Water Heaters

Choosing the Right Size Tankless Water Heater


Choosing the Right Size Tankless Water Heater

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Whether you’re building a new home, upgrading your existing water heater, or simply seeking ways to save on energy, considering a tankless water heater might be on your list. They have garnered attention for their energy efficiency, zero storage accorded by their compact design, and the prospect of unlimited hot water. However, as everything in science, the perfect functionality demands making an apt choice. One critical facet in choosing the right tankless water heater is understanding the size that aligns correctly with your unique hot water demands, commonly referred to as its capacity. So, what size tankless water heater do you need?

Understanding Tankless Water Heater Sizes

Tankless water heaters, as the name suggests, don’t store hot water. Instead, they heat the water as it flows through the unit, providing hot water on demand. Their size is determined by the quantity of hot water they can provide at a given time, rather than the capacity of a tank. Usually, this ‘size’ is measured in gallons per minute (GPM).

Variables To Consider When Determining Tankless Water Heater Size

  • Demand: Calculate the number and types of applications that require hot water simultaneously. Summing up their total GPM will reflect your demand.
  • Temperature rise: Identify the temperature increase needed from the incoming water temperature to your desired temperature. This varies upon location and weather.
  • Flow rate: Determine the flow rates of various appliances, adjusting to the maximum flow rate an appliance may need.

Calculation For Necessary Size

Now that you have identified your household’s demand, temperature rise, and flow rates, you can easily calculate the “size” or capacity of the tankless water heater suitable for your needs. Just multiply the desired flow rate by the required temperature rise.

Sizing For Various Types Of Homes

Small Apartments And Condos

Tankless water heaters that provide around 1.2 to 3.5 GPM should be sufficient for smaller homes with a maximum of two simultaneous demands. These sizes are ideal for running a couple of appliances at the same time.

Medium To Large Sized Homes

For larger homes, you might need a flow of 2.6 to 8 GPM. They can handle more simultaneous demands, such as running a dishwasher and two showers, or even two showers, a washing machine, and a faucet.

Very Large Homes

If your home is large and hot water demand is high, you may need a flow rate of 5 to 10 GPM. This kind of capacity can comfortably handle three showers, a laundry cycle, and a kitchen faucet concurrently.

Other Factors To Consider

Besides sizing, there are other factors to note when choosing a tankless water heater:

  • Fuel type: They can be gas, electricity, or propane-fueled.
  • Energy efficiency: Check for Energy Star rated models that provide increased energy savings.
  • Installation: Some may require upgrades to your home’s electrical system or gas pipeline.

Choosing the optimal size for a tankless water heater is pivotal to make the most of its benefits. Oversizing can lead to unwarranted energy consumption and cost inefficiencies, while undersizing can mean inadequate hot water supply, defeating the system's purpose. Plan your needs, ascertain your key parameters, and rely on professionals to guide your choice if needed.

Frequently Asked Questions About Choosing the Right Size of Tankless Water Heater

What do you mean by "size" when talking about tankless water heaters?

When we refer to the "size" of a tankless water heater, we're primarily talking about its heating capacity, which is commonly measured in British Thermal Units (BTUs) per hour and gallons per minute (GPM). This basically indicates the volume of water the unit can heat within a specific period of time. It doesn't refer to the physical dimensions of the unit.

What factors should I consider when choosing the size of a tankless water heater?

Several factors come into play for the size of the tankless water heater you need. These include the number of appliances and fixtures that use hot water in your home, the number of people in your household, and the average groundwater temperature in your area. All these will help you to determine how much hot water you need at any given time.

How does groundwater temperature play a role in choosing the size of a tankless water heater?

The groundwater temperature is significant because it influences the temperature rise your heater needs to achieve. For instance, if you’re living in colder areas where the groundwater temperature is low, you'll require a larger, more powerful heater to achieve the desired hot water temperature.

What is the importance of flow rate in determining the size of my tankless water heater?

The flow rate, typically measured in GPM, determines how much hot water the unit can produce and deliver at any time. You need to consider the number of devices you may need to run concurrently. For example, if you often have the shower, dish washer and washing machine running at the same time, you’ll need a heater with a high flow rate.

How can I calculate the necessary size of my tankless water heater?

To calculate the size of a tankless water heater you need, you can add up the flow rates of all the appliances you might use at once. Also consider the desired temperature rise by subtracting the average groundwater temperature from your preferred hot water temperature. Professional HVAC consultants or online calculators can help in making these calculations.

What happens if I choose a tankless water heater size that is too small for my household?

If you select a tankless water heater that's too small, it may not be able to produce enough hot water to meet your needs, particularly during peak usage times. This could mean cold showers or dishes that aren’t properly washed, which could be inconvenient for you and your family.

Are there any downsides to getting a tankless water heater that is too large?

