Testing The Thermocouple And Gas Control Valve/Thermostat; Thermocouple Output Test; Gas Control Valve/Thermostat (Thermal Switch Models) - A.O. Smith GCV Series Service Handbook

TESTING THE THERMOCOUPLE AND GAS CONTROL VALVE/THERMOSTAT

IMPORTANT: Before performing any test, check the area around the water heater for any source of a Flammable
Vapor (i.e gasoline, paint thinners, etc.) If any sources are found do not proceed until they are removed.

Thermocouple Output Test

The following test will check the DC voltage generated
by the thermocouple when the pilot light is lit.
1. Disconnect the thermocouple from the gas control
valve/thermostat as shown in the adjacent figure.
2. Using a multimeter with alligator clips leads,
attach the red lead to the body (copper part) of the
thermocouple. Attach the black lead to the end
(silver part) of the thermocouple. Reference the
adjacent image for placement of the leads.
3. Follow the lighting instructions on the front of the
water heater to light the pilot and watch the voltage
readings on the multimeter. NOTE: You will have
to hold the button down to keep the pilot lit for the
duration of the test. After 45 seconds the meter
should read 20-30 millivolts DC or more.
IF . . .
the multimeter voltage reads
at least 20-30 millivolts DC
the multimeter voltage reads
more than 20-30 millivolts DC
Gas Control Valve/Thermostat Test
(Thermal Switch Models)
The following test will check the gas control valve/
thermostat's ECO & TCO (Energy Cut Off & Thermal Cut
Off). NOTE: This test applies to gas valves with wire leads
that attach to the thermal switch on the manifold door.
1. Remove both wire leads from the TCO (thermal cut
off switch) and jumper the two leads to complete
the circuit. Once the test is complete, remove the
jumper and reconnect the wire leads.
2. Follow the lighting instructions on the front of the
water heater to light the pilot.
3. Release the knob and check to see if the pilot
remains lit.
IF . . .
the pilot lights and remains lit
the pilot will not light or remain
lit with the wire leads jumpered
. . . THEN
• check the positioning of the thermocouple tip to the pilot flame. If
necessary position the thermocouple to ensure the flame contacts the
thermocouple's tip. Re-test the thermocouple, and if it fails replace the
thermocouple. (See p. 22)
• the thermocouple is working properly. Proceed to the gas control valve/
thermostat test.
. . . THEN
• the TCO (thermal switch sensor) has tripped OR the TCO is not
working (OPEN). Reattach the wire leads and press the TCO reset
button. Relight the water heater. If the pilot will not remain lit, replace
the TCO. If the pilot does remain lit, the TCO was tripped-possibly
due to a lack of combustion air (see pp 13-16 & 18-19) OR because
of a Flammable Vapor event. Check around the water heater for any
source of fl ammable vapors (i.e. gasoline, paint thinner, etc.).
• replace the gas control valve/thermostat.
(See p. 24)
20
Figure 20
GAS CONTROL
KNOB
TCO
WIRE LEADS
Figure 21
TCO
WIRE LEADS
(JUMPERED)
TCO
SWITCH