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how long does a water heater take to heat up

Whether you’ve installed a new water heater or waiting for your heater to recover, nobody likes waiting for hot water. That’s why one of the most common questions asked is how long does it take a water heater to heat up? As a general rule, you should expect hot water in about 30 minutes for a traditional tank-stye water heater and nearly instantaneously for a tankless model. 

But in reality, it really depends on a number of factors. This article will take a look at the different types of water heaters, and how long each takes to heat up. And, if you want more details, it’ll even walk you through the factors that come in play to determine the amount of time it takes to heat water.


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

As we’ll go into detail below, there are a lot of factors that impact the amount of time it takes a hot water heater to heat up. But this chart should give you a quick glance for a 40-gallon electric and gas tank-style heater (as well as tankless models).


Since tankless water heaters are capable of heating water nearly instantly, this article will focus on the traditional tank-style water heater. These heaters heat the incoming water and then hold it in a tank until there’s a demand.

How Long of a Shower Can You Take with a 40-Gallon Water Heater?

The average shower uses roughly 10-gallons of hot water, so in theory your household should be able to take 4 showers with a 40-gallon hot water tank.

This of course, is assuming that no other hot water appliances or faucets are being used.

Gas Water Heater Heat Up Time

The size of tank plays a role in how long it takes a water heater to heat water. Since larger tanks have more water to heat, it shouldn’t come as a surprise that they take longer to bring the water to temperature.

The average gas tank-style water heater is 40-gallons, so for this example we’ll assume this tank size.

If you’re starting with a cold tank of water, you can expect that it’ll take roughly 30 to 40 minutes to heat 40-gallons of water. Larger tanks will take longer.

A 40-gallon GAS hot water heater will take 30 to 40 minutes to heat up. 

British Thermal Unit

Gas water heaters use a burner located at the bottom of the tank to heat the water. Gas burners are measured in British Thermal Units (BTU). 

BTU measures the amount of heat required to raise a single pound of water by 1°F. Water heaters with a higher BTU rating are capable of heating water faster.

Temperature Rise

Temperature Rise asks the question: “How many degrees does your water heater need to heat the incoming water in order to reach your desired hot-water temperature?” 

In other words, if your water heater needs to heat the incoming water 57° in order to bring the water to 120° it’s going to need to work harder (take more time) than a water heater that only needs to heat the incoming water 33°.

Here’s the Facts:

  • 330 pounds of water to heat (40 gallons x 8.3 lbs per gallon)
  • 60° temperature rise (60°F incoming water / 120° hot water)
  • 40,000 BTU on a 40-gallon tank requires 30 seconds to heat each gallon

= It’ll take 30 Minutes to heat the cold water in a 40-gallon tank to 120°F.

Remember, the temperature rise, tank size and BTU rating will impact the amount of time it takes to heat the water in your water heater. Colder incoming water will have a higher temperature rise, and therefore will take longer to heat.

Smaller sized tanks will heat the water faster than larger sized tanks at the same BTU rating.

Higher BTU ratings will heat the water faster in a 40-gallon tank, and lower BTU ratings will heat the water slower. It’s all relative.

Bottom Line

  • A brand new 40-gallon gas hot water heater will take between 30 to 40 minutes to heat hot water for the first time. This is also the case if all of the hot water in the tank was used.
  • An existing tank-style gas water heater is capable of delivering hot water immediately since hot water is stored in the tank. Keep in mind, the hot water needs to travel through you home’s plumbing, so there will be a slight delay until it reaches the faucet.

Electric Water Heater Heat Up Time

Known to be more economical and environmentally friendly than gas water heaters, electric models unfortunately take longer to heat water. In fact, they take, on average, double the amount of time as a gas heater.

A 40-gallon ELECTRIC hot water heater will take 60 to 80 minutes to heat up. 

Although tank size and the water’s temperature rise will impact the amount of time it takes to heat the water. Unlike gas water heater’s that use a burner, electric models use heating elements.

Electric water heaters typically use two heating elements in order to heat the water. One is located near the top of the tank, and the other, near the bottom.

The heating elements are rated by voltage and wattage, and the higher the numbers the more powerful the element. In other words, it’ll heat water faster.

A word of caution, never replace a heating element with a different voltage. Always use the same voltage as the original element. And, although you can replace an element with a lower wattage, you should never replace it with a higher wattage. 

Bottom Line

  • A brand new 40-gallon electric water heater takes between 60 to 80 minutes to bring a cold tank of water to temperature.
  • Since electric (and gas) water heaters have a tank which holds the heated water, you’ll have immediate access to the hot water after it travels through your home’s plumbing. 
  • If your household has a high demand for hot water, you may want to consider your options. You could purchase a larger tank-sized electric water heater, go with a gas heater, or even move to a tankless system.

How Long Does it Take for Hot Water to Come Back?

Once you’ve drained your hot water tank and your shower has turned cold, your water heater will need to heat the cold incoming water before it can deliver hot water again. This is called Recovery Time.

So when we ask the question: “How long does it take for hot water to come back?” what we’re really asking is “What’s the recovery time?”

You can assume that it’ll take an electric water heater 60 to 80 minutes to fully recover and heat a full tank of cold water back to the set temperature. A gas heater will take 30 to 40 minutes.

These times could be a little less since your water heater actually began heating water while it was draining the hot water. But when the demand is high, it’ll take nearly as much time as it would when you first installed your water heater.

Recovery Time Factors

The amount of time it takes your water heater to recover is influenced by a number of things. Here are the most critical factors:

Fuel Source – As we covered, a gas water heater is capable of heating water faster than an electric heater. 

Tank Size – Smaller tanks simply heat faster since there’s less water to heat. However, you’ll be able to draw more hot water from a larger sized tank, and you many not even realize your water heater is recovering from your shower! Proper sizing is critical when it comes to water heaters – Getting the right sized tank for your household hot water needs ensures you’ll have hot water when you need it.

BTU Rating – The higher the BTU rating on a gas water heater, the faster the heater can heat water. This also translates into a shorter recovery time.

Heating Element Wattage – Electric water heater heating elements with a higher wattage will heat water faster.

First Hour Rating – When purchasing a hot water heater, consider the heater’s First Hour Rating. This important metric is often overlooked, but it’s an important measurement that determines the heater’s performance and recovery. The First Hour Rating assumes that the heater starts with a full tank of hot water, and then measures the number of gallons the heater can supply each hour.

Age – Water heaters don’t last forever, and the age and condition of your unit can definitely impact how fast it recovers. Over time both gas and electric water heaters become less efficient at heating water. Check out this article to learn the signs of when you should replace your water heater.