Water Heater - Gas Type - Detailed Operation Explained with Dia gram
Water
Heater - All the details about how a gas water heater works, with a diagram
showing all the parts. Includes TPR valve, Anode, Thermostat, Thermocouple, gas
valve, dip tube and more.
Water Heater Operation
A gas water heater is the most prevalent type of
heater. Gas water heaters reach full temperature about twice as fast as
electrics, and cost about half as much to operate due to the lower cost
of gas.The tank should be tied to a structure such as the wall in earthquake prone areas to
prevent a serious gas fire if the tank falls over and ruptures the gas
line. Water Heater – How It Works With All the Details (gas)
Most of us take our water heater for granted. At
first glance there doesn’t seem to be much of interest. After all, it’s
basically an insulated tank full of water with a gas flame under it.
Add a thermostat, a pressure temperature relief valve, a dip tube, a
drain valve, and you’ve got a water heater!
A water heater can exhibit some strange behaviors
Not so fast. Water heaters have some interesting
behaviors and can often be quite puzzling when something goes wrong.
Let’s see how a gas water heater works.
A typical 40 or 50 gallon gas water heater consists
of a tall cylindrical tank, with a hemispherical fire box on the bottom
with the flue (chimney) leading from the firebox through the center of
the tank and out the top. The firebox is of course where the gas burner
is located. There is a pilot light next to the burner, and a
thermocouple is placed in the flame from the pilot light.
Thermocouple keeps the pilot light lit
A thermocouple produces a small electric current
that tells the gas control valve that the pilot light is burning. If the
pilot light goes out, the thermocouple stops producing the electric
current, and the gas valve will not turn the gas on to the burners. In
order to produce enough current to hold the valve on, it has to be very
hot. That is why you have to hold in the button for so long when
lighting a pilot light.
The thermostat controls the water temperature
Near the bottom of the tank there is a thermostat
inserted into the side of the tank. The thermostat senses when the
temperature drops below a certain
pre-set level, and this causes the burner to
come on. When the desired temperature is reached, the thermostat shuts off the burner. Typically
there
is a control knob to set the temperature to warm med or hot.
The dip tube is critically important
At the inlet to the water heater there is what is
called a “Dip tube”. The dip tube is a long skinny plastic tube that
reaches down to the bottom of the tank. Incoming cold water travels
through the dip tube down to the bottom of the water heater.
Without the dip tube in place the cold water coming
into the water heater can just travel a few inches to the side and exit
through outlet. Some mixing occurs of course, but the water temperature
is low, and it can seem as though your water isn’t getting hot, just
warm, and/or you run out quickly.
Cold water coming into the water is sent through
the dip tube to the bottom of the tank where the thermostat is located.
Running even just a gallon out of the hot water tap can cause a drop in
temperature thermostat, turning on the gas valve. The pilot light
ignites the gas and water heating begins.
Hot water stacking
The hot water in a water heater tends to form
layers with differing temperatures. When short uses occur frequently the
heater can turn on and off and cause the water at the top of the heater
to keep getting hotter and hotter each time. It’s called “stacking”.
It’s because hot water, like hot air, rises. It can get hot enough to
cause the temperature / pressure safety valve to open and release hot
water from the heater, even with the thermostat setting on low.
The TP valve is a safety device (TPR valve, T/P valve, T&P valve)
The safety valve often referred to as a TP valve,
opens to relieve pressure if either the pressure gets too high or the
temperature gets too high. This prevents the water heater from blowing
up if something goes wrong. The TP valve usually is located on the side
of the tank near the top.
Normally as your water heater heats the water, the
water expands as it’s heated, and as the water expands some of it gets
pushed back into the source, such as the water main. Sometimes there is
a check valve or some other obstruction between the heater and the
source of cold water. In that case, as the water expands, there is
nowhere for it to go, and thus the pressure begins increasing.
Thermal expansion tank
If for some reason the TP valve does not open to
relieve the pressure the water heater becomes a bomb and can explode
with amazing power. One way to avoid problems with the T&P valve in this
situation is to install an expansion tank at the inlet.
An expansion tank is a pressurized tank usually
with a rubber bladder inside or a diaphragm in the middle. As the water
expands it moves into the expansion tank and doesn’t build up to
dangerous pressure levels.
Usually local codes call for the output of the T&P
valve to be piped down to near the floor, or outside to the ground, so
no one gets scalded if they happen to be standing near the water heater
when the valve opens.
What is sediment and where does it come from?
One of the things that happen when you heat water
is that dissolved minerals in the water can precipitate out and settle
to the bottom. That is what sediment is. You sometimes hear people say
that you shouldn’t drink water that has passed through the water
heater. Obviously if some of the minerals settle out, then the hot
water might have a lower mineral content, but that certainly can’t hurt
you.
If it were un-healthy to drink water that has
passed through the heater the NSF or FDA would be advising not to drink
it. There is simply no evidence that it is harmful in any way. Now if
you had lead pipes that would be another story. Hot water could leach
more lead out of the pipes than cold water.
If you live in an area with hard water, over time
your sediment can build up to pretty significant levels. You often hear
that a build-up of sediment reduces the efficiency of your heater, but
it doesn’t have much impact. If the sediment interfered with the
transfer of heat from the firebox to the water, and as a result more
heat leaked out of the firebox or went up the chimney, then the
efficiency would be reduced.
Instead, the heat just passes through the sediment
into the water and no reduction in efficiency occurs. However, if the
buildup is large, the amount of water held by the water heater is
reduced, so you could run out of water more quickly.
Popping noises and other strange sounds
Loud popping and other odd noises occur when gas
bubbles form under the sediment due to the water in contact with the
tank bottom turn to steam. It doesn’t hurt anything, but it can be
annoying. Sizzling noises could be condensation dripping onto hot metal
parts.
Those who live in hard water areas might want to
flush the water heater once a year or so. If you flush it, and there
isn’t much sediment, you might just want to let it accumulate.
The anode protects against corrosion
To protect against corrosion water heaters have a magnesium or aluminum rod
called an anode inserted into them from the top. The idea is that the
anode will corrode instead of the tank.
Smelly hot water -- rotten egg odor
Sometimes some anaerobic bacteria can get into the
tank and will react with the magnesium or aluminum forming hydrogen
sulfide gas which is what gives a rotten egg smell. This is more common
with well water than from water mains.
Some people say that replacing the magnesium rod,
the more common one, with aluminum can help. There are also some special
(and expensive) anodes that are supposed to solve the problem. Anther
method that works is to sterilize the tank and the hot water pipes with
bleach or hydrogen peroxide. However, the bacteria can come back,
especially if you go away for a time and the water just sits in the
heater for days or weeks. Other than the awful smell the bacteria will
not harm humans.
Drain valve - replace it with a ball valve
At the bottom of the tank there is a drain valve.
If you live in an area with hard water and you have a sediment problem,
then you should probably replace the cheap plastic drain valve that most
water heaters come with now days with a full flow ball valve. This will
make the tank much easier to drain and flush. Chilipepper
Sales 10 Greg St., # 110 Sparks Nevada, 89431
PH (775)-359-1223 |
