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Parts of a Water Heater and the Two Basic Types of Water Heaters: Tank Type and Tankless

Tank Type Gas Water Heater

This is the type of water heater everyone has seen and most have in their home. Traditionally they are the most energy inefficient  however new self condensing and power direct vent tank type water heaters have been developed by American/A.O. Smith that bring 95% efficiency to the consumer. The following cross sectional gas water heater parts diagram shows the components common to most tank type heaters.

Gas Water Heater Parts Diagram

Gas Water Heater Parts Diagram

Here is a list of the parts of a water heater and their function common to most gas, propane and electric tank type water heaters

1. Flue Pipe

Every gas fired water heater has a burner were combustion takes place. The purpose of the flue pipe is to evacuate the deadly gasses which contain carbon monoxide created upon combustion.

2. Cold Water Shut Off Valve

This valve shuts the cold water supply down when a heater needs to be drained or replaced

3. Draft Diverter

This is a fitting on the end of the flue pipe that gather the gasses to be safely evacuated into the flue and then to the outside.

4. Temperature and Pressure Relief Valve (T&P Valve)

This is another pretty self explanatory term. This valve allows excess heat or pressure to be released from a hot water tank so it doesn’t explode. They are code on all tank type water heaters. If your T&P valve is constantly discharging you should replace.

Temperature and Pressure Relief Valve (T&P Valve)

5. Overflow Pipe

Put it simply this a pipe that is attached to the T&P valve, in the event that the T&P valve releases excess heat/steam the pipe allows the water to drain safely away from the heater. Some municipalities allow you to pipe the overflow to the floor but some require the overflow to be piped to a floor drain.

6. Hot Water Outlet

It’s the pipe that drops down into the heater that allows the hot water exit the heater in enter into the potable water supply in a home or business.

7. Anticorrosion Anode Rod (Sacrificial Anode)

This is an extremely important part of a tank type water heater. They are installed at the top of the tank and are generally made of magnesium or aluminum with a steel core. Simply put through electrolysis the anode rods will corrode before the exposed metal in the tank. If the anode rod has been corroded the water begins to attack the exposed metals and your water heater will eventually fail. If your water heater has an egg smell and someone tells you to remove the anode rod tell them no. It will void the warranty of every water heater from every water heater manufacturer in the world. (Please see detailed description of anode rods in the profession plumbing section.)

8. Dip Tube

The dip tube is a tube connected to the cold water inlet and extends nearly to the bottom of the tank. Because hot water rises to the top the dip tub allows the cold water to enter at the bottom of the tank where it is heater rising to the top always self circulating. This also helps circulation of suspended solids so they don’t sit at the bottom of the tank. Most water heaters have dip tubes unless the cold water inlet is at the bottom of the heater.

9. Elements 

Elements are only installed in electric water heaters and they are the means by which heat is transferred to the water. Think of the metal rod in an electric oven same concept.

10. Insulation

All tank type water heaters have insulation that wraps the metal tank to keep the heat in the tank the more insulation the less thermal heat loss.

11. Drain Valve

It’s the valve installed at the bottom of a tank type water heater that allows the tank to be drained down.

12. Thermostat or Control Valve

It is a mechanism to control how hot the water gets in a water heater. Most local plumbing codes have minimum/maximum temperatures that water heaters be set. Water heaters typically perform better and longer when set at a higher temperatures but because of scalding issues must be tempered down. In commercial applications the heater is usually set at a higher temperature and tempered down at the source i.e. lavatories showers etc.

Thermostat or Control Valve

13. Burner

The burner ignites natural gas or propane and heats the tank which transfers heat the water inside.

14. Thermocouple

A device that monitors the flame of a pilot on a water heater. If the pilot flame is blown out by a draft or fouling of the pilot orifice, the thermocouple causes the gas valve to shut off the flow of gas to both the pilot flame and the main burner.

Picture of a Water Heater Thermocouple

15. Gas Supply Valve

This valve is used to shut off the gas to the water heater.

Here is the Skinny on Tankless Water Heaters and How They Work

Tankless Water Heaters

Tankless or Instantaneous water heaters differ from conventional storage tank water heaters in that they do not store water in a tank for future use. They literally heat water on demand. When a faucet or shower valve is open in a home and hot water is needed the tankless heater turns on and provides the hot water necessary. Many tankless heaters are around 80% efficient however very recently tankless heater have been improving, climbing into the area of 92% efficiency, self condensing units can be up to 98% efficient.


They save energy in two different ways, first they experience no heat loss from the tank because they store no water, secondly they do not continually heat water all day as the water temperature drops. Here is a blow out of the different parts contained in a common tankless water heater. You’ll find the inside of a tankless water heater looks more like the inside of a desktop PC than a water heater. When choosing the right plumbing professional to install a tankless water heater it is essential that they know exactly how they are installed.

Hot Water Tank

Flue piping can be different sizes or different materials like stainless steel or PVC in self condensing units. Also be aware that the units being advertised from the home centers may not be big enough for your family needs. When you hear 5.6 GPM you have to take into account where it is you live and how cool is the incoming water temperature. Here are some key points to think about when looking to install a tankless water heater, these will also help you in deciding which installer to hire i.e. if they don’t make you aware or ask you some of these questions show them the door.

1. How many bathrooms are in your current home?
2. How many pre-teen/teenagers live in your home?

This may seem like an odd question but it is very important. Teens tend to take long showers. If you have two children ages 10 to 18 and they take showers near the same time you may have a problem with a 5.6 GPM unit. If your shower heads are 2.5 GPM and they are running at the same time and your incoming water temp is 65 degrees you’ve already more than maxed out the capacity of the heater. You haven’t even turned on a kitchen sink or clothes washer and you’re tapped out of hot water.

3. What kind of efficiency do you want, what brand of heater is the plumbing professional trying to sell you?

All tankless heaters are not the same. HINT: Just because they advertise the most doesn’t mean they are the best.

4. How is your existing tank type heater installed in your basement?

Is it on an outside wall or in the middle of the basement next to the stairs? This is important because of venting issues.

5. How is the heater being installed vented?

What size is the flue piping and what material is the piping? If a potential installer never mentions changing the flue piping politely show them the door. Most tankless water heaters need stainless steel flue piping because of the high exhaust temperatures, if an installer intends on using the existing flue which is most likely sheet metal they are already showing their lack of knowledge. Self condensing units can use PVC plastic because their exhaust temperature is so much cooler.

6. What size gas line will the unit use?

Most tankless heaters use ¾” supply piping and if you want to go with a bigger tankless it could possibly need 1”. Your house may only have a ½” gas supply, you cannot undersize the gas line. You will starve the unit and  end up having to have it removed.

7. This is a tough question but a fair one nonetheless, when can you realistically expect a return on your investment?

A realistic answer to that is 3 to 5 years depending on your water usage.

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