The right tankless water heater size depends on your peak hot water demand and required temperature rise. Add the flow rates of hot water fixtures you may use at the same time. Then subtract the incoming water temperature from your desired hot water temperature. Choose a tankless water heater that can handle both requirements.
Household size can provide a starting point, but it does not tell the whole story. A family that regularly runs two showers at once needs more hot water than a household that uses one fixture at a time.
Quick Takeaways
Tankless water heaters are primarily sized by flow rate and temperature rise.- Flow rate is measured in gallons per minute (GPM).
- Add the GPM of fixtures you expect to use at the same time.
- Colder incoming water requires a greater temperature rise.
- A unit’s maximum advertised GPM may not reflect its output at your required temperature rise.
- An undersized unit may struggle when several fixtures need hot water at once.
- A plumber can calculate your home’s demand and match it to the manufacturer’s specifications.
What Size Tankless Water Heater Do I Need?
Most homes need a tankless water heater capable of supplying several gallons of hot water per minute. The exact size, however, depends on how much hot water the home needs at one time.
Navien provides these general starting points based on simultaneous fixture use:
These numbers are useful for preliminary sizing, but GPM alone is not enough. A tankless water heater must heat incoming water to the desired temperature. It must also keep the required flow rate.
That means two homes needing 5 GPM may require different equipment if one home receives much colder incoming water.
How Do You Size a Tankless Water Heater?
Size a tankless water heater by calculating peak hot water demand in GPM and the required temperature rise. Then compare both numbers with the manufacturer’s performance data for the water heater.
The process can be broken into three steps.
Step 1: Calculate Your Peak Hot Water Demand
Peak hot water demand is the combined flow rate of the hot water fixtures you expect to use at the same time.
Flow rate is measured in gallons per minute (GPM). A home does not necessarily need enough capacity to operate every fixture at once. The goal is to account for a realistic period of heavy hot water use.
For example, imagine your household regularly uses:
- Shower: 2.0 GPM
- Second shower: 2.0 GPM
- Kitchen faucet: 1.5 GPM
If all three fixtures may run at the same time:
2.0 + 2.0 + 1.5 = 5.5 GPM
The tankless water heater would need to support about 5.5 GPM of peak demand under those conditions.
This is why household size alone can be misleading. Four people who rarely use hot water simultaneously may have a lower peak demand than two people who frequently run a shower, dishwasher, and faucet at the same time.
Step 2: Calculate the Required Temperature Rise
Temperature rise is the difference between the incoming cold water temperature and the desired hot water temperature.
The calculation is simple:
Desired hot water temperature − incoming water temperature = required temperature rise
For example:
120°F − 50°F = 70°F temperature rise
In that example, the tankless water heater must raise the incoming water temperature by 70°F.
Incoming water temperature varies by location and season. We recommend accounting for groundwater temperature during the colder part of the year when sizing a system.
Step 3: Match GPM With Temperature Rise
Choose a tankless water heater that can provide your required GPM at your required temperature rise. Both numbers need to be considered together.
This is an important distinction because a tankless water heater’s flow capacity changes with temperature rise. Heating very cold incoming water requires more energy than heating warmer incoming water.
Suppose your home needs:
- 5.5 GPM peak flow
- 70°F temperature rise
You would look for a model capable of producing at least 5.5 GPM at a 70°F temperature rise.
Do not rely only on the largest GPM number printed on a product page. Check the manufacturer’s performance or sizing information at the temperature rise your home actually requires. Navien and Stiebel Eltron both use flow rate and temperature rise as core sizing measurements.
How Much GPM Do You Need for a Tankless Water Heater?
Your required GPM depends on which hot water fixtures you expect to operate simultaneously. Each shower, faucet, dishwasher, and washing machine adds to peak demand when used at the same time.
Fixture flow rates vary by product. The actual flow rate listed for your fixtures provides a better calculation than relying on a generic estimate.
For example, if two showers each use 2 GPM, the showers create a combined demand of 4 GPM. Running another hot water fixture at the same time increases the required flow rate.
The key word is simultaneously. You do not need to add every fixture in the house unless you realistically expect all of them to require hot water at once.
Tankless Water Heater Sizing Example
A simple sizing example shows why GPM and temperature rise must be calculated together.
Imagine a household wants to operate two showers and a kitchen faucet at the same time:
Now assume the incoming water is 50°F and the desired hot water temperature is 120°F:
120°F − 50°F = 70°F
The household therefore needs a tankless water heater capable of providing approximately 5.5 GPM at a 70°F temperature rise.
The next step is to compare those requirements with manufacturer performance specifications. If a model cannot maintain 5.5 GPM at that temperature rise, it may be too small for the home’s expected peak demand.
What Size Tankless Water Heater Do I Need for My Family?
Family size can help estimate hot water demand, but simultaneous fixture use is more important. Two households with four people can require different tankless water heater sizes because their hot water habits are different.
