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Home » Water Heaters » Water Heater 101: How Long Do Water Heaters Take to Heat Up?

Water Heater 101: How Long Do Water Heaters Take to Heat Up?

By: David Trinh
Last Updated:
How Long Does It Take For a Water Heater to Heat Up

Distilled water, with its 99% purity level, is a preferred choice for many households and medical establishments. Its uses range from CPAP machines and dentistry to steamers and home water use.

However, the vast array of water distillers and complex technical terms can be daunting.

This guide simplifies the process, reviewing the top countertop water distillers and providing a comprehensive buyer’s guide to help you make an informed choice.

How Long It Takes A Water Heater to Heat Up For The First Time

Water Heater TypeTime Takes to Heat Up
Gas Tank

tank heater
30-40 minutes
Gas Tankless
tankless water heater
0 minutes *
Electric Tank
tank heater
60-80 minutes
Electric Tankless

tankless water heater
0 minutes *

*Tankless water heaters provide almost instantaneous heat if sized and installed properly.

Chart Source

How Long Does it Take for a Gas Water Heater to Heat Up?

tank heater

Your average gas tank heater takes about 30 to 40 minutes to start heating water once it enters the tank. This initial heating up of warm water occurs when new water from your plumbing supply is fed into the tank.

A more specific explanation of why this takes 30 minutes requires some math. The heater’s tank size is obviously a major factor, as more water will take longer to heat. The next major factor is the heater’s BTU (or British Thermal Unit) rating. Simply put, a BTU is the amount of heat required for one pound of water to rise by one degree Fahrenheit. A heater with more BTUs can heat water faster.

For example, the average hot water heater tank is 40 gallons. There are approximately 8.3 pounds of water per gallon, so our example tank has about 330 lbs of water to heat.

40 Gallons x 8.3 lbs Per Gallon = 330 lbs of water

If the water is already 60 degrees and you want it to reach 120-degree hot water, a temperature rise of 60 degrees is needed.

To avoid getting into full-blown thermodynamics calculations, we can simplify and say that a 40,000 BTU system with a 40-gallon tank needs one half of one minute to heat each gallon, resulting in a half hour heat up time.

If you have a smaller tank or higher BTU rating, your hot water heater’s warm-up time will be shorter. If you have a larger tank or lower BTU rating, on the other hand, it will take longer to heat your tank.

If you want an energy efficient water heater which will heat up your water in the amount of time you desire (after it runs out of hot water) as well as store a good amount of hot water, these are the specifications you will need to keep in mind. The storage of warm water will come into play with your heater recovery, allowing for longer showers for multiple users in. your household.

Also, keep in mind that this is the amount of time it takes for new cold water to be heated in your tank. When turning on the hot water for the first time after your tank has been storing hot water already, you should have hot water within just a few moments because tanks store pre-heated water.

This amount of time it will take to heat up new water comes into consideration when all the hot water in the tank is used up. That’s when the gas tank water heaters heat will have to start heating new water again from the incoming groundwater temperature.

A gas tank water heater will take approximately 30 minutes to heat up new incoming water for the first time.

How About Tankless Gas Heaters?

Tankless water heaters warm up your water “on demand”, so the distance from your heater to the appliance being used is the only consideration that determines how long it will take to receive the hot water out of your faucet.

If the system is working properly, this should not be more than a few seconds with a normal sized home. With a large home, it may take a few more seconds to travel through the pipes and reach appliances further away from the water heater.

A tankless gas water heater heats up water instantly so it should only take a few seconds before that hot water travels through your pipes into your fixture.

How Long Does It Take For an Electric Hot Water Tank to Heat Up?

electric water heater hanging on the wooden wall

Electric tank water heaters typically require double the amount of time compared to their gas counterparts. Electric elements, while typically more economical, simply cannot compete with the high performance of gas-fired systems. So, how long does it take an electric hot water tank to heat up? From the time new water enters, it would take about an hour or more for an electric water heater to warm the 40-gallon tank described above.

