How to Fill a Water Heater (8 Step Guide)

Last Updated On June 11, 2024

Updated on July 22, 2021



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How to Fill a Water Heater (8 Step Guide)

Do you want to learn the proper way to fill a water heater? You’ll need to fill your water heater tank when you buy a new unit or after you empty or flush your current unit. 

This PlumbingNav guide will tell you everything you need to know about how to fill a water heater! 

What's In This Guide?

      How to Fill a Water Heater

      Here is a step-by-step guide to help you fill up your empty water heater. This guide assumes the water heater is already empty. 

      Capacity Questions? Check here to see how much water your water heater will hold.

      Step 1: Understand the Components of Your Water Heater 

      Before you get started, you should know some important components of your water heater and what they do. 

      • Cold water shut off valve: this valve stops the cold water flow.
      • Hot water outlet: this is the pipe that sends hot water from the water heater to the appropriate room in the house. 
      • Drain valve: this valve, usually located at the bottom of the unit, allows you to empty the unit. 
      • Relief valve: this valve allows pressure to escape from the water heater. 

      Now that you know the different components, you need to find them on your water heater. If you’re unsure of which valve is which, or where to locate the appropriate valve on your unit, refer to the user manual. 

      Step 2: Cut Off Gas or Electricity

      Water heaters heat the water in the tank through gas or electricity. For gas water heaters, you need to turn off the gas valve. For electric water heaters, you need to turn off the appropriate circuit.

      did you know how to fill a hot water heater

      Step 3: Close Drain Valve and Relief Valve

      The drain valve should already be closed. However, it’s wise to double-check that it’s probably closed. If it’s open, the water will just start pouring out of the tank when you go to fill it. This may not be a big deal if yours is located in the garage, but it will be if it’s in the attic! Check this first.

      You also need to close the relief valve. In most cases, the relief valve will already be closed. The relief valve is usually only opened to check for leaks and relieve pressure in the water heater.  

      Step 4: Open Cold Water Shut Off Valve

      Next, you want to activate the cold water shut-off valve. You will find this near the water meter in your home. Turning the valve clockwise will tighten it, preventing water inflow. You want to turn it counterclockwise to loosen it. 

      This is the step that starts filling up the tank. You should start to hear water flowing into the tank. 

      Step 5: Turn Hot Water Faucets On

      Go through your home and activate some of the hot water faucets. This allows airflow to run through the pipes, presenting an air block. It also allows ventilation for the water heater. 

      Since the water heater is empty, you will likely notice the faucets not releasing water. It may sputter a bit if anything at all. This is normal. 

      Step 6: Wait For Hot Water to Flow Properly

      As the tank fills up, the water in the open hot water faucets should start to gain momentum. When the hot water faucets are flowing at full capacity, you know your hot water heater tank is finally full. The water should be cold at this point. 

      Step 7: Restore Power

      Next, you need to heat the water up by restoring the power to the water heater. Flip the circuit breaker to “On”. Verify the power did indeed return to the unit.

      You can tell the unit has power by noise or the digital control panel illuminating again. See these details for turning on your electric water heater. Have a gas unit? We cover pilot light questions here.

      Step 8: Wait For Water to Heat Up

      After you fill the water heater, you need to wait for the water to heat up. Generally, this will take 2 – 3 hours for a residential water heater. Various factors come into play to determine how quickly the water will heat up, including:

      • Tank size
      • Tank insulation
      • Power 
      • Temperature of water in the tank 

      Check the water heater’s first-hour rating (FHR) to learn how much hot water you can expect after an hour. An 80 gallon water heater with an FHR of 40 gallons will take 2 hours to heat up. An 80-gallon water heater with an FHR of 80 gallons will heat up in one hour.

      If the water never gets hot… 

      If after 2 – 3 hours the water still hasn’t heated up you may have a problem. First, check that your electric water heater has power and your gas water heater has a pilot light. If that’s not the issue, it’s time to look into alternative causes of the problem. 

      Common causes of cold water in electric water heaters include a faulty heating element or a faulty thermostat. 

      Common causes of cold water in gas water heaters include a faulty or dirty thermocouple or poor ventilation.

      Read more about how long it takes for your water heater to get hot, here.

      FAQS For Filling A Water Heater

      How can you tell when a hot water heater is full?

      You will know the hot water heater is full when the water starts flowing freely from the hot water faucet that you left open for just this purpose. Just because the water from the hot water may be cold at this point does not matter. The tank is full. You just need to heat up the water in it. 

      Can you manually fill a hot water heater?

      No.  Manufacturers design hot water heaters to fill up using the cold water valve, and breaking away from its intended processes can damage the unit. While technically physically possible, filling a water heater manually would require removing the cold water input and attaching it again, which could lead to all sorts of problems (and probably void the warranty). 

      Not to mention, using the cold water valve will save a lot of time and energy as opposed to manually filling it yourself. 

      If for some reason you don’t want to fill the hot water tank using the cold water valve, correct that problem first. Then fill your water heater as usual. 

      How long does it take to fill a water heater?

      One of the largest factors in how long it will take your water heater to fill up involves the size of the tank. The larger the tank, the longer it will take to fill up. Another factor at play is the speed of the cold water entering your tank.

      In most cases, it should only take a couple of minutes to fill your water heater tank. If it still doesn’t appear full after a half-hour, you may want to troubleshoot further. 

      Why isn’t my water heater filling up?

      If you followed the instructions perfectly and still don’t have a full water heater, you may have a problem with the unit yourself. You know the water isn’t filling properly if you notice puddles around the water heater, extremely low pressure, or weird noises. 

      The most common reasons a water heater won’t fill up include:

      • Open drain valve

      The drain valve is designed to allow the water in the water heater tank to drain out of it. If you accidentally left the drain valve open, you should notice water coming out of the pipe when the cold water valve is open. Shut the drain valve and try again. 

      • Leak

      A leak can definitely prevent the water heater from filling up. To determine if your water heater has a leak, look for puddles of water on the floor or under the pipes. You should also check the walls if the pipes go into the walls, or the ground outside if the pipe goes outside. 

      If you find a leak, you can try to tighten the connections to solve the problem. If that doesn’t work, you can find a temporary solution by applying putty, a clamp, or coupling for small leaks. If that still doesn’t work, call a plumber to fix the leak for good. 

      • Clogged pipes

      If you notice a lot of sediment in your water heater, this sediment may be filling up your pipes as well, making it difficult for water to get through. Sometimes, flushing the water heater can get rid of the clog. If the pipes still seem clogged after you flush the water heater, it’s time to call for a professional plumber.

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