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How Does A Hot Water Heater Work?

One of the systems that keeps your home comfortable, and your family happy, is your water heater. This familiar fixture provides your home with the hot water you need to take showers, use your dishwasher, and run your washing machine.

When your water heater isn’t working correctly, it can cause a lot of headaches, which is why it is important to have a general idea of how your water heater works. A basic knowledge of this important household system is important for a number of reasons including allowing you to make an informed decision when it comes to purchasing a new water heater or troubleshooting your current system. Having a basic understanding of how your water heater works will eliminate ambiguity and help you find the right solution should any problems arise.

What is a Hot Water Heater?

A water heater is essentially a large metal cylinder with heating equipment inside. The purpose of a water heater is to heat water, and keep it warm until it is ready for use. One of the downsides of this design is that energy must continuously be expended to keep the water warm, and if you use too much water too quickly, you can run out of hot water. When this happens, you need to wait until more water can be heated.

Newer water heaters tend to use the “tankless” design. Instead of holding hot water until it is needed these systems instead heat your water on demand, ensuring an unlimited supply of warm water.

What Types of Hot Water Heaters Are There?

Water heaters tend to fall into two broad categories: Tank-type water heaters and tankless water heaters.

Tank-type Hot Water Heaters

Tank-type water heaters are most common in older homes and can run on fuel or electricity. Attached to the cylindrical tank, you will find water supply and delivery pipes. These bring cold water into the tank to be heated and move hot water from the tank to the various faucets and appliances in your home. The supply pipe routes cold water to the bottom of the tank through the drip tube, putting it close to the heating element. As the water warms, it rises in the tank until it is removed from the tank by the delivery pipe.

All tank-type water heaters are equipped with a safety valve, called a T&P (temperature and pressure) valve. If either the temperature of the water or the pressure in the tank exceeds safety limits the valve opens, releasing water into a pipe that runs down the outside of the tank and ends about 6 inches above the floor.

It is a good idea to place a sturdy bowl or bucket under this pipe so that if the safety valve releases the water doesn’t spill all over your floor. The T&P valve should not be directed at a drain because you won’t be able to tell if the valve has been released. This could mask problems, allowing them to snowball.

Hot water tanks are typically made of steel to prevent corrosion. One of the main reasons water heaters fail is rust. If a hole forms in your tank it can be fixed temporarily with a patch, but the tank will have to be replaced to solve the problem entirely. To fight corrosion hot water tanks are equipped with a magnesium anode rod, which is designed to corrode instead of the tank. The rod can be easily replaced when necessary.

Finally, tank-type water heaters are equipped with a drain cock and a valve. The drain cock allows you to drain the water heater, and the valve allows you to shut down the hot water supply without affecting your home’s cold water supply.

Tankless Hot Water Heaters

Tankless water heaters are useful because they do not require energy to keep your hot water supply warm while it waits to be used. However, because the boiler must be fired up every time hot water is needed this system can be wasteful in the warm summer months. Also, because the water is heated far hotter than necessary, it is imperative to ensure a cold-water mixing valve is also installed to reduce the likelihood of scalding.

There are two types of tankless water heaters: boiler mounted and stand-alone units

Boiler Mounted Tankless Hot Water Heaters

Boiler mounted tankless water heaters feature a coil of pipe that connects the cold water supply to the hot water delivery pipe. When hot water is needed the boiler turns on and heats the water in the coil before it is delivered to your faucets and appliances.

If more hot water is required than the boiler can heat in the coil the temperature of the hot water available drops. To counteract this problem, some tankless systems are equipped with storage tanks. These tanks are connected to the heating coil and are used to increase the amount of hot water available at any one time.

Stand-alone Tankless Hot Water Heaters

Stand-alone tankless water heaters do not rely on your boiler to heat water. These systems are typically powered by gas-fired units, which use a coil and heat exchanger to heat water as it is required. Like other tankless systems, stand-alone units don’t waste energy maintaining a large reservoir of hot water. However, one downside of stand-alone systems is that they typically have a lower flow rate than boiler mounted systems, so they may not be able to heat enough water during periods of high demand.

