A tankless water heater provides hot water on demand, but choosing the right size requires calculating your home’s peak flow rate and required temperature rise.
Key Takeaways
- Tankless water heaters heat the water as it flows through the unit, so there is no tank to run out, but if too many fixtures run at once, the water temperature or pressure can drop.
- To size a tankless unit correctly, add up the gallons per minute (GPM) of the fixtures you expect to run at the same time during your busiest hour.
- Subtract your incoming cold water temperature from your desired hot water temperature to find the required temperature rise, then choose a unit that can meet both your GPM and temperature rise at the same time.
A tankless water heater heats water only when you need it, instead of storing hot water in a large tank as a traditional unit does. Instead, cold water flows into the tankless unit, gets heated quickly, and then moves to your faucet or shower. Because there’s no storage tank, you won’t run out of hot water, but that doesn’t mean all tankless units will work for your home.
If you use more water than the tankless unit can process, you’ll notice the water temperature or flow will drop. To prevent this, it’s important to calculate what size tankless water heater you need so it can keep up with your home’s hot water demand.
How Do You Determine What Size Tankless Water Heater You Need?
To calculate what size tankless water heater to get, look at these five key factors.
1. Number of Fixtures Running at the Same Time
Start by thinking about your busiest hour of hot water use. Do you run the shower while someone is using the kitchen sink? Does the dishwasher run at the same time?
Tankless systems are sized based on simultaneous use, not daily use. If two showers and a sink may run at once, your system needs to handle all of them together.
Make a list of the fixtures you expect to use at the same time. That number plays a big role in determining how to size a tankless water heater.
2. Total Flow Rate Needed
Each fixture in your home has a flow rate, which is measured in gallons per minute (GPM). For example, a showerhead might use about 2.5 gallons per minute. A bathroom faucet may use around 0.75 gallons per minute.
If you don’t know the flow rate, you can measure it. Place a bucket under the showerhead and see how much water fills it in one minute.
To find your total flow rate, add up the GPM of all fixtures running at the same time. If you run a 2.5 GPM shower and a 0.75 GPM faucet together, you’ll need a unit that can handle at least 3.25 GPM. That total number tells you how much hot water your tankless heater must produce at once.
3. Required Temperature Rise
Next, you need to know how much the heater must warm the water, which is called “temperature rise.” To do so, check your incoming water temperature by running cold water from a faucet for a minute and placing a thermometer under the stream. In many areas, incoming water is between 40°F and 60°F, depending on climate and season.
Most homes want hot water set around 120°F. To find the temperature rise, subtract the incoming temperature from the desired temperature.
For example: 120°F (desired) – 50°F (incoming) = 70°F temperature rise
Keep in mind that the higher the temperature rise, the harder the unit has to work.
4. Tankless Heater Output (Gas vs. Electric)
Gas and electric tankless heaters don’t heat water at the same rate.
Gas models usually handle higher flow rates with larger temperature rises. Electric units often produce lower flow rates at the same temperature rise.
If your home needs a high GPM and a large temperature increase, a gas unit may be more suitable.
5. Compare Manufacturer’s Specs To Find a Tankless Water Heater
Now that you know your total flow rate (GPM) and your required temperature rise, you’re ready to compare models.
Tankless water heaters are rated by how many gallons per minute they can deliver at a specific temperature rise. When you look at product specs, you’ll usually see a chart that shows how much hot water the unit can produce at different temperature increases.
For example, if your home needs 4 gallons per minute at a 70°F temperature rise, the unit you choose must be able to meet or exceed both numbers at the same time.
If the heater can produce 4 GPM but only at a lower temperature rise, it may not keep up during colder months. Both numbers matter when sizing your tankless water heater.
All Aboard Can Help You Choose the Right Size Tankless Water Heater for Your Home
All Aboard Services helps homeowners in Northern Utah choose the right size tankless water heater based on their home’s exact needs, so you get steady hot water without performance issues.
With more than 45 years of experience, our licensed technicians install, repair, descale, and fine-tune water heaters for reliable, energy-efficient operation. We also account for Utah’s hard water conditions, helping protect your system from mineral buildup and extending its lifespan.
Are you ready to upgrade? Schedule your tankless water heater installation today and get a free quote with clear communication, same-day estimates, and workmanship backed by a one-year warranty.