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How Long Does a 40 Gallon Water Heater Take to Heat Up?

Fact checked by Stephen Conklin

how long does a 40 gallon water heater take to heat up

A 40-gallon water heater is just right for a small household, but many often wonder, “How long does a 40 gallon water heater take to heat up?”

Gas water heaters require 30 to 40 minutes to heat water, while electric versions might need about 60 to 80 minutes. Unfortunately, these numbers only reflect the average. Your experience might yield a different result (it can be faster or slower to heat).

Please keep reading to learn what influences water heater heating times.

Table of Contents

40-gallon Water Heater Heating Times


You can expect a gas water heater to raise the water temperature to the desired level within 30 to 40 minutes. On the other hand, an electric 40-gallon water heater might take at least 60 minutes to heat the water.

So, why the discrepancy?

Typical Heating Times

Before we understand the variables influencing the average heat-up time, you might want to look at the following table.

Type of Water Heater Heating Time
Tankless (electric and gas) Instantaneous (0 minutes)
Gas storage tank-type 30 to 40 minutes
Electric storage tank-type 60 to 80 minutes

The table implies that gas-powered water heaters have a shorter heating speed than electric versions. Moreover, tankless systems heat the water instantaneously, eliminating the long wait.

The information underscores the influence of several factors on a water heater’s average heating time.

Factors Influencing Water Heater Heat-up Time

Maybe you’re wondering why hot water takes an hour to come back after shower while your mom on the other side of the country only requires 30 to 45 minutes. The discrepancy underscores the influence of the following factors.

1. British Thermal Unit (BTU) Rating

Gas-powered water heaters have a BTU rating, describing their water heating speed. A single BTU can raise water temperature (a pound) by one degree Fahrenheit.

Hence, a 10-BTU device will heat a pound of water to 10 degrees Fahrenheit.

  • But how do we convert gallons to pounds?

We must multiply our 40-gallon water heater by 8.34 to get its pound equivalent. And that’s 333.6 or 334 pounds (40 x 8.34 = 333.6).

Calculating water heater heating duration using the BTU method is quite technical. But we’ll try to simplify. We can divide 334 pounds of water by 40,000 BTUs/hour to get 0.00835.

Suppose we want to increase the water temperature by 60 degrees. In that case, we can multiply 0.00835 by 60 degrees to get 0.501 hours. Multiplying this by 60 minutes gives us 30.06 minutes.

  • What if you have a 100,000-BTU 40-gallon water heater?

Following our procedure, we would get a hot water heater time of 12 minutes.

So, what’s the rule?

The higher the 40-gallon water heater’s BTU rating, the quicker the water heating capability.

  • But how about electric water heaters with wattage ratings instead of BTUs?

A single watt equals 3.41 BTUs/hour. Hence, a 4,000-watt 40-gallon water heater is similar to a 13,640-BTU appliance (4,000 watts x 3.41 BTUs = 13,640 BTUs).

And that’s why gas water heaters are quicker to heat the water than electric versions.

2. Water Temperature Entering the Water Heater Tank


How much hotter you want the water dictates water heater heat-up times.

For example, suppose the cold water entering the water heater is 40 degrees Fahrenheit, and you want to shower at 130 degrees. In that case, the temperature rise will be 90 degrees.

Let’s try calculating using our formula. Multiplying 0.00835 by 90 degrees will give us 0.7515 hours or 45 minutes. That’s 15 minutes longer than the 60-degree temperature rise in our first example.

Hence, the lower the incoming water temperature, the longer it takes for the water heater to heat up.

3. Water Heater Age


Wear and tear can influence water heating times. Sediment buildup, failing heating elements, and corrosion can affect how well the water heater raises the water temperature.

Unfortunately, neglect (i.e., lack of maintenance) only worsens aging effects.

4. Water Heater Distance to Hot Water Fixtures


An often-overlooked factor, the distance between the water heater and its receiving fixtures can spell the difference between instant and delayed hot water.

For example, a shower 10 feet from the water heater will receive hot water faster than a hot water faucet 20 feet away. The hot water storage tank might fill up quickly, but the distance to the hot water fixture can influence the hot water delivery time.

Hence, even a tankless water heater might take some time to deliver hot water if the distance between it and the shower is substantial.

5. Pipe Diameter


Homeowners might want to pick the right size pipe diameter for their water heater to avoid water heating time issues after installation.

Many homeowners believe that large-diameter pipes are better in delivering more hot water to the shower. Unfortunately, they don’t realize that increasing the pipe diameter increases water volume.

The more water in the pipe, the longer the water heater takes to heat the water and build sufficient pressure to push the hot water through the plumbing network.

How to Speed Up the Heat-up Time


Water heater heating times are variable. They could be as fast as 12 minutes or as slow as 90 minutes, highlighting the factors influencing heating time. So, can you speed it up? If so, how? Please consider the following.

  • Insulate the pipes to reduce standby heat loss and allow the water heater to deliver hot water quickly to showerheads and other fixtures.
  • Hire a professional to install a recirculation pump. This system channels all unused hot water back to the water heater, keeping the water temperature more stable and allowing the appliance to deliver hot water faster when needed.


How long does a 40 gallon water heater take to heat up? Although the average for gas units is 30 to 40 minutes and 60 to 80 minutes for electric heaters, the water-heating speed would still depend on the appliance’s BTU or wattage rating.

Moreover, temperature rise, water heater age, pipe length, and pipe diameter can influence water heating speed.

Hence, we recommend planning your 40-gallon water heater installation to account for these factors. After all, you don’t want to wait “forever” to enjoy a warm, refreshing shower.

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