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How Long Does It Take to Fill a Hot Water Heater?

Fact checked by Stephen Conklin

how long does it take to fill a hot water heater

Nothing could be more annoying than enjoying a refreshingly warm shower only to be doused with cold water after a few minutes. It begs you to wonder, how long does it take to fill a hot water heater?

Unfortunately, several factors can influence how long for hot water to come back, producing highly varied answers. For example, a 40-gallon unit might need half an hour, while an 80-gallon version requires an extra ten minutes.

Keep reading to learn the juicy details.

Table of Contents

How Long to Fill a Water Heater


Determining how long for hot water heater to refill can be hit-or-miss if you cannot appreciate how these appliances fill back up with water.

1. How a water heater “refills”


Storage tank-type water heaters fill up with water almost instantaneously. When a hot water faucet, fixture, or appliance (i.e., tap, shower, or dishwasher) is open, hot water flows from the water heater. Meanwhile, cold water enters the system to refill hot water heater.

Unfortunately, mixing cold and hot liquid lowers the tank temperature. That is why the water is not as hot after a bath compared to the first time you hit the shower.

So, the next logical question is, how long does water heater take to fill and reach its ideal temperature? Homeowners would want to know how fast their respective water heaters can “return” or “recover” the hot water.

2. Recovery time


Filling the hot water tank for the first time differs from refilling the unit for continuous use. As explained, recovery time underscores a reduction in water temperature. However, the temperature would still be warmer than the cold water entering the tank.

For example, initial tank filling would mean putting 60-degree water into the system. On the other hand, recovering hot water implies heating the liquid (which is often over 60 degrees) to the desired temperature (i.e., 120 degrees).

Moreover, when you fill a 40 gallon water heater for the second time, you don’t fill all 40 gallons. Maybe you only need to refill about 70 percent or about 28 gallons. This figure may or may not equal the water heater’s draw efficiency, which is the maximum hot water amount you can use before the temperature stops being warm.

This scenario illustrates why initial tank filling takes longer than succeeding refills. There is more cold water to heat than a mixture of warm and cold water.

Of course, other factors can influence how quickly a hot water cylinder recovers.

  • Water heater type


An electric water heater is twice slower than a gas-powered unit. For example, suppose we have an 80-gallon water heater and the water temperature entering the system is 62 degrees.

In that case, a gas-powered water heater can recover hot water within 60 to 70 minutes. On the other hand, an electric version needs about two hours to heat the water to desirable levels.

Suppose you ran out of hot water how long to wait depends on your water heater type.

But why?

Gas water heaters feature burners with BTU ratings. Meanwhile, electric units have heating elements with wattage values. Comparing these two attributes reveals that one BTU equals 0.293W.

Suppose you have a 40-gallon gas water heater with 40,000 BTUs and an electric version with 4,500 watts. Converting BTUs to watts, and vice versa, reveals that the 40000-BTU unit is worth 11,723 watts. Meanwhile, the 4500-watt water heater is only 15,358 BTUs.

Hence, you might have identical 40-gallon heaters, but the electric version heats the water slower than its gas-fed counterpart.

  • Tank size (capacity)


Common sense dictates that larger-capacity appliances need more time to complete their tasks than smaller-capacity units. It’s the same story with water heaters. For example, a 30 gallon water heater will heat the water faster than a 50-gallon water heater (about 20 minutes faster).

The following table reflects the influence of water heater capacity or tank size in determining the time needed for hot water to return.

Water Heater Capacity (in Gallons) Number of Hot Water Users Average Recovery Time (in Minutes)
30 to 40 Up to two 20 to 30
50 3 to 4 50
60 5 80
70 to 80 More than six <120

Water heater type and capacity are not the only parameters influencing hot water recovery times. So, what other factors should we consider?

Factors Affecting Water Heater Recovery Time


How long you must wait for hot water to come back in the shower also depends on the water heater’s First Hour Rating (FHR), fuel type, and water temperature rise.

1. First hour rating


All storage tank-type water heaters have a sticker on the side like the one below. This “EnergyGuide” information label describes a water heater’s most valuable attributes, including its First Hour Rating (FHR).

You will notice an FHR value of 70 gallons in the sample image. This number describes the hot water volume the appliance can deliver within an hour starting at full tank.

So, how does FHR relate to recovery times? The higher the FHR value, the faster the water heater recovery time. For example, an 80-gallon FHR water heater should recover hot water faster than a 60-gallon FHR unit.

2. Fuel type


Although we already mentioned this in the preceding section under “water heater type, it doesn’t hurt to review what we know.

If you know a gas water heater’s recovery time, you can take the number and multiply it by two to get a clue about an electric version’s hot water recovery performance (i.e., 50 minutes for gas and 100 minutes for electric).

3. Temperature rise


We mentioned in “how water heaters fill” that the water temperature entering the appliance is lower than the desired hot water temperature. The difference between these two numbers (or water temperatures) is what we call the “temperature rise.”

As a rule, the greater the temperature rise, the longer the water heater recovery. For instance, suppose you want 130-degree water in an apartment shower, and the groundwater temperature is 60 degrees. The temperature rise is 70 degrees.

If the water inlet temperature is 80 degrees, the difference is only 50 degrees. Your water heater can recover 50 degrees faster than 70 degrees.

What Are the Common Problems That Slow Down the Recovery Time?


A hot water tank not filling or recovering as expected might indicate a problem caused by the following.

  • Sediment accumulation

Look at the image below and appreciate the various heating components. Imagine if limescale, heavy minerals, sediments, and particulates accumulate at the tank’s bottom and coat the different heating elements.

Unsurprisingly, the resulting thick crust can impair the appliance’s ability to recover hot water as fast as possible.

  • Incorrect water heater size

No hot water will reach the tap or the shower if the water heater is not accurately sized (i.e., it cannot accommodate your hot water needs).

For instance, you cannot expect a 30-gallon water heater to supply continuous hot water for a family of six. You need at least 80 gallons for such a household.

  • Problems with water-heating components

Although water heaters can last many years, heating components remain subject to wear and tear. Resistance coils and burners can fail over time, undermining the water heater’s hot water recovery performance.

  • Broken dip tube 

The dip tube conveys incoming cold water to the water heater tank. Hot water recovery times will be longer with a broken dip tube because cold water mixes with hot water at the break.


We understand why many newbie water heater owners ask the question, “How long does it take to fill a hot water heater?” Unfortunately, no definitive answer exists because everything depends on several factors.

However, several attributes can facilitate faster hot water recovery. They include a high First Hour Rating (FHR), a lower temperature difference (between water inlet temperature and preferred hot water temperature), and a gas-powered water heater.

Tank size also matters, with smaller units faster to recover hot water than high-capacity versions. However, you must be cautious in choosing the correct capacity for your household.

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