Express Sewer & Drain Blog

Tankless water heaters, which are becoming more and more popular every day, are a great way to save money on your energy bills. But, with so many different sizes and models available, it can be difficult to determine which one is right for you. 

If you need help figuring out what size tankless water heater you need for your home or business, keep reading to understand the benefits of using a tankless water heater and some of the factors you should consider when making your purchase.

How Tankless Water Heater Sizing Works

When selecting the ideal water heater size for your needs, it's crucial to consider two main factors: the maximum flow rate and the temperature rise.

The maximum flow rate, measured in gallons per minute (GPM), is determined by the simultaneous use of hot water fixtures and their individual flow rates. Additionally, the size of the system's water pipes, particularly the incoming service supply, can impact the overall flow volume.

Temperature rise is equally important and refers to the difference between the water heater's set temperature and the temperature of the incoming water supply. For instance, if you set your residential water heater to 120 degrees Fahrenheit and the incoming water is at 55 degrees, the temperature rise is 65 degrees (120-55=65). Residential settings typically range from 120 to 125 degrees, while commercial settings may vary from 140 to 180 degrees, depending on the specific requirements. It's important to calculate temperature rise using the coldest incoming water temperatures experienced during the year to ensure adequate heating.

To find the right water heater, you must align your required flow rate and temperature rise with the capabilities of tankless water heaters, which are rated by their gas energy input in British Thermal Units (BTU). A higher BTU input means a higher maximum flow rate that the unit can achieve. This is a key consideration when comparing models, such as those from Navien Tankless Water Heaters, ensuring you select a unit that meets your hot water demands efficiently.

What to Consider for Finding the Right Size Tankless Water Heater

The size of your tankless water heater will be determined by a number of factors, including the number of bathrooms in your home or business, the number of people who live there, and the climate you live in. You will also need to consider the peak hot water demand, which is the maximum amount of hot water you would need at any given time.

For example, if you have a large family and everyone is taking a shower at the same time, your peak hot water demand will be higher than if you only have one or two people in your household.

Number of Bathrooms

The number of bathrooms in your home is perhaps the most important factor to consider when determining what size tankless water heater you need. This is because each bathroom will have its own fixtures, such as a shower, sink, and toilet. If you have multiple bathrooms, you will need to add up the flow rate of all the fixtures to determine your peak hot water demand. For example, if you have two bathrooms, each with a shower and a sink, your peak hot water demand would be 4.5 gallons per minute (GPM).

Number of People

The number of people in your household will also play a role in determining the size of your tankless water heater. If you are looking into tankless heaters for your business, consider the number of people who work there or who use your facilities if you have public restrooms.

This is because the more people you have, the more likely it is that multiple fixtures will be in use at the same time. If you have a large family, you will need to consider the flow rate of all the fixtures in your home to determine your peak hot water demand.

Climate

The climate you live in can also impact the size of your tankless water heater. This is because hotter climates will require less hot water than cooler climates. If you live in a climate that is prone to freezing temperatures, you will need a larger tankless water heater to prevent your pipes from freezing.

Temperature Rise

The temperature rise is the difference between the incoming cold water temperature and the desired output water temperature. The higher the temperature rise is, the more hot water you will need, and the larger your tankless hot water heater will need to be. For example, if your incoming incoming water temperature is 70 degrees Fahrenheit and you need your hot water to be 120 degrees Fahrenheit, your temperature rise would be 50 degrees.

Flow Rate

The flow rate is the amount of water that flows through your fixtures per minute and how much water you'll need at any given time. The higher the flow rate, the more hot water you will need, and the larger your tankless water heater should be. For example, if you have a shower with a flow rate of 2.5 gallons per minute (GPM), your tankless water heater would need to be able to heat 4.5 gallons of water per minute to meet your peak hot water demand.

How to Calculate the Tankless Water Heater Sizing You Need for Your Home and Business

To calculate the size of the tankless water heater you need, start by multiplying the number of bathrooms in your home or business by 3. This will give you the gallons per minute (GPM) flow rate that you need. Next, multiply the GPM by 60 to get the maximum flow rate in an hour. Finally, divide that number by 1000 to get the maximum kilowatts (kW) required.

For example, if you have a three-bedroom home with three bathrooms, you would need a tankless water heater that can handle 9 GPM. That’s because 3 x 3 = 9. So, you would need a unit that has a maximum flow rate of at least 9 GPM.

Once you have the GPM, you can determine what size tankless water heater you need by checking the flow rate on the model’s specifications. Most models will list their maximum flow rate in GPM. For example, a Rinnai RL94iN tankless water heater has a maximum flow rate of 9.8 GPM.

Contact Express Sewer for Help With Water Heater Repair or Installation

Choosing the right size tankless water heater is a critical decision that can significantly impact your energy savings and ensure a constant supply of hot water. By considering factors such as the number of bathrooms, the number of people in your household, climate, and temperature rise, you can select a unit that meets your needs efficiently. 

Remember, a properly sized tankless water heater not only provides the convenience of endless hot water but also contributes to the overall energy efficiency of your home or business. If you're unsure about the best option for you, consulting with professionals like Express Sewer & Drain can help you make an informed decision and enjoy the benefits of a tankless water heating system.