# Can an Electric Tankless Water Heater Fill a Bathtub?

Many people wish to use an electric tankless water heater to fill a bathtub. However, they worry if an electric tankless water heater can cope with so much water. So, I did some calculations.

A 24 kW electric tankless water heater can fill a standard 40 gallons bathtub in about 7 to 15 minutes time depending on the cold water temperature. It is unlikely that you can use an electric tankless water heater to fill a large bathtub within an acceptable duration.

Using an electric tankless water heater to fill a standard bathtub is fine. But, if you have a large bathtub, you need to reconsider if an electric tankless water heater can do the job.

## How Long Does It Take for an Electric Tankless Water Heater to Fill a Bathtub?

Usually, we feel right if it takes around 10 minutes to fill up our bathtub. If the filling time is too long, first is we need to wait and second, the water becomes cold.

So, if an electric tankless water heater can fill a bathtub in about 10 minutes, it is good to go.

Now, the time required for an electric tankless water heater is depending on two things; a) water flow rate and b) cold water temperature.

Water flow rate is given by the water heater. The larger the water heater power, the greater the water flow rate. So, we need to keep in mind the water flow rate.

Next is the cold water temperature. Cold water temperature is the temperature of the water right before it enters the water heater. This temperature varies depending on where you live, the seasons and the weather.

However, the cold water temperature in the United States is averaged between 45°F and 75°F. So, if you don’t have a thermometer to measure, we can estimate based on that.

Now, an electric tankless water heater works by taking in the cold water and raising the temperature of the cold water by a certain degree. The larger the water heater’s power, the greater the degree to which it can raise the temperature of the water.

The target hot water temperature we want for a nice bath is around 105°F.

So, if the cold water temperature is low, more work has to be done and a higher power electric tankless water heater is needed.

However, the largest commonly available residential electric tankless water heater is 36 kW only. So, if your bathtub is too large, the filling time is going to be too long.

Below is the electric tankless water heater size needed to fill a 40 gallons bathtub in around 10 minutes time based on different cold water temperatures:

As you can see, if the cold water temperature in your house is high, you can use a smaller size water heater to fill a 40 gallons bathtub in about 10 minutes time.

Conversely, if your house has chilling cold water during the winter, you need to account for that and use a big power water heater in order to fill a 40 gallons bathtub in about 10 minutes time.

Of course, you can choose to upgrade the size of the electric tankless water heater if you want to shorten the filling time and vice versa.

Now, what if your bathtub holds more than 40 gallons of water? Let’s see.

Below is the selected electric tankless water heater size based on different bathtub sizes and the expected filling time:

Based on the above table, it is unlikely that an electric tankless water heater can fill a bathtub that holds more than 80 gallons of water within an acceptable duration.

If a 36 kW electric tankless water heater is used to fill a 100 gallons bathtub, it will take 14 minutes if the cold water temperature is 75°F. But, if the cold water temperature is 45°F, it’ll take 25 minutes.

Remember that our target hot water temperature is 105°F.

So, if you want to bathe in hotter water, it’ll take even longer because the water heater needs to slow down its flow rate in order to produce hotter water and vice versa.

Just to clarify, what I’ve just shown you is based on single-point usage. If you want to use an electric tankless water heater to fill a bathtub and shower concurrently, it’ll not be hot enough for you.

## Is an Electric Tankless Water Heater Good for a Bathtub?

So, you might ask if an electric tankless water heater is still good for a bathtub?

Actually, if you have a small bathtub like a 40 gallons or 60 gallons bathtub, I would recommend you use an electric tankless water heater as opposed to an electric storage water heater.

With a tankless water heater, you essentially have unlimited hot water. Whereas if you have a storage water heater, 60 gallons for example, you’ll use up nearly 70% of the stored hot water in a single bath.

Electric storage water heaters can’t provide you with instantaneous hot water like electric tankless water heaters do. They need time to replenish the hot water.

So, others will have to wait for a while before they can shower with enough hot water.

However, if you have a large bathtub like 100 gallons or more, I will suggest you use an electric storage water heater because it’ll take too long for a tankless one too long to fill up the water.

## Conclusion

An electric tankless water heater can be used to fill a bathtub if sized properly. However, an electric tankless water heater is probably not suitable for large bathtubs. Nonetheless, places with high cold water temperatures may find an electric tankless water heater good for even a large bathtub.

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Yu Chang Zhen

Yu worked as a professional in the HVAC industry for more than 7 years. He is an engineer who has a passion for blogging. Ever since he created this website, Yu has helped hundreds of homeowners and engineers on HVAC related matters.