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How Long Should a 40 Gallon Water Heater Stay Hot?

Fact checked by Stephen Conklin

how long should a 40 gallon water heater stay hot

Many first-time water heater owners wonder how long should a 40 gallon water heater stay hot. The average is about 45 to 60 minutes. However, common sense and experience tell us that several factors can impact these figures.

So, keep reading in order to learn the nitty-gritty of how long you can enjoy hot water, how long the heater takes to raise the temperature to the ideal levels, and more.

Table of Contents

How Long Should a 40-gallon Water Heater Maintain Its Ideal Temperature?


A 40-gallon water heater can deliver about 45 to 60 minutes of continuous hot water use before running out.

It is worth mentioning that storage tank systems differ from tankless water heaters.

As you use hot water for a shower, the tank’s 40-gallon content dwindles down. Eventually, cold water enters through an inlet valve and tops up the water.

Unfortunately, mixing cold water with in-tank hot water lowers the water temperature. Hence, the water heater tries to “recover.”

This cycle provides a glimpse as to how long a 40-gallon water heater must stay hot.

Now, let us look into various factors impacting hot water’s longevity.

1. Water Heater Type


A 40 gallon natural gas hot water heater can heat the water faster than an electric version.

For example, an electric water heater might take 90 minutes to two hours to reach the ideal temperature. Meanwhile, deduct these numbers by about 40 to 50%, and you’ll get the time necessary for a gas-powered 40 gallon water heater to heat water.

However, we must also consider the gas water heater’s BTU rating and the electric water heater’s wattage. These factors can potentially impact how long a water heater should stay hot.

As a rule, a water heater with a higher wattage rating or BTU will reheat faster than those with lower wattage or BTUs.

A gallon of water is about 8.3 pounds. Hence, 40 gallons of water is 332 pounds.

British Thermal Units describe how much heat an appliance needs to produce to increase the temperature of 1 lb of water by 1 degree Fahrenheit.

Suppose you need to heat 70-degree water to 120 degrees Fahrenheit for a temperature rise of 50 degrees. Let us also assume your 40-gallon water heater produces 40,000 BTUs/hour.

Using an online calculator, this 40-gallon gas water heater will require about 26 minutes to reheat the water.

If we use the same example but replace it with a 40-gallon, 4,500-watt electric water heater, the heating time will be one hour, six minutes.

2. Hot Water Consumption

Water consumption rates can also impact how long water stay hot in water heater.

Suppose you are taking a hot shower, another family member is doing the dishes, and a third person is at the laundry. Three people are using hot water from the 40-gallon heater simultaneously.

We can expect hot water to run out quicker (shorter than 45 minutes) than if only one person uses hot water.

However, you can extend the hot water lifespan by mixing cold water with hot water at the faucet or showerhead (50-50).

3. Water Pressure/Flow Rate


Plumbers say water flow is directly proportional to water pressure. The higher the water pressure, the greater the water flow rate (in gallons per minute or GPM).

We know that a higher flow rate leads to higher hot water consumption. Hence, it is not unusual for households with high-flow, high-pressure water fixtures (i.e., faucets and showerheads) to consume and use hot water quicker than families with low-flow, low-pressure systems.

4. Water Heater Insulation

Insulating water heaters does more than save you money, increasing your energy savings by seven to 16 percent annually. It can also extend the time the water in the water heater stays hot.

High-quality insulating materials wrapped around water heaters without gaps can lower standby heat losses by as much as 45 percent.

Hence, a 40-gallon electric water heater stay hot longer if it’s covered rather than bare.

5. Uniform Energy Factor (UEF) Rating


To qualify for EnergyStar certification, gas-fired storage tank water heaters with a maximum capacity of 55 gallons should have a UEF rating of at least 81% or 0.81, effective April 18, 2023. The current minimum requirement is 64%.

In general, water heaters (whether gas or electric) often have a maximum UEF value of 93%. However, electric versions with integrated heat pump water heating technologies can have UEF of at least 330% or 3.3.

As a rule, a water heater with a higher UEF value will be more efficient in heating without wasting a lot of energy.

While insulation reduces standby heat losses, UEF describes a water heater’s efficient use of energy to heat the water (i.e., how much heat goes to the liquid and not lost in the environment).

6. Recovery Rate

A water heater’s recovery rate describes its ability to “heat” the water within an hour after emptying the storage tank.

Hence, a 40-gallon water heater should have a recovery rate of at least 40 gallons. Otherwise, you will end up with shorter shower times or hot water that does not stay hot long enough.

7. First-Hour Delivery Rate


Some homeowners confuse FHD with recovery rates because they both describe a water heater’s ability to produce hot water.

However, it is worth pointing out that the first-hour delivery rate refers to the amount of water that the appliance can produce/hour with water still present in the water tank.

Unsurprisingly, FHD values are higher than recovery rates. For example, the formula for FHD is “tank capacity x 70% + recovery rate.”

Suppose you have a 40-gallon water heater with a 40-gallon recovery rate. If we multiply 40 gallons by 70 percent, we will get 28 gallons. Adding this number to the recovery rate (40 gallons) produces 68 gallons per hour.



How long does a 40 gallon water heater last?

A 40-gallon water heater should have the same longevity (about eight to fifteen years) as a 30-gallon tank or a 50-gallon version, provided you observe proper care and maintenance.

Electric water heaters typically last longer (about 10 to 15 years) than gas-fired water heating systems (8 to 12 years).

Although water heater warranties can estimate a product’s longevity, homeowners should still care for their appliances.

The water heater might already require a replacement if it is no longer producing hot water or the water does not stay hot as long as before.

How long of a shower can you take with a 40-gallon hot water heater?

The average person spends about eight minutes showering, consuming about 20 gallons of hot water or 2.5 gallons per minute.

If you have a 40-gallon water heater, it is safe to assume you can shower for 16 minutes (40 gallons ÷ 2.5 GPM = 16 minutes). Suppose you have a 50 gallon tank, your continuous shower time can extend to 20 minutes.

However, lowering the showerhead’s flow rate to 1.5 GPM can extend shower time (26.67 minutes for a 40-gallon water heater and 33.33 minutes for a 50-gallon unit).

Why does your hot water run out so fast?

Sediment buildup in the hot water storage tank is the leading cause of hot water running out faster than usual. For example, you only have 30 gallons of hot water if the sediment occupies 10 gallons of the tank.

Hot water can also run out faster than usual because of a defective lower heating element due to sediment buildup, electric shorts, and a faulty thermostat. The heating element cannot warm the water without power.

A defective dip tube also prevents cold water from reaching the tank’s bottom. Instead, the cold water mixes with the already-heated water at the top.


Asking, “how long should a 40 gallon water heater stay hot?” requires understanding several factors.

Although the consensus is between 45 and 60 minutes, water heater type, water flow rate, hot water consumption, and the water heater’s technical ratings can affect hot water “longevity.”

Temperature rise and BTU/wattage ratings can also impact how fast your appliance heats the water and, by extension, keeps it hot enough for everyone in the family to enjoy.

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