Troubleshooting Basics


If your water heater is leaking make sure you check to make sure your T&P valve is off, your pipes are securely connected, and your inlet valve is sealed correctly. Most leaks can be repaired by tightening loose parts or patching small holes. However, if your actual hot water tank is leaking your system will need to be replaced.

No Hot Water

If you aren’t getting any hot water, the culprit might be your breaker. Check your breakers to make sure they have not been tripped. If your breaker has not been tripped you should check your thermostat to make sure there are no issues with the limit/reset switch. It may have been tripped because the water got too hot, or need to be repaired or replaced.

If your breakers and thermostat are fine, the problem may lie with your heating element. If your heating element has failed, it will need to be replaced.

Not Enough Hot Water

If your water isn’t getting hot enough, the problem likely lies with the thermostat. Correcting this problem may be as simple as adjusting your desired water temperature. However, on electric models, the thermostat is often hidden behind an access panel and is not designed to be changed once the unit has left the factory.

On gas models, you may need to increase the temperature on the thermostat during the winter months as hot water cools more quickly when it travels through cold pipes.

If your water heater is still not producing enough hot water, the problem may be a result of a faulty thermostat, a faulty element, or loose wiring. If any of these parts are the culprit, you will need to have them repaired or replaced.

Your hot water tank may also be too small to meet your family’s needs.

Water is Too Hot

If your water is too hot, it may be because your thermostat is set too high. Adjust your thermostat accordingly, and make sure you adjust the temperature to account for the transitions between winter and summer.

If you are unable to turn your thermostat down low enough, it may be an indication of a wiring issue. If this is the case, then you should not try and fix the wiring yourself, but instead, consult a professional. You may need to replace your thermostat entirely.

Water Takes Too Long to Heat

Consult your owner’s manual to determine how long it takes for your water heater to replenish its supply of hot water. If your water heater is taking longer than usual to replenish its supply, it may be an indication that there may be a problem with the heating system (such as a build-up of sediment on the heating element) or an issue with the thermostat.

If your water heater is no longer able to meet your family’s hot water needs, you may want to consider investing in a system with a larger tank or a tankless system. You can also supplement your current water heater by installing a point-of-use water heater near the water source you use most (such as your shower).

Low Hot Water Pressure

Low hot water pressure is a problem that plagues a lot of older homes. A lot of older homes were built using ½ inch diameter galvanized pipes running to and from the water heater. Since water pressure is automatically limited the only way to rectify this problem is to replace those pipes with modern ¾ inch diameter pipes so more water can flow through.

Another culprit may be sediment buildup in the pipes. Calcium deposits and rust inside your plumbing or sink aerators can limit the flow of water and decrease your water pressure.

Strange Noises

A scale build-up is the most likely cause of strange noises such as banging, hissing, or knocking on your heating element or sediment in the bottom of your tank. These noises could also be caused by a leak or a build-up of pressure inside the water heater’s tank. Chances are the explanation is harmless, but it is always a good idea to consult a professional just in case.

Dirty or Rusty Water

Dirty or rusty water is usually an indication that your anode or tank has become rusted. Have your water heater checked out by a professional right away. If the tank itself is corroded, it will need to be replaced. If the culprit is your anode rod, it can easily be replaced, adding years of life to your water heater.

If the water appears to be cloudy, but not dirty, it may be a result of scale build-up on your heating element or sediment that has made its way into the hot water outlet.

Smelly Water

The most common cause of smelly water is a bacteria. Flushing your tank will temporarily fix the problem, but to ensure your water stays fresh smelling you will want to replace your anode rod.

Your water heater isn’t something you likely think about often, but when something goes wrong, it can be very disruptive. By having a basic understanding of how your water heater works, and some common problems, you can better diagnose problems when they occur. You can also make a more informed decision when it comes time to replace your water heater and select a style that suits your family’s needs.

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