This is why homes with larger water demands usually decide to purchase a whole house gas tank water heater instead of an electric model. Electric models are excellent for smaller homes and smaller water demands.

An electric tank water heater takes 60-80 minutes compared to 30 minutes that a gas tank heater takes to heat water.

How About Tankless Electric Heaters?

Just like tankless a gas water heater, a tankless electric water heater will only begin to warm up your water once an appliance demands it. In other words, the water is not warmed until you turn on the dishwasher or turn in the faucet.

In most cases, an electric tankless heater will provide hot water within moments, but they can take a tiny bit longer than gas systems, due to the power of gas heat.

A tankless electric water heater heats up water instantly so it should also only take a few seconds before the hot water travels through your pipes into your fixture.

Factors That Affect Heat Up Time

In addition to the factors we’ve already explored, such as tank size and BTU rating, there are other circumstances that could dictate how long your water heater takes to warm up water for the first time.

Tank Size

The size of a water heater tank directly influences its heat-up time: the larger the tank, the longer it takes to heat the water. This is because a larger volume of water requires more energy and time to raise its temperature to the desired level.

BTU Rating

The BTU (British Thermal Unit) rating of a water heater impacts its heat-up time significantly. A water heater with a higher BTU rating will heat water faster than one with a lower rating, as it has a greater energy output per hour.

Incoming Temperature

For both tankless and tank-style water heaters, the temperature that the water starts at will help determine heat up time. Because tank heaters store water and keep them heated, the incoming temperature shouldn’t affect it so greatly. Tankless heaters, however, feed incoming water on demand just moments before it comes out of your faucet. This means that if the groundwater temperature is very low, the water may not get as warm as fast. Both types of heaters can be affected by extremely cold ambient temperatures in the room or area where they are stored.


Although water heaters seem relatively simple when compared with other household mechanicals, they often have more to them. If your heater isn’t working, a professional may need to come to check out any settings or calibration that could be negatively impacting its performance.  

Age / Maintenance Issues

Just like any other mechanical equipment, the age and condition of your water heater could eventually affect its performance, including how long it takes to heat up.  A lack of general maintenance, particularly failing to clean up sediment that may be in the pipes could cause performance issues as well. Pipe sediment is more likely in areas with hard water.

Distance from Appliance

Sometimes it’s easy for the end user to forget, but your hot water is traveling from the ground through the water heater and pipes in your home before reaching the appliance you are using. The farther your appliance is from the water heater, the longer it could take for the heated water to reach it. A savvy installation should account for this when setting up your system, so it should not be too much of an issue.

Pipe Diameter

In addition to the length of piping, the width of your water pipes could affect how long it takes the water heater to heat up. A wider pipe is beneficial as it carries more water, but it will require more water to be heated before the pressure builds enough for it to push through the remaining pipe system.

Factors That Slow Down Heat Up Time

In addition to the factors we’ve already explored, such as tank size and BTU rating, there are other circumstances that could dictate how long your water heater takes to warm up water for the first time.

Sediment Buildup

Sediment buildup occurs when the dissolved calcium and magnesium minerals settle at base of your water heater. Over time, this creates a crusted layer that restricts your water heater’s ability to heat water.

Broken Dip Tube

The dip tube is component that pushes cold water to the bottom of the tank to be heated. If the dip tube is damaged then cold water will mix with heated water, decreasing your overall water temperature.

Worn Heating Elements

If your heating elements like gas burners and electric resistance coils get damaged over time, your tank water heater may have trouble reheating.

Final Thoughts

In summary, there’s a water heater perfect for everyone out there. Whether it’s a classic tank or a tankless model, think about your needs before selecting one.

Now that you know how long it takes both gas and electric water heaters to heat up, see our review of the best models on the market.

With great brands like Bosch, Rheem, and Takagi, you’re sure to find a match!

Photo of author
David Trinh
David is an expert in all things plumbing, heating, cooling, and water treatment. He got his start in the plumbing business working on fixing all types of home improvement issues including water leaks, broken toilets, appliance installation, and more. Over time, he learned a ton about installing and choosing the correct water treatment products for homeowners.

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