A tankless water heater that is too large for your needs might initially seem like a good idea for ensuring plenty of hot water. However, it uses more energy than a smaller unit, making it less cost and energy efficient. It will also take up more space and may require a more extensive installation process.

I live alone. What size of tankless water heater do I need?

If you live alone, a smaller tankless water heater will probably suffice. Average single person households rarely use more than two or three hot water appliances at the same time, resulting in a lower required flow rate. However, if you plan on expanding your household in the future, consider a larger unit for preemptive convenience.

What if I want to add more hot water appliances in my house in the future?

If you're planning to expand the number of appliances or fixtures that will require hot water (like adding an extra bathroom), you should take this into consideration when sizing your tankless water heater. It often makes sense to invest in a larger unit that can accommodate these future needs.

Can I get professional assistance in sizing a tankless water heater for my needs?

Yes, you can. A professional plumber or a qualified HVAC consultant can help assess your household hot water needs accurately to determine the right size of tankless water heater for your home. They can also guide you through the selection process and clarify any of your questions or doubts.

Pros and Cons: Determining the Size of the Tankless Water Heater You Need

Pros of Determining the Appropriate Size of Tankless Water Heater

1. Energy Efficiency

Opting for the right size of a tankless water heater will significantly enhance energy efficiency. If the size of the tankless water heater matches your needs, you can prevent energy wastage, thus reducing energy bills.

2. Sustained Hot Water Supply

Choosing the proper tankless water heater size for your demand ensures that you have a constant supply of hot water, even during peak times. Thus, you can avoid the worry of running out of hot water during crucial moments.

3. Increased Durability

The proper size of a tankless water heater means that the unit is not overstressed. Consequently, the lifespan of your heater is increased, reducing your long-term replacement and service costs.

4. Streamlined Installation

Installing a correctly-sized tankless water heater is easier and more straightforward. This reduces the initial costs and the likelihood of future complications caused by a mismatch in the sizes.

  • Broader choice of installation locations: if the size is right, you'll have more options for where you can install it in your home.
  • Less intrusive: an appropriately sized unit will be less likely to disrupt the aesthetic of the house or to take up unnecessary space.

Cons of Determining the Correct Size of Tankless Water Heater

1. Initial Cost

Getting the right size of a tankless water heater for your demands might be more expensive initially. Larger units tend to cost more upfront, but it could be a long-term investment.

2. Complexity in Calculating the Required Size

Determining the size of the tankless water heater you need can be challenging. Factors such as the number of appliances, simultaneous use, and groundwater temperature come into play. Errors in these calculations may result in getting the wrong size.

3. Installation Difficulties for Larger Units

Larger tankless water heaters, although more efficient, may present installation difficulties. They usually require larger gas lines and adequate ventilation, which might necessitate significant adjustments to your home.

  • Space constraint: although tankless water heaters are generally compact, larger units might still require a sizable amount of space, which could be a challenge in smaller homes or apartments.
  • Increased utility requirements: larger units may require an upgrade to your home's electrical system or gas supply, leading to additional costs.

4. Difficulty in Predicting Future Needs

Accurately predicting future hot water requirements to determine the size of the tankless water heater you need can be challenging. Household demands may change over time. A unit that's correctly sized for you now may become insufficient in the future, or vice versa.

In conclusion, determining the size of tankless water heater you need has its pros and cons. Weighing these against your current and future needs can help you make an informed decision that will drive value in the long run.


To answer the question, "What size tankless water heater do I need?", you need to take into consideration your simultaneous usage and flow rate. It's essential to choose the correct size to ensure it meets your household's hot water demands. Remember, if the unit is too small, it may not provide sufficient hot water, while an overly large unit will result in unnecessary energy usage and higher bills. Selecting the right size is all about balance.

Understanding what size tankless water heater you need is essential for smooth operation. Do your due diligence by gauging your peak water demand and understanding the ground water temperature in your area. We hope our guide helped you realize how important it is to get this decision right. This decision will ensure you have hot water when you need it, without paying for more than you need.

All in all, choosing the right tankless water heater can be a bit complex. But once you've determined your household’s peak hour demand and factored in the temperature rise needed, you'll be able to identify the correct size unit for your needs. Don't forget that what size tankless water heater you need should align with both your budget and household usage to avoid unwanted costs or inconveniences.

About Perfect Air & Water Services

Welcome to Perfect Air & Water Services, your friends in Lakewood, NJ for all your home climate control needs. We've been providing our neighbors with top-notch heating, cooling, and water system services for years. We're not just technicians and engineers, we're family. Our team of experts are dedicated to ensuring your environment is just right, offering innovative solutions tailored to your specific needs. So, whether you need a routine service call or an emergency intervention, we're always here to make sure your home is the perfect place to be.