hot. A temperature balance shower valve
will automatically mix in cold water to reduce the hot water
temperature. In the event of any temperature instability at a fixture
using a temperature balancing valve, refer to the valve manufacturer
for instructions on internal adjustment setting. An adjustment should
be made to minimize the amount of cold water the valve is adding.
Additionally, the temperature setting on the heater can be lowered to
prevent the temperature balance valve from mixing in too much cold.
4. Inlet water pressure is erratic due to fluctuating supply water
pressure. For installation on a private well system with the use of a
pressure tank, the lowest pressure range setting recommended is 40
- 60 psi (2.75 - 4.15 bar). Consult your installer or local plumber for
effective ways to maintain constant water pressure to the appliance
when on a well system.

Noisy burner/heater during operation

1. Sealed combustion leak. Make sure combustion cover is securely
fastened. Ensure the exhaust vent adaptor is properly sealed with
supplied gasket. Leaky seals create improper combustion resulting
in noise.
2. Blocked venting. Ensure exhaust gases are properly discharging to
the outdoors via the louvered openings on the outdoor cap and/or
recess box.
3. Lack of adequate combustion air. Drawing combustion air from a
room area of inadequate size will result in unstable burner flames and
noise. Improper piping of combustion air to the outside or
insufficient space will result in unstable burner flames and noise.
Ensure adequate combustion air is provided from the unit in
accordance with specifications in Section 5, page 11 Combustion Air
4. Lack of gas pressure. Inadequate gas pressure will cause the fuel-to-
air mixture (CO
) to be out of adjustment. This will result in unstable
burner flames and noise. Ensure gas pressure is in accordance with
specifications in Section8.6, page 23 Measuring Gas Pressure. A gas
pressure reading is needed to proceed further. Contact your original
installer or a local certified gas technician to obtain this reading.
5. Verify proper CO
readings per chapter 11.4, page 30. CO
adjustments must be done by a certified gas technician with a
calibrated combustion gas analyzer.
940 ESO
Troubleshooting | 35
6 720 644 956 (2016/05)



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