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How to Tell if Your Hot Water Heater is Full? – 3 Methods

Fact checked by Stephen Conklin

how to tell if your hot water heater is full

If you’re like many first-time water heater owners, there’s a good chance you don’t know how to tell if your hot water heater is full.

Experience tells us that a steady hot water stream without spurting (indicating air in the pipes) after opening the water tap means our electric or gas hot water heater is full.

But if you’re curious how to tell how much hot water is left in your water heater, please continue reading. We offer three proven ways to check water level in water heater units.

Table of Contents

Ways to Tell if Your Hot Water Heater is Full


How do you know if your water heater has reached capacity and it’s time to stop refilling it? Knowing how to check water level in water heater units enables you to assess the appliance’s performance and function. Here are three proven techniques to help you out.

Method 1. Check water heater-specific water level visual aids


Some water heater manufacturers integrate a system for indicating storage tank water levels. Some brands use a sticker, while others print the information in the User Guide or Owner’s Manual.

You can check these documents for specific information on recognizing a water heater empty unit or a full one. If you’re lucky, you might even find a visual aid for knowing when to refill a hot water heater.

Note that manufacturers vary in visual aid systems. Hence, you can contact your authorized distributor or dealer for more information. Alternatively, reaching out to the water heater company should give you a more accurate answer.

Method 2. Check the information on the digital screen


Suppose your water heater doesn’t have a visual aid for telling how long to fill hot water tank (the answer is 20 to 60 minutes) or knowing if the device is full or empty. Maybe you misplaced or lost the Owner’s Manual or User Guide of your 40 gallon water heater, and you cannot contact the brand or dealer.

In that case, you can check water heater for a digital display on the side or wall. Like method 1, not all water heaters have this technology. And even if they do, the system would vary across brands.

Some water heaters might offer only basic water heater parameters on the LCD screen or LED panel. Others might present more in-depth information about your unit. Brands such as Bradford White, for example, might issue warning signs hot water heater going out, allowing you to rectify the situation before it worsens.

Advanced digital displays might also inform you whether the tank’s full, which is helpful when you fill a water heater for the first time.

Unfortunately, digital display technologies are almost non-existent in residential water heaters, unless you buy a commercial-grade unit for your home.

Method 3. Perform an old-school manual water level assessment


Don’t fret if the first two methods didn’t work or you cannot perform them. There’s always the old-school trick to execute. Although this technique can be inconvenient, it remains one of the best and most trusted ways to check if the water heater is full. Here’s how.

  • Go to the nearest faucet and open the hot water tap.
  • Leave the water flowing for about two to three minutes.
  • Close the hot water tap and wait several seconds.
  • Open the same hot water faucet and observe if air or water comes out first.
  • Your water heater is full if you notice a steady water flow without air pockets.
  • Your water heater still has room for extra liquid if air comes out of the hot water tap.

When and How Often Should I Drain and Refill the Water Heater?


Experts recommend an annual draining and refilling of water heaters to remove any sediment and particulate buildup at the tank’s bottom. Flushing these substances down the drain also gives you an idea on when to stop filling the water heater tank – once the water runs clear.

However, households in areas with high levels of sediments, iron, sulfates, particulates, calcium carbonates, heavy minerals, and particulates might need to drain, flush, and refill their water heaters as frequently as once every four to six months to remove these substances from the appliance.

You can also look for signs of sediment buildup to know when to drain, flush, and refill the water heater. These signs are:

  • Unexplained water temperature fluctuations
  • No hot water or the water heater works more slowly than expected
  • Odd sounds from the water heater (i.e., cracking, popping, knocking noises)
  • Leaks next to the drain valve
  • Water discoloration and offensive smells

Frequently Asked Questions


Why should I drain and flush my water heater?

Draining and flushing your water heater improves its performance, allowing it to heat the water more efficiently and increasing its longevity.

The most significant reason for flushing a water heater is sediment and particulate accumulation. For instance, rust, limescale, and other substances can build up over time. Flushing removes these compounds from the water heater.

Water discoloration (i.e., murky, hazy, or rust-colored water) and offensive smells can also warrant draining and flushing the water heater.

How do I know if my water heater is at risk of exploding?

The following are signs your water heater is going to explode.

  • An uncanny odor that smells like a rotten egg potentially indicates a gas leak.
  • Popcorn machine-like thumping or cracking sounds are a sign that the water heater storage tank’s bottom is full of sediment and other particles.
  • Problems with the water heater’s electrical connections, control interfaces, fuel links, and other components can also increase the risk of water heater explosions.
  • A leaking or worn-out temperature and pressure (T&P) relief valve can also lead to explosions because of increased intra-tank pressure.


Learning how to tell if your hot water heater is full or empty is as straightforward as looking for visual aids on the device. The task is simpler with a high-end, commercial-grade water heater featuring a high-tech information-displaying digital screen.

Of course, not everyone has units with these modern water level-sensing and indicating technologies. Oddly enough, turning on a hot water faucet, closing it, and reopening it should give you a clue about the water heater’s fullness.

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