Basic Parts For Gas Water Heater

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Most of us will have a gas water heater in our homes, but few of us ever really think about them.

These devices are responsible for supplying hot water to a home, so it can be used for showers, taps and other appliances such as dishwashers.

Many of us will take our gas water heaters for granted, which is why it can be tough when they finally break. 

When a gas water heater (see also ‘Water Heater Smells Like Burning‘) stops functioning as it should, it can lead to a lot of problems.

Possibly the biggest of these issues is that your house will no longer have a constant supply of hot water, which means no more warm showers, amongst other things.

If you believe something has gone wrong with your water heater, then you should call a professional plumber immediately to take a look at it. 

That said, if you are DIY minded or just want to know more about how these devices work, you could try having a look at it yourself.

While we wouldn’t recommend attempting your own repairs, if you can find out what the problem is, then you can make valuable suggestions to your plumber to help make their job easier.

In this guide, we will be taking you through all the different parts that make up your gas water heater (see also ‘ Does A Gas Water Heater Need Electricity? ‘), telling you what they do and how they work. 

How Do Gas Water Heaters Work?

Before we get into the different components that make up a gas water heater, it will help to have a basic understanding of how they work.

All gas heaters will share a few common components, although some of these components may work differently depending on the model.

We will go into greater depth about these individual parts later on, but for now we will try to give a general overview of how a gas water heater works. 

These devices heat water by the process of convection. Cold water enters the main tank via the cold water inlet at the top.

It is then heated up by the burner unit at the bottom that runs on a steady supply of natural gas.

Often there will be a flue and chimney in the unit for carrying away toxic by-products created by the complete combustion of natural gas.

These byproducts include carbon monoxide, sulfur oxide, and nitrous oxide to name just a few. 

When the water is heated, it rises further up the tank as it is displayed by the cold water which falls to the bottom.

The reason this works is that cold water is more dense than hot, which causes it to sink to the bottom of the tank, forcing the hot water to the top where it leaves through the main outlet.

This is a basic principle of convection and by this method your gas heater (see also ‘Gas Hot Water Heater Troubleshooting‘) can provide a continuous supply of hot water.

Because the tank is constantly replenished with cold water which sinks to the bottom to get heated up, you often won’t have to wait to get warm water out of your taps or showers. 

The burner is often controlled by a thermostat that will turn it off when the water inside the tank is too high to save on energy.

When the water temperature falls below a certain level, the thermostat will turn the burner back on to start heating up the tank again. 

Now that we understand the basic principles behind how a gas water heater works, let’s go into depth about the various components that make them up. 

Basic Components Of A Gas Water Heater

Basic Parts For Gas Water Heater

Not all gas water heaters are built in the exact same way, although many of them will share common components.

Some parts are substantially more complex than others, but all of them are equally important to ensure that your water heater functions as it should (see thermostat) without any problems.

In this section, we will cover all the basic parts that make up a gas water heater, before going into more depth on how some of the more complicated parts work. 

The Water Tank 

Naturally, the most essential part of any water heater is the main tank for holding the water.

The tank on your water heater actually consists of two tanks placed inside each other.

There is an outer steel jacket that encloses the pressure-tested steel tank to provide extra protection against leaks.

To prevent corrosion or rust, the inside of the inner layer is coated with vitreous glass or plastic. 

Between the two tanks, there is a layer of insulation to prevent heat loss and reduce the amount of work your boiler has to do to heat your water.

If you want to save even more on your energy bills, then you can add a secondary fiberglass insulation jacket that you can wrap around your water tank. 

The water tank will have a secondary compartment at the bottom called the combustion chamber.

This is where the burner arrangement is often located.

Between the combustion chamber and the main tank, there is a plate of metal that is called the heat transfer surface.

This is often made of a metal that conducts heat very well so that it will be quickly heated up by the burner, so it can start boiling your water. 

The whole tank sits in a drip tray, which is there to catch small amounts of water that leak out of the tank, as well as condensation that builds up on the surface.

While you shouldn’t have too much water building up in this pan, it is useful if your tank ever needs to relieve its internal pressure by opening its TP valve. 

