How Do Gas Water Heaters Work? | Repair and Replace


Hi, I’m Vance and welcome to Repair and Replace. Your water heater works hard to give you a
steady supply of hot water. In this episode we’ll learn how gas water
heaters work and then we’ll look at some of the differences between
standing pilot and power vent models. Lets begin. A standard water heater burns gas to heat
the water stored inside the insulated tank. When hot water is used, cold water enters
through a dip tube and fills the bottom of the tank. The gas control valve uses a built in thermostat
to monitor the water temperature. When the thermostat senses the cold water,
it opens up the main gas valve. Air is drawn from below the tank and mixes
with the gas. The main burner ignites, and the exhaust fumes
rise up through the flue vent. The burner continues to heat the water until
the set temperature is reached. The temperature and pressure relief valve is by far the most important safety
device in water heaters. If the pressure or temperature inside the
tank gets to high, the relief valve will open and release water, preventing the
tank from exploding. Additionally every water heater has a drain
valve at the bottom which can be used to flush the tank of sediment. If too much sediment builds up it will act
as an insulator reducing the efficiency. This is why its best to flush the tank once
a year during regular maintenance. Regardless of the type, all water heaters
need protection against corrosion and rust. The tank is coated with a thin glass lining
but it doesn’t always cover 100% of the tank. Also the glass can become cracked, exposing
more of the tank walls. On the top of the tank sits the anode rod. Like a lighting rod the anode attracts the
corrosive elements in the water, sacrificing itself to protect the tank. If the anode dissolves completely, or if the
rod is completly incased in calcium, then the corrosive elements will start attacking
the exposed metal. Most anode rods will last 4 – 6 years, but
this depends on the pH and purity of the water. This is why its best to check the anode rod
during regular maintenance. All water heaters have a relief valve, drain
valve and an anode rod, but ignition and venting is different in Standing
Pilot, and power vent models Older homes will generally have a standing
pilot water heater, with an atmospheric vent. They use a small pilot flame that burns continuously, igniting the main burner when heat is needed. This is the standard design that has been
used for decades and is easily identified by the metal exhaust
pipe. Since its the pressure from combustion that
pushes the exhaust upwards, these heaters must be vented vertically. Any restriction in the airflow might
cause backdrafting, which can extinguish the pilot flame. These tanks are generally replaced with newer
standing pilot models, since connecting to the existing vents keeps
replacement costs low. Standing pilot water heaters will either use
a conventional or an electronic gas valve. Conventional gas valves are powered by a thermocouple. The thermocouple sits in the pilot flame, and generates a small amount of
electricity when heated. About 20 – 30 millivolts. This produces enough electricity to power
the gas valve. The thermocouple also acts as a safety switch. If the pilot light goes out, then the thermocouple
will cool down, the voltage will drop and the gas valve will shut off. This prevents unburnt gas from being released
into your home. Electronic valves require more power as they
have additional circuitry and diagnostics. These valves are powered by a thermopile,
which is a pile of thermocouples bundled together. When heated by the pilot flame thermopiles
generate around 600 – 750 millivolts. Electronic valves will also flash an error
code when something goes wrong. And these are often listed on the valve. Now if the pilot isn’t staying lit, then it’s
most likely a problem with the thermocouple or thermopile. Alternatively it could be an issue with the
thermal cutoff switch. The thermal switch protects against overheating. If the temperature in the combustion chamber
gets too high, the thermal switch will shut off the gas valve. The thermocouple, thermopile and thermal switch
can all be tested. You can learn more in the troubleshooting
videos linked in the description. Unlike standing pilot heaters, power vent
models don’t rely on atmospheric pressure. Instead they use a draft inducer blower to
push the exhaust through the vent. This makes them useful in newly constructed
homes as they can be placed virtually anywhere. When the thermostat detects cold water, the
draft inducer pulls fresh air into the burner. The pressure switch then verifies that there’s
enough airflow for combustion. Next the hot surface ignitor heats up and
ignites the burner. The flame sensor monitors the burner to confirm
that there is a flame. The burner runs until the water in the tank
reaches the set temperature. After the burner shuts off, the inducer will
stay on for several minutes to purge the system of exhaust gases. Power vent water heaters are similar to high
efficiency furnaces. If any one of the safety switches trips or
is faulty, then the water heater will shut down. It will attempt ignition several times before
going into a hard lockout. The water heater will use flashing lights
as codes to describe the source of the error. It’s best to check your manual to see what
these codes mean. You can learn more in the videos linked in
the description. Hopefully this has given you a better understanding
of how water heaters work. For more troubleshooting on water heaters,
furnaces and appliances then subscribe to our channel. And if you need help, you can call or visit
an AMRE location to talk with our knowledgeable staff. Thanks
for watching.

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