A household where people shower at different times may have relatively modest peak demand. A household that routinely runs two showers while using the dishwasher may need considerably more capacity.
When estimating demand, consider:
- Number of bathrooms
- Number of showers that may run at once
- Kitchen hot water use
- Dishwasher use
- Washing machine use
- Typical morning and evening routines
- Future changes in household size or hot water demand
The goal is not to buy a water heater based on the number of people under the roof. The goal is to provide enough hot water during the home’s highest realistic period of simultaneous use.
Does Climate Affect Tankless Water Heater Size?
Yes. Climate can affect tankless water heater sizing because incoming water temperature affects the required temperature rise. Colder incoming water requires the water heater to do more work to reach the same outlet temperature.
For example:
- 60°F incoming water heated to 120°F requires a 60°F rise.
- 45°F incoming water heated to 120°F requires a 75°F rise.
A tankless water heater may produce a lower flow rate when it must create a larger temperature rise.
This matters in Everett and the Pacific Northwest. Sizing should account for colder incoming water. Do not rely on performance figures based on warmer conditions. Manufacturer sizing information should be checked against the home’s expected cold-water temperature.
Gas vs. Electric Tankless Water Heater Sizing
Gas and electric tankless water heaters both need enough capacity to meet peak flow and temperature-rise requirements. The way each system provides that heating capacity is different.
Gas tankless water heaters are commonly rated using BTUs in addition to flow performance. Electric tankless systems depend on available electrical capacity and may require substantial amperage for larger whole-home applications.
The home’s existing gas supply, electrical service, installation requirements, and hot water demand can all affect which type makes sense.
Regardless of fuel type, the basic question remains the same: Can the water heater deliver the required GPM at the required temperature rise?
Can a Tankless Water Heater Be Too Small?
Yes. An undersized tankless water heater may struggle to maintain the desired water temperature when too many fixtures run at once.
For example, a unit capable of supplying 4 GPM under your home’s conditions cannot reliably meet a 6 GPM demand. The system may reduce flow or fail to provide the expected hot water temperature during periods of heavy use.
Consistently pushing an undersized system can also place greater demand on the equipment. Exceeding a unit’s capacity can contribute to premature wear.
Correct sizing is one factor that can affect long-term performance. If you’re also comparing system lifespan, see how long tankless water heaters last and what affects their service life.
Can a Tankless Water Heater Be Too Big?
A tankless water heater can have more capacity than your home actually needs. Extra capacity is not necessarily harmful, but paying for significantly more output than the household will use may add unnecessary upfront cost.
Bigger is not automatically better.
The better approach is to calculate realistic peak demand, account for temperature rise, and choose equipment with enough capacity to handle those conditions.
Some larger homes or unusually high-demand properties may need more than one tankless water heater. Multiple units can be connected for applications where a single water heater cannot meet peak demand.
FAQs
Is a 7 GPM Tankless Water Heater Enough?
A 7 GPM tankless water heater may be enough for a home that requires no more than 7 GPM at its actual temperature rise. The 7 GPM rating alone does not determine whether the unit is large enough.
Check how much flow the specific model can produce at your required temperature rise. Colder incoming water can reduce the available flow rate.
What Size Tankless Water Heater Do I Need for Two Showers?
Add the flow rates of both showers and any other hot water fixtures likely to run at the same time. Two 2 GPM showers create a combined demand of 4 GPM before accounting for faucets, appliances, or other hot water use.
The water heater must then provide that flow at the home’s required temperature rise.
Can a Tankless Water Heater Run Two Showers at Once?
Yes, a properly sized tankless water heater can run two showers at once. The unit needs enough flow capacity to supply both showers while producing the required temperature rise.
If two showers each use 2 GPM, they require approximately 4 GPM together. Other simultaneous hot water use increases the total demand.
Should I Size a Tankless Water Heater Based on Bedrooms or Bathrooms?
Bathrooms and expected fixture use are more useful than bedroom count, but neither replaces a GPM calculation. The best sizing method calculates the flow of fixtures that may require hot water simultaneously and combines that number with the required temperature rise.
Does a Larger Tankless Water Heater Give You Hot Water Faster?
Not necessarily. A larger tankless water heater provides more heating capacity, but it does not automatically make hot water reach the faucet faster. The distance between the water heater and fixture, plumbing layout, flow rate, and whether the home has a recirculation system can affect how long you wait for hot water.
Get the Right Tankless Water Heater for Your Everett Home
Correct sizing helps a tankless water heater keep up with your household’s hot water demand without paying for capacity you do not need. Apollo Plumbing can calculate peak flow, temperature rise, and the installation requirements for your home.
Our tankless water heater services in Everett, WA include professional help with tankless water heater installation, replacement, and service throughout Everett and surrounding areas.
Apollo Plumbing is here when you need us. Plumbers. Ready Right Now.