The Flue

In the center of the main tank, there is a chimney called the flue that takes away toxic byproducts produced by the combustion of natural gas.

This is an essential component for any gas water heater (see also ‘ A Review And Buying Guide To GE Gas Water Heater ‘), as otherwise your home would fill up with toxic exhaust fumes. 

Inside the flue, there is a steel baffle filter that is designed to remove harmful chemicals from the exhaust fumes.

To feed the burner and push the fumes up the flue, there is normally an air intake located at the bottom of the tank where the combustion chamber is. 

At the top of the tank, there is a vent hood that lets in more air to make the fumes shoot up the chimney.

It is vital for your flue to properly vent all the gasses it carries outside and away from your home.

If exhaust fumes from burning natural gas build up inside your home, they can be very dangerous and potentially lethal.  

The Anode Rod 

Inside your water tank, you will find a solid metal rod that protrudes deep into the water in your tank.

This is commonly referred to as the anode rod or sacrificial rod.

Its purpose is to attract corrosive elements that would normally cause the inside of your tank to rust, thus protecting it from corrosion. 

To attract these corrosive elements, a small electric current is passed through the rod.

This charge is what enables it to serve its purpose of removing rust-causing particles from the water in your tank. 

Sacrificial rods can be made out of Magnesium, Zinc, or Aluminum and may need to be replaced every few years.

You should check on your anode rod every 1-3 years, depending on how hard or soft your water is.

Once more than 50% of it has been consumed or corroded, you should switch your rod out for a new one. 

Cold Water Inlet

This is where most of the water enters your tank so that it can be heated up and re-distributed around your home.

This pipe will often have a shut-off valve near where it connects to your tank, which can be identified by its blue handle. 

When you need to perform maintenance on your tank, or replace broken components, you will need to use this valve to shut off the flow of water. 

The cold water pipe (see also ‘Banging Water Pipes: Causes And Simple Fixes‘) continues into the tank and almost reaches the bottom.

This is to make sure that the cold water enters at the bottom of the tank, which forces the warm water to the top where it will enter the hot water outlet. 

As such, the inlet valve is often made deliberately narrow so that the water is fired into the tank under pressure.

This pressure will help to force the hot water back out of the tank through its outlet. 

Hot Water Outlet 

This is one of the most crucial parts of your gas water heater, and it is responsible for carrying hot water back out of the tank, so it can be circulated around your home.

The hot water inlet pipe may also have a shut-off valve, although not all models do.

You can tell it apart from the shut-off valve on the cold water pipe because it will have a red handle instead of a blue one. 

The Pressure And Temperature Relief Valve

All water tanks must be fitted with a temperature and pressure relief valve which is generally located at the top of the tank connected to a long pipe.

Any water tank is going to be very hot and very pressurized and while most are built to handle this, occasionally they will need to release excess pressure or heat. 

That’s what this valve is for, and it is designed to automatically open once the temperature in the tank exceeds 170 °F or the pressure goes above 50 psi.

If either of these conditions is met, the valve will open, allowing water to exit the tank via the discharge pipe where it will collect in the drip tray at the bottom. 

To test if your temperature and pressure relief valve is still working, you can slightly lift the handle up to see if water from your tank comes out of the discharge pipe.

If you don’t see any water, then your TP valve is likely damaged and will need replacing. 

Tank Drain Valve

At the very bottom of the water tank on your gas water heater, you will see a small valve for draining your tank.

Over the years, sediment from your water supply can gradually accumulate at the bottom of the tank.

If this gets too severe, it could cause corrosion on the inside of your tank or other issues that damage its structural integrity. 

As such, it is often a good idea to drain your tank once every few years to remove any sediment that has built up.

Doing this is very easy and most of the time won’t even require you to call out a plumber.

To drain the tank on your gas water heater, you simply need to follow these basic steps. 

  • Step 1 – Turn off the gas control valve by switching it to the pilot setting. 
  • Step 2 – Shut off the control valve on the cold water inlet to prevent more water entering the tank. 
  • Step 3 – Turn on the nearest hot water faucet to remove any hot water in the system and prevent backflow into the tank. 
  • Step 4 – Attach a garden hose to the drain valve and put the other end of it in a suitable drain or utility sink. 
  • Open the drainage valve and watch the end of the hose until the flow of water stops. 

