4 Steps To Determine The Right-Size Tankless Water Heater

Installing a tankless water heater in your home has many advantages over the traditional storage-tank system. Tankless systems provide hot water only when you need it, so there’s no standing loss of heat and no wasted energy costs associated with keeping it heated. In addition, they are more durable and will last longer than a standard water heater.

Choosing the right size is essential to get the most out of your tankless system; an undersized model won’t keep up with demand, while an oversized one will use unnecessary energy and cost more than necessary. Learning how to calculate the size of a tankless water heater that’s perfect for your home will ensure you get the most out of your new system without wasting money or time.

4 Steps to Determine the Right Size of a Tankless Water Heater

Step 1: Determine the Mounting Space 

Tankless water heaters come in various sizes. The amount of space you have in your home largely determines what size of the water heater should come in. Most innovative water heaters can provide heated water for your home at once. So, going for a small-sized heater can be ineffective.

The water heater size you choose should not be too small that it cannot provide water to all areas needed in the home.

Step 2: Measure Your Cold-Water Temperature 

You have to determine the amount of heat your water heater needs to produce the required heat in the water outlets. This is determined by the temperature of the water that flows into your home and the temperature of the water you need to flow out of the outlets.

Measure Your Cold-Water Temperature

For instance, if the water flowing into your home is 45°F, and the showerhead is supposed to produce water at 105°F, the needed temperature rise is 60°F.

Step 3: Determine Your Water Flow Rates 

Tankless water heaters are different from traditional tank heaters because they steam as they enter your house. Thus, the rate of water from your tap is determined by the flow rate your heater supplies. This is a crucial factor in knowing the size of the water heater for your home. This factor allows you to run multiple water outlets simultaneously without running slowly.

Water Usage and Flow Rates for Different Fixtures:

FixtureAverage Flow Rate (GPM)Average Temperature (°F)
Shower2.5 – 3.0104
Washing Machine2.0120
Kitchen Sink1.5110

Simultaneous Water Needs and Suggested Tankless Water Heater Size:

Simultaneous UsageTotal Required GPMSuggested Tankless Heater Size
2 Showers5.0Sized to meet or exceed 5.0 GPM
1 Shower + 1 Washing Machine4.5Sized to meet or exceed 4.5 GPM
1 Shower + 1 Kitchen Sink4.0Sized to meet or exceed 4.0 GPM
1 Shower + 1 Dishwasher4.0Sized to meet or exceed 4.0 GPM

Remember, these figures are rough estimates and should be adjusted based on your preferences and usage patterns. If you frequently have multiple fixtures running simultaneously, it’s a good idea to size the tankless water heater to meet your peak hot water demand comfortably.

Lastly, as noted, tankless water heaters are designed to heat drinking water, and the inlet water to the unit should not be pre-heated. This is important to ensure the proper functioning and efficiency of the heater.

Step 4: Consider the Ground Water Temperature

Consider the Ground Water Temperature

The groundwater temperature of different states differs from each other since some states are colder or warmer than others. Some states have a temperature of 75°, while others can be as low as 35°. The groundwater temperature helps to calculate how hard your heater needs to work to produce hot water.

However, if you do not know the ground temperature of your state, you can assume it is 40° and work with that.

Factors That Affect Tankless Water Tank Performance

Factors That Affect Tankless Water Tank Performance

Tankless water heaters have become popular over the years based on their functionality and benefits. These appliances are portable and are installed in the home or the office.

They provide instant hot water as they enter the home and can expand to meet the hot water requirements of your house. However, despite all these benefits, some factors affect the performance of tankless water heaters.

Mineral Deposits

The minerals found in tap water can get stuck in the water heater. Minerals like magnesium and calcium in hard water can affect the performance of the tankless water heater. This problem is solved by cleaning the heaters frequently.

If the mineral deposits are left too long in the heater, their capacity to heat water will decrease. Most modern heaters come with a digital meter that works as an alarm to give you a signal when the mineral deposits are much in the heater.


If the tankless water heater’s heat gets corrupted, it can affect the quality performance of the heater. The metal parts of the heater get corroded when condensation drops on them, which can cause the heater to stop functioning. This problem is challenging to manage and will need the services of a professional to fix it.


The modern tankless water heater is more complicated than the traditional one. It helps to save energy costs, but the installation and maintenance are quite expensive. It requires professionals to install and maintain it on time. So, we can conclude that modern tankless water heaters are costlier in the long run. 

Improper Venting

Tankless water heaters can stop functioning if the ventilation system is poorly installed. The heater needs gas or electricity to supply hot water, which is impossible without proper ventilation. If the water heater gets too hot, it is essential to restore it immediately. Ensure to get professionals to work on the ventilation system so you get a proper installation.


The water heater’s sensor helps ensure that water is provided at the same temperature as required. If the sensor fails, it affects the water heater’s performance in providing hot water. You must have experienced professional checks on the sensor every few months.

Heating Overload

If the water heater produces more hot water than the capacity it is supposed to, it can affect the performance and stop working. This can happen when you want your heater to supply hot water at different places simultaneously. The system can break down if the tasks are too much for their ability. Ensure that your water heaters can perform the tasks you’re giving them. You can install more than one heater in your home to reduce the load on one.

Flame Failure

This can occur due to low gas pressure or electrical issues. The system warns you when this happens and tells you that the flame can no longer generate heat. This issue requires the service of a professional. Trying to solve it yourself might result in more problems for the heater.

Frequently Asked Questions

1. Why is My Tankless Water Heater Not Working?

If you have an electric water heater, ensure the breaker box is in perfect condition. Your heater will not work with a burnt fuse in the outlet. If it is a gas water heater, be sure the gas supply is on.

2. Can You Run a Tankless Water Heater on Propane?

Yes, there are water heaters that run on propane. They have almost the same features as natural gas water heaters. The propane heaters do not suffer heat loss and heat water only when needed.

3. Why is My Tankless Water Heater Not Getting Hot?

This is usually caused by a blocked vent or plugged heat exchanger. The ventilation system might be blocked due to too much heat and can not provide the necessary air for the heater. The heater won’t get hot with this. It requires that you call an experienced professional to look into the problem.

4. Can a Homeowner Install a Tankless Water Heater?

Water heaters come in different types and have different installation processes. It is not advisable to install your water heater if you have no experience with it. An experienced professional is the best person to install your water heater so you don’t have issues afterward.

5. What is the Downside of a Tankless Water Heater?

The commonest disadvantage of a tankless water heater is the cost. Getting a size that effectively produces hot water in the home costs a lot.


A tankless water heater is an appliance that makes getting hot water comfortable in the home. The water gets heated at the point of entry into the house by the tankless water heater. The water heaters come in different sizes and for various purposes. You must know how to size a tankless water heater and the right size for your needs. You won’t face any trouble now that you have read my article.


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