A.O. Smith GCVL-40 Technical Documents | Page 8

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

B

ASICS

Scalding Risk
A water heater 
can make water 
hot enough to 
cause severe burns 

instantly, resulting in severe injury or 
death.  Higher temperatures increase 
the risk of scalding, but even at 120°F, 
hot water can scald.  See Table 2. 

 

Thermostatic mixing valves at each 
point of use reduce the risk of scalding.  

VAC

HONEYWELL® GAS CONTROL VALVE/THERMOSTAT

GAS CONTROL/TEMPERATURE KNOB

120°F

MARK

§130°F

§140°F

§150°F

§155°F

§55°F

TEMPERATURES SHOWN ARE APPROXIMATES AND MAY VARY.
SOME MODELS ARE CERTIFIED FOR 180°F.  SEE RATING PLATE. 

STATUS
LIGHT

IGNITER

Figure 3. 

Table 2.   

Water 
Temperature

Time to Produce 
a Serious Burn

120°F (49°C)

More than 5 minutes

125°F (52°C)

1½ to 2 minutes

130°F (54°C)

About 30 seconds

135°F (57°C)

About 10 seconds

140°F (60°C)

Less than 5 seconds

145°F (63°C)

Less than 3 seconds

150°F (66°C)

About 1½ seconds

155°F (68°C)

About 1 second

IMPORTANT: Water temperature 
in the tank can be higher than the 
thermostat setting.  Some of the 
reasons are:
•  Incoming water temperature is 

higher than the set point (e.g., 
solar-heated water).

•  The water heater and/or pipes 

are located in an area with high 
ambient temperatures (e.g., an 
attic).  See also “Attic Installations” 
on page 23.

•  Certain usage patterns, such as 

short hot water draws or no hot 
water use for long periods of 
time, can cause the tank water 
temperature to rise higher than the 
thermostat setting.  Modern water 
heaters are very well insulated 
and retain heat so efficiently that,  
under certain circumstances, 
the tank water temperature can 
increase over time, particularly 
when hot water is not being used.  
This “heating” effect can be caused 
by the pilot flame itself.  

CombusƟ on  Chamber 
with TCO Switch
Combustion air is drawn through the 
base-ring filter and flame arrestor and 
into the combustion chamber.  (See 
Figure 5.)  Sufficient combustion air is 
critical to the operation of the water 
heater.  

IMPORTANT: The flame arrestor 
and filter should be inspected and 
cleaned during each service call.  See 
page 18 for instructions.

Also, each water heater covered by 
this handbook is equipped with a 
resettable TCO switch (thermal switch), 
designed to shut off the gas supply in 
the event that the water heater has 
been exposed to high combustion 
chamber temperatures caused by one 
of the following events:

• 

Flammable vapors (e.g., spilled 
gasoline)

• 

Poor combustion caused by a 
blocked vent

•  Insufficient combustion air.  

A resettable thermal switch is 
illustrated in Figure 4.  Its reset button 

is located in the center of the switch, 
directly between the terminals.

IMPORTANT: If the thermal switch has 
been tripped, determine the cause 
and correct it before returning the 
water heater to service.  Ensure that 
there are no flammable vapors 
present.
  However, a tripped thermal 
switch is most likely caused by poor 
combustion (e.g., blocked vent or dirty 
filter/arrestor). 

THERMAL 

SWITCH 

(TCO)

THERMAL SWITCH (TCO) LOCATION

Figure 4. 

Figure 5.

FLAME ARRESTOR

COMBUSTION CHAMBER

BASE-RING
FILTER

 

 

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