The sediment may cause the water to appear dirty.

If this discoloration is very severe, then it will be necessary to close the drain valve again and allow the tank to refill with fresh water before draining it again.

However, provided that you empty your tank on a regular enough basis, this shouldn’t be necessary most of the time. 

Remember, dealing with pressurized water mains in your home can be risky, and if anything goes wrong it could cause extensive damage to the plumbing and infrastructure in your home.

If you are uncertain about anything when it comes to performing DIY repairs on your boiler, stop and call a professional. 

The cost of hiring a plumber will always be far less than the cost of repairs if anything goes wrong while you are trying to fix your water heater by yourself. 

The FV Sensor

This is a unit located on the front of your water heater tank near the bottom where the combustion chamber is located.

It has one of the most important jobs in the whole system of detecting the presence of flammable gasoline vapors. 

Unburned gasoline vapors are the most common causes of fires relating to water heaters, and if any are present, they could be ignited by the burner.

This is why the FV sensor (see also ‘What Is A Flammable Vapor Sensor?‘) is there to turn off the supply of gas if these vapors are detected. 

If you suspect that your sensor is blocked or not working for any reason, then the easiest way to test it is with a simple multimeter.

You will know there is a problem with your sensor if it won’t reset after tripping, even if there are no flammable gasoline vapors in the air. 

Many units will also have an FIVR sensor that will detect the presence of other volatile gasses other than gasoline fumes. 

Gas Valve Control

Operating A Tankless Gas Water Heater Without Electricity.

This component controls the flow of gas to the burner and pilot light.

It is usually located on the outside of the combustion chamber, with a thermostat probe that goes into the main water tank.

Gas valve controllers can be either mechanical or electrical and while both versions share many similarities, they also have a few differences. 

These controllers are connected to your main gas inlet pipe and as such they are a vital component.

If anything goes wrong with the gas control, then you could end up with natural gas leaking into your home, which is very dangerous. 

Since this is such an important part of your gas water heater, we thought we would break it down for you, part by part, so you can get a good idea of how it all works. 

The Main Gas Line

This is the black metal pipe that supplies your home with natural gas, and it will probably have other branches leading to cookers, heaters and any other appliances that require gas as fuel. 

It has its own dedicated shut off valve for cutting off the flow of gas to your gas controller, which is essential when you need to repair or replace it.

The bottom of the pipe continues past the turning point, where it feeds into your gas valve controller.

This is the sediment trap, which collects any particulates or impurities contained in your gas before they can reach the burner. 

Without a sediment trap, these impurities may clog up your gas controller, which will impede its performance.

Always be very careful when touching or handling the main gas line that feeds into your water heater. 

Never work on it near an open flame and if you start to smell gas leaking into the room, even after you have closed the shut-off valve, get out and call a professional plumber. 

The Valves

Gas travels through your control unit through two primary valves.

The first is the safety, or pilot valve, that controls the flow of gas into the control unit itself.

When open, this valve will only be able to supply gas to the pilot light until the secondary manifold valve is opened as well. 

The manifold valve supplies gas to the main burner unit.

Since natural gas is incredibly toxic to humans, these valves need to be kept closed when neither the burner nor pilot light is in use. 

That is why, both of the valves are controlled by a type of electromagnet called a solenoid.

When a current is passed through these solenoids, they generate a magnetic field that will open the valves, and close them again when current through the magnet stops. 

In mechanical gas valve controls, these magnets are controlled by the temperature cut off device or TCO.

For electrical valve controls, there is also an energy cut off located inside the main body of the controller itself.

Alongside these controllers, there are different types of temperature sensors that help to tell them what the temperature is in the main water tank and the combustion chamber.

With this information, your gas control valve will know when to supply gas to the pilot light and burner and when to stop doing so. 

The Thermostat 

On the back of your gas valve control unit, there is a large thermostat, often made from copper.

