A.O. Smith GCVL-40 Technical Documents | Page 12

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

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Table 3. 

 

Gas Pressures

IF . . .

. . . THEN

...the supply gas 
pressure is lower 
than required...

•   increase the 

supply gas 
pressure regulator 
setting

 and/or

•   increase supply 

gas piping size.

...the supply 
gas pressure 
is higher than 
required...

•  add a gas pressure 

regulator.

•   reduce the setting 

on the existing 
regulator.

...the manifold 
gas pressure 
is not within 
+/- 10% of the 
value shown 
on the gas 
pressure label... 

•  ensure that there 

is adequate supply 
gas pressure

•  ensure that the 

main burner 
orifice is the 
correct size for 
the model being 
tested.

•   if the above 

tests have been 
performed and 
the results were 
correct, replace 
the gas control 
valve (p. 20).

NOTE: The gas 
pressure label 
lists the gas 
valve outlet 
pressure.  This 
is the same as 
the manifold 
pressure.  See 
Figure 10.

 Thermopile  Check 
(Open Circuit)
The following test will check the DC 
voltage generated by the thermopile 
when the pilot light is lit.

1.  Remove the connectors from the 

gas control valve/thermostat (red 
and white wires).  Use needle-nose 
pliers.

2.  Insert multimeter leads into the 

connectors.  The multimeter must 
be set to read millivolts DC (on a 
scale that can read 750 millivolts).  
Reference Figure 12.

USE A MULTIMETER TO CHECK THE OUTPUT OF 
THE THERMOPILE (MEASURED IN MILLIVOLTS).

SHOWN ABOVE - MULTIMETER PROBES ARE 
INSERTED INTO THE THERMOPILE PLUGS.  THESE 
PLUGS WERE REMOVED FROM THE RECEPTACLE AT 
THE BOTTOM, RIGHT-HAND CORNER OF THE GAS 
CONTROL VALVE/ THERMOSTAT. 

Figure 12.  

3.  Follow the lighting instructions to 

light the pilot.  Once the pilot is lit, 
continue to hold the gas control 
knob in until this test is complete.  
You may need assistance from 
another person to do this.  The 
voltage reading should rise 
gradually as the thermopile heats 
up.  After a few minutes, you should 
have a reading of at least 350 mV 
DC.  A reading above 350 mV DC 
indicates that the thermopile is 
working properly.

4.  When you are finished, plug the 

connectors back into the gas 
control valve/thermostat.  If the 
connectors are separate (not a 
double connector), ensure that the 
red wire connector is to the left and 
the white wire connector is to the 
right.  See Figure 2B, page 7.  If 
the status light does not blink once 
every three seconds, replace the 
gas control valve/thermostat.  

Thermopile Check 
(Closed Circuit)
Follow the lighting instructions to light 
the pilot.  Once the pilot is lit, you can 
check the output of the thermopile 
with one of the following tests.  A 
reading above 350 mV DC indicates 
that the thermopile is working 
properly.  

INSERT MULTIMETER LEADS INTO 
THERMOPILE PLUGS AT CONTROL.
PILOT MUST BE LIT.

MULTIMETER SET TO READ mV.

MINIMUM ACCEPTABLE 
MEASUREMENT: 350 mV DC.

Figure 13.

INSERT ONE MULTIMETER LEAD INTO 
THE NEGATIVE PLUG ON THE CONTROL 
(WHITE WIRE).

 

TOUCH THE OTHER LEAD TO A 
TERMINAL ON THE THERMAL 
SWITCH (TCO).

MULTIMETER SET TO READ mV.
PILOT MUST BE LIT.  
MINIMUM ACCEPTABLE 
MEASUREMENT: 350 mV DC.

Figure 14.

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