Water Is Too Hot; Water Is Not Hot Enough; Low Water Flow/Pressure; Hot Water Temperature Fluctuates At Tap - Bosch Greentherm T9800 SEO199 Installation And Operating Instructions Manual

for water flow at each tap. There should be no water
flowing. Any continuous flow of water, small or large,
indicates a crossover is present and must be corrected.
Consult a professional plumber for help in correcting a
crossover. Failing single lever faucet mixing valves are
common causes of plumbing crossovers.
8.3

Water is too hot

1. Selected temperature on the unit may be too high. To lower
output temperature, see chapter 6.4, page 28.
2. Confirm the heater's gas type coincides with the type of gas
being supplied. See fig. 2, page 11 for the location of the
rating plate.
3. In areas where the water has a high mineral content,
periodic descaling may necessary. See chapter 7.3,
page 35 for directions.
8.4

Water is not hot enough

1. Selected temperature on the unit may be too low. To raise
the output temperature, see chapter 6.4, page 28.
2. Confirm the heater's gas type coincides with the type of gas
being supplied. See fig. 2, page 11 for the location of the
rating plate.
3. Check the inlet gas particle screen for blockage at inlet of
the gas connection on bottom of unit.
4. Verify the gas pressure is in accordance with specifications
in chapter 4.13, page 25. A gas pressure reading is
needed to proceed further. Contact your original installer
or a local certified gas technician to obtain this reading.
5. Cold water is mixing into the hot water lines (plumbing
crossover). A plumbing crossover can unintentionally mix
cold water with the hot water leaving the heater. The end
result is a cooler water temperature than desired. To check
for a plumbing crossover, shut off the cold water supply to
the water heater. Then open all of the hot water taps served
by the heater. Wait 10 minutes and check all taps for water
flow. There should be no water flowing. Any continuous
flow of water, small or large, indicates a crossover and must
be corrected. Consult a professional plumber for help in
correcting a crossover. Failing single lever faucet mixing
valves are common causes of plumbing crossovers.
8.5

Low water flow/pressure

1. Too many hot water applications are being used
simultaneously or too much flow is demanded. The water
heater will effectively support two 2.0-2.5 GPM shower
heads simultaneously or multiple sink applications. Greater
draws will result in a water pressure drop and reduced flow
at taps.
Greentherm T9800 SEO 160/199 | SECO 199 – 6 720 817 281 (2018/12)
2. If the selected temperature on the unit is set too high for the
demanded flow rate, the water heater will close its
motorized water valve, reducing the hot water flow rate in
an attempt to reach the selected output temperature.
Lowering the selected temperature will allow the motorized
water valve to open up for increased water flow rate.
3. Clean the inlet water filter screen per chapter 7.1, page 34
to increase flow through the heater.
4. Inspect the water path for obstructions. Make sure all
shower heads, faucet aerators and whole house filters are
clear of debris. It may be necessary to upgrade to higher
flow rate shower heads if allowed by local code.
5. In areas where the water has a high mineral content,
periodic descaling may necessary. See chapter 7.3,
page 35 for directions.
8.6

Hot water temperature fluctuates at tap

1. If the water heater set point temperature is high, mixing at
the faucet could deactivate the water heater. The addition
of too much cold will overpower hot water flow from the
tankless water heater reducing its flow below the activation
point.
If this slows the flow through the tankless water heater
below its activation point, it will shut off the burners.
2. Unbalanced pressure in water lines. Any restriction in the
water heater, such as a clogged inlet water 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 that the inlet water filter
screen is clean and clear of debris. See chapter 7.1,
page 34 for inlet water filter cleaning instructions.
3. The water heater may be deactivated by temperature
balancing valves. If the water heater's set point
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 water heater
can be lowered to reduce the amount of cold water that the
temperature balance valve mixes in.
Troubleshooting
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