This protrudes into the main water tank to detect how hot or cold the water is.

Inside the copper casing, the thermostat contains a variable resistor with a small energy cut off device attached to it.

Changes in temperature will cause the resistance of the thermostat to increase or decrease, allowing it to control the flow of electricity to the magnet in the manifold valve. 

The variable resistor usually has a cut-off point of 120 °F.

As long as the water is below this temperature, current will be able to flow through the thermostat, keeping the manifold valve open.

As long as the pilot flame is lit, the burner will not turn off. 

When the water temperature rises above 120° F, the resistance in the thermostat becomes too great for current to move through it.

This cuts off the flow of electricity to the electromagnet, shutting off the manifold valve. 

The main safety valve that allows gas into the controller is controlled by one of two different temperature sensors called a thermocouple or a thermopile (see also ‘ Thermopile Voltage Low – What To Do? ‘).

These are located right next to the pilot light and can detect changes in temperature to shut off the flow of gas through the control, extinguishing both the burner and pilot light at the same time. 

The energy cut off (ECO) inside the thermostat is there to act as a fail-safe, closing the safety valve and subsequently the manifold valve if the temperature inside the tank gets too high. 

These ECOs are often one use devices that will require you to replace the thermostat if they are tripped.

As such, they have a high cut-off point to make sure they only activate in an emergency.

Typically, this temperature is around 180 °F which is a remarkably high water temperature even for a gas water heater. 


Between the control unit and its thermocouple or thermopile is the TCO. The TCO is a basic thermal switch that detects changes in temperature.

When the pilot light heats up the tip of the thermocouple or thermopile enough, it will pass a small current through the TCO, keeping the main safety valve open. 

The purpose of the TCO is to regulate the temperature inside the combustion chamber, and prevent the water in the tank getting so hot that it will trip the ECO inside the main thermostat.

This is why it has a cut-off point between 160 and 200 °F. 

If the cut-off point is reached, the TCO will turn off, interrupting the signal from the thermocouple to the gas control unit.

This will cause the pilot light and burner to both be extinguished as the main safety valve is closed. 

If your TCO is tripped, it will often reset itself, once it falls below 120 °F. This will open up the main safety valve again, allowing gas to flow to the pilot light, where it is lit by the igniter.

By this time, the water temperature should have fallen enough to trigger the thermostat into opening the main manifold valve so that the burner can ignite, starting the process all over again. 


Thermocouples are typically found on heaters that have a mechanical gas valve control.

They take the form of a simple metal rod that is made up of two different metals.

When the tip of the thermocouple is heated by the pilot flame, it generates a small electrical current. 

This current is relayed back to the TCO on the front of the combustion chamber.

When the TCO receives the current from the thermocouple, it will open the main safety valve, allowing gas to enter the manifold valve opened by the thermostat.

This lets gas reach the burner, where it is ignited by the pilot light. 

When this happens, both the thermocouple and water in the tank will heat up.

Once the temperature in the combustion chamber reaches a certain level, it will trigger the TCO to shut off, shutting the safety valve.

This will extinguish both the pilot light and burner, causing the water in the tank to cool along with the air in the combustion chamber. 

This will trigger the thermostat to open up the manifold valve again.

The main safety valve will stay closed until the TCO determines that the temperature in the combustion chamber has sufficiently gone down.

It will then reopen the safety valve and trigger the igniter to turn on the pilot light and start the whole process again. 

Due to the way that they work, thermocouples can take a long time to cool down.

This delays the shutting of the safety valve, ensuring that enough fuel remains in the burner to push exhaust gasses up the chimney. 


A thermopile is made up of multiple thermocouples joined together in series.

The more thermocouples a thermopile contains, the more current it is able to generate. 

As such, the main advantage of these devices over a single thermocouple is that they are able to produce enough power for a basic circuit board.

That is why they are commonly used with electric gas valve controls, where the current they produce is used to power the unit’s ECO. 

The ECO is a basic circuit board that is designed to monitor the water temperature and pressure inside the tank.

It acts as a secondary fail-safe, working alongside the TCO to make sure that the main safety valve is closed when the temperature of the tank is high enough. 

