At Heritage Services, one of the most common questions we get from homeowners looking to upgrade their water heating system is: “What size tankless hot water heater do I need?” And honestly, it’s a smart question to ask.
In contrast to conventional water heaters, which keep hot water in a tank, tankless water heaters heat water as needed. That means no storage tank taking up space and no energy wasted keeping gallons of water warm all day long. But to make the switch successfully, you’ve got to choose a properly sized water heater. A unit that’s too small won’t keep up. A unit that’s too large might cost more than you need to spend.
Let’s walk through how to determine the correct size tankless water heater for your home, based on flow rate, temperature rise, and your household’s unique hot water needs. In this article, we’ll explain the key differences between tankless and traditional water heaters and explain how to choose the right size unit for your needs.
Tankless Water Heaters Work Differently
If you’re used to a standard water heater with a 40–50-gallon tank, the switch to a tankless unit is a bit of a change. There’s no hot water stored—just instant heat when and where you need it. Tankless units produce hot water on demand, ensuring you have hot water whenever you need it. They generate hot water as needed, without relying on a storage tank. The key to getting enough hot water at the right time is proper sizing.
Your tankless water heater size should match your peak demand. That means calculating how much hot water you typically use at once and how much temperature rise your system will need to heat incoming water. With the right size, your tankless water heater will continue to meet your household’s hot water needs without interruption. Keep in mind that hot water needs can vary depending on your household routines. You can simply follow a few steps to make the sizing process straightforward.
Know Your Flow Rate
First, you need to add up the flow rate of all the fixtures you might use at the same time. Flow rate is measured in gallons per minute (GPM). The total flow rate will vary depending on which fixtures are used simultaneously. Here are some average numbers to help you estimate:
- Shower: 2.0 – 2.5 GPM
- Bathroom sink: 0.5 – 1.0 GPM
- Kitchen faucet: 1.0 – 2.2 GPM
- Dishwasher: 1.5 – 2.0 GPM
- Washing machine: 2.0 – 2.5 GPM
Example: If you typically run two showers and a bathroom sink in the morning, your peak demand might be 5.5 to 6.0 GPM. The required flow rate and sizing will also depend on the specific application, such as whether the tankless water heater is serving the whole house or just a single bathroom.
This is where a properly sized unit really matters. A small unit might only generate 3.5 GPM, meaning you’ll be running out of hot water fast.
Figure Out Temperature Rise
Next, subtract the incoming water temperature from your desired output temperature. That difference is known as temperature rise. The range of incoming water temperatures can vary significantly throughout the year, especially between winter and summer.
In Oklahoma, the incoming water temperature during the coldest days can drop to about 40°F. Most people want their hot water to be around 105°F. So:
105°F – 40°F = 65°F temperature rise
For more accurate sizing, reference a guide or chart that lists the average winter water temperatures in your area.
If your incoming water is warmer in summer, the unit won’t have to work as hard. But your heater still has to be able to handle the maximum temperature rise needed year-round.
Match the GPM and Temperature Rise to the Right Unit
Every tankless unit has a performance chart or spec sheet that shows how many gallons per minute it can handle at a given temperature rise.
Note: These flow rates are estimates and may vary by manufacturer. Flow rates and temperature rise capabilities can also vary significantly between different models and brands.
Let’s say your household needs 6.5 GPM and has a 65°F temperature rise. You’d need a tankless water heater model that can deliver that GPM at that temperature difference. Some condensing models can meet that need easily. Non-condensing models may struggle at that capacity.
If you visit the manufacturer’s website, they often have sizing charts or tools to help. Or better yet—ask us. We’ve installed every kind of unit out there and know which ones perform reliably for Oklahoma homes.
A Few Other Sizing Notes
- For small homes or apartments, a lower GPM unit may work fine, especially if your peak demand is low.
- For larger families, higher GPM is almost always necessary.
- Tankless water heaters can be installed in parallel systems (multiple units) if one unit isn’t enough.
- A professional installation is highly recommended—not only for performance but to meet local building codes and safety standards.
Unlike storage water heaters, which are rated by their first hour rating (the amount of hot water they can supply in a single hour starting with a full tank), tankless water heaters provide a continuous supply of hot water as long as demand does not exceed their capacity. Tankless units are typically rated for DHW (domestic hot water) capacity based on their BTU/h input, which determines their flow rate and ability to meet your household’s hot water needs.
Consider the Type: Condensing vs Non-Condensing
Condensing tankless water heaters are more efficient and better at recovering heat. A condensing unit generates hot water more efficiently than a non-condensing unit. That can be a big help when you need hot water on colder mornings or for multiple showers in a row.
Non-condensing models are usually less expensive upfront, but may have lower efficiency and require additional venting. When comparing units, check whether the model you’re considering is condensing or not—it can make a big difference in both cost and performance.
Don’t Forget Installation Factors
Even with the right tankless water heater size, installation plays a big role in how well the system works. Some things we look at during installation:
- Gas lines: Does your existing line supply enough fuel?
- Electrical system: For electric models, can your panel handle the load?
- Ventilation: Especially important for non-condensing units
- Water quality: Hard water can impact performance—filters may be needed
- Access: Is there sufficient access to joists, an unfinished basement, or other structural areas needed for proper installation?
At Heritage Services, we handle all the above and make sure your system is set up for top-tier performance.
The Right Size Means Lower Costs Over Time
A properly sized unit won’t just keep your showers hot—it will run efficiently, cut down on waste, and lower your monthly energy bills. Oversized systems often cost more upfront and don’t always work better. Undersized systems, on the other hand, wear out quickly and leave you in cold water—literally.
A good installer can figure the right size, help determine necessary upgrades, and take care of the job from start to finish. (Yes, that’s us.) When purchasing a tankless water heater, relying on professional advice ensures you make an informed purchasing decision that fits your household’s needs.
When in Doubt, Let Us Help
Still asking yourself, what size tankless hot water heater do I need? Don’t stress over GPM charts and temperature rise tables—just give us a call.
At Heritage Services, we make it easy to choose the right system for your house. We’ll assess your fixtures, water usage, and layout to recommend the perfect unit. Whether you’re installing for the first time or replacing an older system, we’re here to help from the first service call to the final turn of the faucet.
For more detailed information about tankless water heater sizing, visit our dedicated page.
Call Heritage Services for a Professional Installation
We’ve spent over 50 years helping homeowners get the most from their water heaters, and tankless systems are one of our favorite upgrades. Why? They save space, offer great energy efficiency, and—when sized correctly—give your home hot water any time you need it.
Don’t guess your way into buying the wrong unit.Contact Heritage Services, and we’ll make sure your tankless water heater size is a perfect fit.
