6 • Condensing Models: On-Demand Water Heater Service Handbook
B
ASICS
Becoming familiar with how a tankless
water heater normally operates may
help to troubleshoot it. Assuming it
is properly installed with appropriate
gas, water, and electrical connections,
it should operate as follows:
Ac va on
1. A hot water tap is opened. The
flow sensor detects a flow rate
through the heater that is greater
than the 0.5 GPM activation point.
The system also verifies that the
temperature difference between
the incoming water and the set
temperature is great enough
to meet the 15,000 BTUH input
requirement.
2. The fan activates after the
conditions in the previous step are
met.
3. Igniter activates. You can hear the
buzzing of the spark igniter.
4. The main gas valve, proportional
valve, and solenoid gas valves will
open. You will hear a deep “clunk
clunk” noise.
5. Once a flame is detected, the green
“In Use” LED will activate. (This green
LED is located on the temperature
controller or remote controller.)
Opera on
1. The proportional gas valve and
fan will modulate based on the
amount of hot water demanded
and the temperature rise needed.
2. You may notice that only partial
sections of the burner will be lit.
This is normal operation. There
are three sections on the burner
assembly, and the computer
controls the number of sections
needed based on the flow rate
and temperature rise required.
3. The water control valve within the
unit may restrict water flow to
ensure that the set temperature
will be met.
Shutdown
1. The heater will shut down when
the water flow rate drops below
the deactivation point of 0.4 GPM.
2. The heater will close the main
gas valve and solenoid gas valves,
then extinguishing the flame.
3. When the flame disappears, the
green “In Use” LED will turn off.
4. The fan will increase in speed
to purge the venting of any
remaining exhaust gases. The
length of post-purge can last up to
1 ½ minutes.
5. The heater goes into standby and
waits for the process to begin
again.
NORMAL OPERATION
Figure 2.
The purpose of this diagram is to illustrate tankless water
heater design concepts, and may not be accurate to the
unit’s physical description.
Burners
Exhaust
Secondary
heat exchanger
Primary
heat exchanger
Igniter
Gas valve
Fan motor
Flow sensor
Condensate
drain port
Gas inlet
Cold water inlet
Thermistor
Hot water outlet
Thermistor
Intake port
Exhaust
Thermistor
Water control
valve
Computer
board (PCB)