These ECOs are great for making sure that the main safety valve stays shut in the event of an emergency.

This means that even if something goes wrong with the TCO, the ECO will be able to still halt the flow of gas. 

The Piezo Igniter 

Underneath the thermocouple, or thermopile, and the pilot light is the igniter which is connected by a wire back to the main gas control.

This produces the spark that lights the pilot light, so it can ignite the main burner unit.

As such, it is another component that is essential to the proper operation of your gas water heater

You can check your igniter is working by pressing the button for it on the front of your gas controller.

When pressed, you should be able to see a small spark through sight glass into the combustion chamber.

If your Piezo igniter isn’t working, then you will need to get it replaced as soon as possible. 

Difference Between Different Models Of Gas Water Heater

As we said earlier, while all water heaters will work using the same basic principles, they can still differ in the technology that they use.

There are two main types of gas water heaters (see also ‘Gas Water Heater Vs. Electric‘) commonly found in houses across the country.

These include atmospheric vent water heaters and power vent heaters (see also ‘Power Vent Water Heater: Why You Should Buy One‘) .

Both of these models have their advantages and disadvantages, which we will cover in this section. 

Atmospheric Vent Heaters

Atmospheric vent heaters are often found in older buildings and use mechanical gas controls so that they don’t need to be powered by mains electricity at all.

As such, they can continue to heat your water even during a power cut. 

The mechanical gas control is connected to a thermocouple which controls when the safety valve is opened or shut.

These units have an exhaust flue that vents directly outside through the roof and work by using the air pressure in the combustion chamber to send fumes up the chimney. 

This is why they use thermocouples, which take longer to cool down after the pilot light turns off.

This delays the shutting of the safety valve, providing extra fuel to the burner to force any remaining fumes up the exhaust. 

These heaters don’t require power to operate, which is a significant advantage, especially in the instance of a power cut.

They are also much easier to replace and generally cost less  than their more modern counterparts. 

The disadvantage with this system is that the combustion chamber needs to stay hot for longer to keep the exhaust gasses rising up the chimney.

This means that your burner needs to stay on for a bit longer, and the fuel it is burning isn’t even really being used to heat your water. 

As such, these heaters are less energy efficient and may cost slightly more to run due to the excess gas they use. 

Power Venting Heater

Power venting heaters use electric gas controls connected to a thermopile and have a blower located at the top  of the water tank.

This blower is controlled by the thermostat and turns on at the same time as the main burner. 

Its purpose is to suck in air and blow toxic exhaust fumes up the chimney to their outlet vent.

This means that you don’t need to waste as much gas keeping the combustion chamber warm to force more exhaust fumes up the chimney. 

Thanks to this fan, the exhaust gasses produced by these heaters are not as hot, which allows them to cut back on the cost of some components.

The fan is better at providing more consistent pressure for pushing the gasses out of your home, which means that these devices are less likely to catastrophically fail than their mechanical counterparts. 

Sadly, what you save on natural gas, you may end up paying for on installation and the electricity required to run the fan.

These units need to be installed near an electrical outlet, and won’t function efficiently if the power goes out.

They also require slightly more expensive ducting and ventilation, making them the more expensive option to install in your home. 

If you want to swap out your water heater for a different type, then this can be an expensive process since you will have to swap out the entire unit.

You can’t really add a mechanical gas control to a power vent heater, just like you can’t add an electric gas control to an atmospheric vent system. 

Final Thoughts

Hopefully, after reading this guide, you now have a better understanding of how the gas water heater in your home works.

This should help you to perform some basic DIY repairs, such as being able to know when your TCO is damaged, or when the thermostat needs a new ECO. 

However, plumbing is complicated, and it isn’t a good idea to play with anything connected to the mains water system unless you know what you are doing.

A single mistake could lead to all sorts of problems, so if you are uncertain of anything, you should call a professional plumber immediately. 

That said, understanding your gas water heater will help you to know when something has gone wrong with it.

This will help you to make an informed decision of what you can do about the problem, so you can get it resolved as quickly as possible. 

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