How Does Cold Water Move Through Hot Water Under Pressure?

Sean Jarvis
by Sean Jarvis

It is hard to imagine how cold water could prompt hot water to move through your water lines. Hot water heaters are a very simple appliance that does this very thing, giving your home access to hot water when you need it most.

When cold water is fed into the bottom water heater tank, the pressure causes hot water at the top of the tank to be pushed out. The cold water enters the tank through the dip tube, which goes down the length of the internal side of your water heater. That is why when you run out of hot water, the water doesn’t simply stop. It becomes lukewarm and then cold until you give your water heater enough time to heat up the cold water.

The Flow of Water

City Water Pressure

Every home is connected to the city water supply, through its own water main. This water main line is located near the edge of your property.

Your water main receives pressurized water from the pumping stations, through underground pipes. This water is treated, in different ways depending on the source and your city, before it reaches your home.

The pressure that is applied to the water through these water lines has enough pressure to travel to your home and all other homes within your city. This is why when a water main breaks, you see a big explosion of water shooting into the air.

This pressure is facilitated through pumps at the water station as well as pumps underneath the ground. You will see marks on the street that indicate where water lines are. Fire hydrants are also tied into these lines and provide a massive amount of pressure in order to make it through the thick hoses that firefighters use.

Low Water Pressure

The further you live from the city water pumps, the lower your water pressure may be. The city tries hard to mitigate these issues as best it can.

The reason this happens is because the pressure and momentum that is provided by the pumps is dissipating as it travels through the water lines. If your home has consistently low water pressure, you can have professionals install water pumps on the main water line.

This will help to re-pressurize the water in order to give it enough force to circulate to your home and through your pipes.

From the Street to Your House

Once the water enters your water main, it moves through a large pipe into your home. These pipes are typically ¾” or larger, depending on what is needed. This pipe is called the water supply line.

The water supply line will carry the water to the mainline in your house. The engineers who put in these pipes bury the supply line far enough under the ground to withstand temperature changes. This protects the line from freezing during the winter.

Supply lines are typically made from thick plastic, galvanized iron, or copper. From the supply line to the water main line, the water is split into two different paths. One of these lines will run directly to the water heater, the other will head into your house.

Cold Water Lines

The cold-water service line runs directly to every faucet and water appliance in your home. These pipes are installed in straight lines, with 90degree angles and slight downward slopes to make for an easier flow.

Intake pipes attached to every water appliance in your home allow for the reception of the cold-water line. Your home is therefore filled with a network of pipes, all running through various parts of the building to give you water.

Hot Water Lines

The water line that is connected to the water heater will supply it with fresh cold-water to heat, for use in your home. From there, the water will be sent into hot water lines throughout the walls of your home.

The hot water lines run parallel to cold water service lines. If you look at the plumbing behind your walls, you will be able to see the two pairs of pipes that run side by side with each other.

Hot and Cold Appliances

When you turn on a faucet you are opening the valve at the end of one service line intake pipe. This is why it is important to shut these valves off before doing any plumbing work. The high-pressure system will otherwise allow water to flow freely out of the pipe.

Since every water appliance in your home has two separate lines for the hot and cold, the only time that these lines mix with each other is when they enter a single spout. Hot and cold never mix with each other within water lines that exist behind your walls.

Hot Water Heaters

A water heater is the way that your house turns cold or cool water from the city, into hot water. Used for many different things, they are an essential part of any home.

Tankless Water Heater

Newer systems create hot water on demand. These are referred to as tankless water heaters and heat the water directly, without the use of a water storage tank.

When a hot water tap is turned on, cold water travels into the unit. From there, a gas or electric element heats the water as you use it. The downside to these units is the upfront costs and that they cannot service many appliances like a water tank can.

Tank Water Heater

The most common water heater is the tank. This is the one that looks like a big, metal cylinder. These store hot water for use later, rather than on-demand, and can service many different hot water requirements at the same time.

How A Water Heater Tank Works

The inner shell of a water heater has a protective liner that holds 40 to 60 gallons of hot water, at 50 – 100 PSI. The exterior of the tank is covered in an insulating material and then an outer shell.

The drum of this water heater is filled with water and a heating mechanism is located inside, on the bottom.

Water enters the tank through a dip tube, which extends to the bottom of the tank. When the water is heated it will travel to the top of the tank.

Gas water heaters use a burner and chimney system to heat the water. This is why in some homes you may need to relight the pilot if you notice you no longer have water. There are also electric water heaters that rely on a steady electrical supply.

Cold Water Creating Pressure for Hot Water to Move

Once the water heaters thermostat is set, the water heater will begin heating the cold water within. The dip tube feeds cold water from your water lines into the bottom of the tank.

The heating mechanism will stay on until the water in the tank reaches the required temperature. As the water heats, it rises to the top of the water tank.

When you turn on the hot water, the water will exit the tank out of the heat-out pipe. Therefore, the water at the top of the tank is always the hottest.

The principle aspect of the Tank Water Heaters design is that the pressure from the cold water is what pushes up the hot water and into your hot water supply lines.

Related Questions

What is More Efficient: A Tank or Tankless, Water Heater?

For homes that use 41 gallons or less of hot water daily, tankless water heaters can be up to 30% more efficient. Gas-fire tankless water heaters can save homeowners over $100 a year. Electric tankless heaters save about $44 a year.

What Causes Low Hot Water Pressure?

The most common cause of low hot water pressure is corrosion. Over time the inside of a water tank can corrode, with the material settling to the bottom of a tank and blocking the lines. In this situation, you would need to flush your system to get rid of the materials in the tank.

Can I Turn Off My Gas Water Heater?

If you are going out of town for an extended period, it is always a good idea to shut off your gas water heater. This will save you money on gas and prevent overheating problems that might come up while you aren’t there to take care of them.

Sean Jarvis
Sean Jarvis

Sean Jarvis is an interior decorator, writer, and expert handyman. Well versed in everything home improvement, he is a savant at manipulating words and spaces and upgrading everything around him. Sean specializes in writing concise guides about appliance repair and installation, home and lifestyle, and other residential projects.

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