Hot Water Tank Not Filling Up? (Possible Causes & Fixes)

Jessica Stone
by Jessica Stone

Water heaters are essential in modern homes, but they are as prone to problems as any other modern commodity. Nothing is worse than when your hot water tank won’t fill up when you need it most. So, why is your hot water tank not filling up?

Leaks in the water tank, pipes, or even underground can stop your water tank from filling up. An airlock will prevent your water tank from filling when pockets of air get stuck within pipes or the tank. The buildup of sediment, dust, and dirt can clog your pipes and your hot water tank won’t be able to fill up.

You must contact a plumber if you find evidence of an underground leak or leak within the wall. Both scenarios can be dangerous and lead to expensive problems such as mold and wood rot. Follow along as we explore why your hot water tank won’t fill up.

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How to Know if Your Hot Water Tank Isn’t Filling Up Completely

It’s unlikely that you regularly look into your water heater tank to make sure that it’s consistently filling up all the way, especially if you aren’t familiar with the various components. While this may be the easiest way to find out if you’re having an issue with your hot water tank filling up, there are other signs that you can keep a lookout for.

The following are some of the most common issues that could signify that your hot water tank is not filling up as expected:

  • Low hot water pressure.
  • Puddles of water accumulating around the tank or anywhere near the outside of the tank.
  • A reduced amount of available hot water in your home.
  • Little to no hot water in your sinks, bathtubs, and showers.
  • Unusual noises coming from the tank, including knocking, banging, creaking, whining, or rattling.
  • Uncharacteristic lack of sound coming from your water heater, as you can usually hear when a water heater is filling and operating normally. If you don’t hear this as often, this may be a sign the tank isn’t filling up completely.

If you’ve noticed any of the aforementioned issues and you’ve come to the conclusion that your hot water tank is not filling up, let’s explore some of the most common causes to help you narrow down the problem and find a solution quickly.

Why Isn’t My Hot Water Tank Filling Up?

Clogged pipes and leaks are the most common reasons that your hot water tank isn’t filling up. However, complex problems such as an airlock can also keep your hot water tank from filling with water. Let’s take a look at the most common causes and solutions for a hot water tank that won’t fill up.

1. Leaks

A leak within the pipes or hot wat water tank can prevent the tank from filling with water. You can visually inspect the area around the tank and look for puddles of water on the ground. This will indicate that there is a leak at the base of the tank or within the pipes.

Check the walls behind the heater for signs of water stains. This can indicate that the pipes within the walls are leaking. Leaks within the walls are dangerous because they can cause electrical damage and allow mold and mildew to grow.

Some hot water tanks have pipes that filter underground. In this case, you may notice pools of water on the exterior wall of your home behind the hot water tank.


Call a professional if you determine that an underground leak is the problem because they are equipped to fix it. You can easily fix other leaks if it is a matter of a loose valve or pipe. Simply tighten the loose part so that water doesn’t leak out.

2. Airlock

Pockets of air can lodge within your hot water tank’s pipes. This is called an airlock and can prevent your hot water tank from filling up. An airlock will either prevent any water from getting through or limit how much water enters the tank.

This will make it impossible to fill your hot water tank on time or at all. You can find out if you have an airlock if you check the water temperature at one of your faucets. Turn the hot water handle on your faucet and check to see if the water is as hot as it should be.

Cold or lukewarm water that comes out of your faucet can indicate an airlock. Another sign is that the water pressure is reduced and only trickles or comes out in drips.


There are two ways to remove an airlock from your water heater. First, cover the opening for any faucet in your house and open the hot water valve. Now, open the cold water valve and keep your hand over the faucet’s opening and run the cold water.

Allow the cold water to run for 15 seconds, then shut it off. This can get rid of the airlock in many cases without using any tools. The other method is to take a 12” hose and put one end on the cold water tap and the other end on the hot water tap.

Apply tape to hold the hose in place, and open the hot and cold valves for 5 seconds. Turn off the valves and test the hot water on another faucet in your home to see if you removed the airlock. Repeat the process until hot water comes out at a consistent pressure.

3. Hot Water Tank Filling and Draining Repeatedly

Another possible cause of your hot water tank not filling up is because it is continuously filling and draining. This means that it is maintaining the same water level. To determine if this is the problem, you’ll likely be able to hear water always running in the tank, even when faucets or appliances are not being used.


In most cases, this happens because the TPR (Temperature Pressure Relief) valve is stuck in the open position. This can cause water to drain from the tank, which results in the system constantly resupplying water to prevent the tank from emptying. Stuck TPR valves usually happens with older hot water tanks and can often be remedied by simply closing the valve or replacing it.

This can also occur when a faucet has been left on in your house, whether it’s a faucet that gets rarely used or it simply remained on undetected. In this case, once you turn off the faucet, the issue should be fixed.

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4. Clogged Pipes

It can be difficult to determine if the pipes in your hot water tank are blocked just by simply looking at the tank. The only way to find out if you have a clog is to empty the tank and inspect the base. If you find excessive sediment in the bottom of the tank, it’s likely that there’s more clogging pipes in the system to prevent the tank from filling.


The best way to fix this issue is to drain your hot water tank. Although, you can also remove clogs caused by sediment by backflushing your water heater.

  • Shut off the power to your hot water tank at the circuit breaker and, for gas units, switch the thermostat to “pilot position.”
  • Attach one end of a hose to the drain valve at the base of the take and place the other end into a bucket.
  • Shut off any recirculation pumps, as well as the supply valve for cold water.
  • Turn the hot water on at your nearest sink faucet.
  • Then, turn the hot water on at the faucet furthest from the hot water tank.
  • Open the TPR valve lever.
  • Open the drain valve and allow one to two gallons of water to drain out until no sediment is present.
  • Close the drain valve, remove the hose, close the TPR valve, and open the cold water valve.

Before turning the system back on, turn off the faucets first once them reached a steady flow.

Summing It Up

A hot water tank won’t fill up if there is sediment and debris in the pipes. Inspect your water heater tank for leaks and pools of water at the base. Look at the wall behind the tank for signs of water damage that indicate a leak within the wall.

Otherwise, it is likely a case of an airlock that won’t allow your hot water tank to fill up. You can fix an airlock without tools if you open the valves on a faucet, run the water, and cover the faucet opening.

Jessica Stone
Jessica Stone

Jessica considers herself a home improvement and design enthusiast. She grew up surrounded by constant home improvement projects and owes most of what she knows to helping her dad renovate her childhood home. Being a Los Angeles resident, Jessica spends a lot of her time looking for her next DIY project and sharing her love for home design.

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