What Size Tankless Water Heater Do I Need?
Choosing the right tankless water heater size is one of the most important steps in ensuring year-round comfort and energy efficiency in your home. If the system is too small, you’ll be left with cold showers when multiple taps are running. If it’s too large, you’ll overspend on equipment and energy. To make the best decision, you’ll need to consider your household size, hot water demand, flow rate, and temperature rise. Let’s break it down.
Understanding Tankless Water Heater Sizing
Unlike traditional water heaters that store hot water in a tank, tankless systems heat water on demand. This makes them more energy-efficient, but it also means they must be properly sized to keep up with your household’s peak hot water usage.
Two key factors determine the right size:
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Flow Rate (Gallons per Minute – GPM): This measures how much hot water your home uses at once. For example, a shower typically requires 2.0–2.5 GPM, while a dishwasher may use 1.5–2.0 GPM.
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Temperature Rise: This is the difference between your incoming water temperature and your desired hot water temperature. If your incoming water is 50°F and you want 120°F at the tap, you’ll need a unit that can handle a 70°F rise.
By calculating these numbers, you can match your hot water demand to the right tankless water heater size.
Step 1: Calculate Your Household’s Hot Water Demand
To estimate your home’s hot water demand, add up the flow rates of the appliances and fixtures you might use at the same time. Here’s a quick example:
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Shower: 2.5 GPM
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Kitchen sink: 1.5 GPM
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Dishwasher: 1.5 GPM
If all three are used simultaneously, your home requires 5.5 GPM of hot water.
Step 2: Consider Household Size
Your family size is a strong indicator of how much hot water you’ll need.
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Small households (1–2 people): Typically need 3–5 GPM, which supports one shower and a small appliance running at the same time.
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Medium households (3–4 people): Usually require 6–8 GPM, enough for two showers or a shower plus multiple appliances.
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Large households (5+ people): May need 8–12 GPM or more, especially if multiple bathrooms and high-demand appliances are in use simultaneously.
Step 3: Factor in Temperature Rise
The colder your incoming water, the harder your tankless unit must work. Homes in warmer climates need less heating power than those in colder regions.
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If your groundwater is 70°F, raising it to 120°F requires just a 50°F rise.
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In colder regions where groundwater may be 40°F, you’ll need an 80°F rise.
Manufacturers provide sizing charts showing maximum GPM at different temperature rises—always check these to ensure the unit you choose can keep up.
Examples for Different Home Sizes
Small Home (1–2 people, 1 bathroom)
A couple living in a one-bathroom home might need hot water for a shower (2.5 GPM) and a sink (1.5 GPM). Their total demand is around 4 GPM, so a compact tankless water heater rated at 4–6 GPM should be sufficient.
Medium Home (3–4 people, 2 bathrooms)
A family of four could easily run two showers at once, plus a kitchen sink. That’s about 6.5–7.5 GPM. In this case, a mid-range unit rated at 7–9 GPM would provide reliable comfort.
Large Home (5+ people, 3+ bathrooms)
For larger households, multiple showers, laundry, and dishwashing may run at the same time. Hot water demand could reach 10–12+ GPM. A high-capacity tankless water heater—or even two units installed in parallel—may be the best solution.
Benefits of Choosing the Correct Tankless Water Heater Size
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Consistent Comfort: No more running out of hot water mid-shower when appliances are in use.
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Energy Efficiency: A properly sized unit avoids overworking and prevents wasted energy.
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Cost Savings: Right-sizing ensures you don’t overspend on equipment that’s larger than necessary.
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Longer Lifespan: When your tankless water heater isn’t overworked, it lasts longer and requires fewer repairs.
Your Expert Team of Plumbers
Sizing a tankless water heater isn’t one-size-fits-all. By carefully calculating your hot water demand, factoring in household size, and accounting for temperature rise, you can choose a system that perfectly balances comfort and efficiency.
If you’re still unsure, it’s best to consult a licensed professional who can assess your home’s unique needs and recommend the ideal system. With the right tankless water heater size, you’ll enjoy endless hot water while saving on energy costs.
