BASICS
6 • On-Demand Water Heater Service Handbook for 240, 340 & 540 Condensing Models
If any problem occurs, turn off all hot
water taps and turn off the gas. Then
call a trained technician or the gas
company.
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:
Activation
5. A hot water fixture is opened.
The flow sensor must detect a
flow rate through the heater
that is greater than the 0.5 GPM
before the heater will begin
operation. The temperature rise
must be great enough to meet
the minimum input requirement
which is 15,000 BTUH for natural
gas models or 13,000 BTUH for
propane models. Otherwise, the
heater will remain in standby.
6. The fan activates to purge the system.
7. The Igniter generates a spark
between the spark electrode and
the burner to ignite the gas.
8. The gas valve will open. You will
hear a deep “clunk clunk” noise.
9. Once a flame is detected, the
green “In Use” LED will activate.
(This green LED is located on the
temperature controller or remote
controller.)
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)
Bypass valve
540 (T-H3)
models only
Figure 1.
The purpose of this diagram is to illustrate tankless
water heater design concepts, and may not be
accurate to the unit’s physical description.
Operation
10. The proportional gas valve and
fan will modulate based on the
amount of hot water demanded
and the temperature rise needed.
11. 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.
12. If the water flow exceeds the water
heater’s capabilities to heat to the
desired temperature, the flow control
valve will restrict the flow to ensure
delivery of water at the desired
temperature. This will result in a
loss of pressure at hot water fixtures
whenever flow is restricted through
the water heater.
NOTICE: 540 (T-H3) models
include a bypass valve.
This component acts like
a mixing valve. It helps
to reduce the risk of flue
gases condensing on the
copper heat exchanger
which could lead to
corrosion, damage, and
water leaks. The bypass
valve is also used when
multiple heaters are linked
together.
Shutdown
1. The heater will stop heating 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,
extinguishing the flame.
3. When the flame sensor no longer
detects a flame, the green “In
Use” LED will turn off.
4. The fan will increase in speed
to purge any remaining exhaust
gases. The length of post-purge
can last up to 1-½ minutes.
5. The heater will enter standby
mode.
NORMAL OPERATION
