How to size a tankless water heater for a four-person household
Gallons per minute, temperature rise, and what happens if you undersize the unit.
By Trey · · 4 min read
When you switch from a traditional tank water heater to a tankless model, the biggest mistake homeowners make is guessing at the size. You can't just buy what looks right or what the big-box store recommends. A tankless water heater needs to match your household's peak hot water demand, or you'll run out of hot water mid-shower. Getting the sizing right means understanding your flow rate and temperature rise, then matching it to the unit's capacity. In Magnolia, where we deal with well water and municipal supply alike, this calculation matters even more because your incoming water temperature changes with the season.
Flow Rate Is Your Starting Point
The first number you need is your household's simultaneous hot water demand. For a four-person home, start by listing every fixture that might run at the same time: the shower, the kitchen sink, the washing machine, maybe a bathroom sink. Each has a flow rate measured in gallons per minute (GPM). A standard showerhead uses 2.5 GPM. A kitchen faucet pulls about 1.5 to 2 GPM. Your washing machine draws around 2 GPM. Add them up based on what actually runs together in your house. Most four-person households hit 4 to 5 GPM when they're using hot water simultaneously, though some reach 6 GPM if someone showers while laundry runs and the kitchen sink is hot.
Temperature Rise Changes Everything
The second number is your temperature rise, measured in degrees Fahrenheit. This is the difference between your incoming water temperature and the temperature you want at the tap, usually 120 degrees. In Magnolia, your incoming water temperature varies. In winter, it might be 55 degrees, meaning you need a 65-degree rise. In summer, it could be 75 degrees, requiring only a 45-degree rise. Tankless units are rated at a standard 35-degree rise, but real-world performance depends on that incoming temperature. If you have well water, check your water temperature or ask your well contractor. Municipal water temperatures are fairly consistent, but they still shift seasonally.
The Math Works Against You in Winter
Here's where sizing gets real. A tankless unit rated for 5 GPM at a 35-degree rise will not deliver 5 GPM at a 65-degree rise. The unit's heating capacity is fixed. When you push a larger temperature rise through it, the flow rate drops. If you need 5 GPM of 120-degree water in winter with 55-degree incoming water, you're looking at a unit that handles roughly 7 to 8 GPM at standard conditions. That's a larger, more expensive unit than summer demand alone would suggest. Many homeowners in Texas undersize because they think about summer comfort and then get surprised when winter showers run cool. In Magnolia, where winters are mild but not warm, you still see this problem.
Whole House or Point-of-Use
A whole-house tankless unit handles all your hot water needs from one location, usually near your water heater. A point-of-use unit sits under a specific sink or near a shower and heats only that fixture's water. For a four-person household, a whole-house unit makes sense if you want hot water everywhere and you're replacing an old tank heater. The trade-off is higher upfront cost and the need to run gas and water lines if you're switching from electric. Point-of-use units cost less and install faster, but you're paying for multiple units if you want hot water in several places. If your current tank is in the garage and you're tired of waiting for hot water at the master bath, a point-of-use unit in that bathroom might solve your problem without a full replacement.
Get Your Unit Sized Right
Take your peak flow rate in GPM and your winter temperature rise. Call a local plumber and ask them to size the unit based on those numbers, not a guess. In Magnolia, we see a lot of undersized units that run fine in summer but struggle when it's cold. The spec sheet for any tankless unit shows its performance curve, which tells you the actual flow rate at different temperature rises. If you're buying online or at a big retailer, you won't get that kind of attention. A proper sizing also accounts for your incoming water pressure and whether you have hard water, which affects heat exchanger performance over time.
Bradbury Brothers Cooling, Plumbing & Electrical has sized and installed tankless systems for hundreds of homes in the Magnolia area. We know the local water conditions and can tell you exactly what size unit will keep your family in hot water year-round. Call us to discuss your household's needs and get a real recommendation based on your home's actual demand.