Main Burners Go Out During Hot Water Use; Hot Water Temperature Fluctuates At Tap - Bosch 520-HN -L Manual

28 | Troubleshooting
Fig. 25 Electrical wiring diagram
[2] Spark electrode
[3] Flame sensor
[18] Microswitch
[23] Ignition unit
[25] Burner electrovalve
[28] Pilot electrovalve
[31] Temperature limiter/ECO
[32] Flue gas sensor
[33] Gas valve
[34] Main burner operation LED
[35] On/Off button
[50] Hydrogenerator
[111]Temperature limiter/ECO
7.5

Main burners go out during hot water use

1.Hot water flow rate is dropping below required activation
flow rate. Turn flow control knob all the way clockwise. Fully
open a hot water faucet and fill a quart container. If the
container fills in 36 seconds or less, the flow rate (0.5 gallon
per minute) is sufficient to activate the water heater. If flow rate
is not sufficient, the water heater's inlet filter should be
inspected and cleaned. See chapter 5.2.
2. Close installer supplied cold water shut off valve (if none
installed, install before proceeding). Open every hot water tap
supplied by the heater. Wait 5 minutes and check all taps. Any
water running, even a trickle, is a sign of a plumbing crossover.
Consult local plumber or service person for help correcting a
plumbing crossover.
3. The minimum water pressure required is 30psi. For
installation on a well system with use of a pressure tank, the
lowest pressure range setting recommended is 30-50 psi
(2.07-3.45 bar).
4. Hot water is very hot out of the tap, requiring a lot of cold
water to be added to it in order to attain a useable hot water
temperature. The addition of too much cold water will slow the
6 720 644 942 (2014/11)
flow within the tankless heater, decreasing it below activation
point, which shuts off the burners. The end result is nothing but
cold water coming out of the outlet. Reduce the amount of cold
water to be mixed by turning the gas control knob clockwise for
lower hot water temperatures. See chapter 4.4 for details in
lowering temperature.
5. A temperature limiter (ECO) is tripped. The water heater
does not have a thermostat. If the inlet water is preheated, the
unit will overheat, stopping the flow of gas. Supply heater inlet
with cold water only.
6. Flue gas sensor is tripped. Lack of adequate combustion air
or venting improperly by reducing pipe diameter, improper use
of elbows or exceeding maximum vent length will trip the flue
gas sensor. Confirm that venting and combustion air meet
requirements in this manual. Refer to service bulletin CT-03.
7. If the water has a high mineral content, the heat exchanger
may be scaled internally. This restricts the water path, causing
the water to overheat and trip the temperature limiter (ECO).
To descale the heat exchanger, consult section 5.6.
8. The water valve assembly, which allows gas flow to the
burners, requires periodic maintenance (every 2-5 years
depending on water quality and use). See section 5.2.
7.6

Hot water temperature fluctuates at tap

1. Close off installer supplied cold water shut off valve (if none
installed, install before proceeding). Open all hot water taps
supplied by the heater. Wait 5 minutes and check all taps.
Water running is a sign of a plumbing cross-over. Consult a local
plumber or service person for help in correcting a plumbing
crossover.
2.Check for a clogged inlet water filter screen. See chapter 5.2.
3. Hot water is very hot out of the tap, requiring a lot of cold
water to be added to it in order to attain a useable hot water
temperature. The addition of too much cold water will slow the
flow within the tankless heater, decreasing it below activation
point, which shuts off the burners. The end result is nothing but
cold water coming out of the outlet. Reduce the amount of cold
water to be mixed by turning the gas control knob clockwise for
lower hot water temperatures. See chapter 4.5. If the problem
persists, go to www.bosch-climate.us for service bulletin CT-
07.
4.The water heater is designed to modulate gas flow to the
burners when water flow varies. If the water pressure in the
home is erratic and the water flow is not consistent while a tap
is opened, then the temperature of the hot water will fluctuate.
The minimum water pressure for the home must be 30psi or
greater. For installations on a private well system with the use
of a pressure tank, the lowest pressure range setting
recommended is 30-50psi (2.07-3.45 bar). The use of a
pressure reducing/regulating valve before the water heater
520 HN