A.O. Smith GCVL-40 Technical Documents | Page 11

A.O. Smith Water Heater Technical Documents - GCVL-40.
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Residential Standard Gas Water Heater Service Handbook • 11

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TEM CHE

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SYSTEM CHECKS

Explosion Risk

WARNING!  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. 

 DraŌ  Test 

WARNING! Burn Hazard. Do not 

touch the vent.  Doing so can cause 
burns. 

After successfully lighting the water 
heater, allow the main burner to 
operate for five minutes.  Close all 
windows and doors and turn on all 
gas appliances and air-moving devices 
(range hoods, bathroom exhaust fans, 
ceiling fans, etc.) in the home. Also, 
close any fi replace dampers. 

After five minutes, pass a newly 
exƟ nguished match about one inch (1”) 
from the draŌ  hood’s relief opening. 
Smoke from the match should be steadily 
drawn into the opening, indicaƟ ng proper 
draŌ  and a funcƟ oning vent system.  

MATCH

DRAFT HOOD

RELIEF 

OPENING

Figure 8.

If the main burner has been operaƟ ng 
for  fi ve minutes, but the match smoke 
driŌ s around or is blown away from 
the opening, turn the gas control knob 
OFF.  Do not operate the water heater 
until proper adjustments or repairs are 
made to the vent pipe system and/or air 
supply (p. 21).  Always check the filter 
and flame arrestor first and clean them, 
if necessary (p. 18).

 ConnecƟ ons Check
Some issues can be caused by faulty 
connections.  Refer to the following list 
of connections when troubleshooting 
the system:

•  Igniter wire (Figure 9).  See also 

“Testing the Igniter System” on 
page 19. 

•  Thermopile/thermal switch wires 

at the gas control valve/thermostat 
(Figure 9). 

•  Wire connections at the thermal 

switch (TCO).  A disconnected 
thermal switch wire can cause the 
pilot to become inoperable.  See 
Figure 4, page 8.

See also “Diagnostic Flash Codes” on 
page 10.

Igniter Wire

 

From Thermopile (White)  

 

From Thermal Switch (Red) 

 

Manifold 

Pressure Tap 

(1/8” NPT-RH) 

 

Manifold Flex Tube Nut:
ͻWƌŽƉĂŶĞ͗>ĞŌͲŚĂŶĚƚŚƌĞĂĚƐ
ͻEĂƚƵƌĂů͗ZŝŐŚƚͲŚĂŶĚƚŚƌĞĂĚƐ

Figure 9.  

 Gas Pressure Checks
Gas pressure checks are made with 
flowing gas.  Use a gas pressure gauge 
capable of reading pressure in inches 
of water column. 

•  Supply gas pressure checks are 

measured before the gas control 
valve/thermostat and as close 
to the water heater as possible. 
NOTE: The dynamic gas pressure 
(burner on) should be measured 
with all appliances operating that 
use the common gas supply.

• Manifold (main burner) gas 

pressure is measured at the 
pressure tap on the bottom of 
the gas control valve/thermostat.  
See Figure 9 and Figure 11.  Use a 
3/16” Allen wrench to remove the 
pressure tap plug, then attach the 
gas gauge. 

NOTE: Gas outlet pressures will be listed 
on the gas control valve/thermostat’s 
pressure label.  See Figure 10.  Also, 
gas supply pressure requirements are 
shown on the water heater labeling.

 

VAC

GAS 

PRESSURE

GAUGE

CONNECTED AT

PRESSURE TAP

MANIFOLD GAS PRESSURE TEST

GAS PRESSURE LABEL (OUTLET PRESSURE)

Figure 10. 

CONNECTED AT PRESSURE TAP

Figure 11. 

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