Water Heater - Gas Type - Detailed Operation Explained with Diadiagram of a natural gas water heater - gas 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.















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