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Tankless sizing is a math problem before it is a product choice. Count the fixtures likely to run together, measure or estimate their flow, determine the cold-water temperature and select equipment that can create the required rise at that total flow.
Bathroom count alone is not enough.
Step 1: establish the desired outlet temperature
Many residential systems are set around 120°F, but fixture mixing, codes, manufacturer requirements and household needs vary. Higher storage or delivery temperatures increase scald risk and require appropriate controls.
Step 2: use a realistic winter inlet temperature
Ground and municipal water temperatures change by location and season. A design based on warm summer inlet water will overstate winter flow. Measure or use reliable local design information.
Temperature rise equals desired outlet temperature minus entering-water temperature. If water enters at 50°F and the heater supplies 120°F, the required rise is 70°F.
Step 3: total simultaneous flow
List the uses that can overlap. Typical efficient showerheads may flow up to 2 gallons per minute under the WaterSense specification, but actual fixtures and pressure differ. A body-spray shower can use far more than one standard head.
Example demand might include:
- Two showers at 2.0 gpm each.
- A kitchen sink using 1.0 gpm of hot water.
- Total mixed-water flow of 5.0 gpm, adjusted for the hot/cold mix.
The tankless unit must meet the required hot-water portion at the design rise. Use the manufacturer's chart rather than a single headline number.
Custom showers can exceed one heater
Multiple body sprays, a rain head and hand shower may create a demand that one residential gas unit cannot serve at Central Ohio winter inlet temperatures. Options include reducing fixture flow, using staged tankless units, adding storage or choosing another water-heating design.
The supply piping and drain system also must support the fixture flow. A larger heater does not correct a restricted service line.
Gas, venting and electrical requirements
High-input tankless units can require more gas than the old storage heater. The entire gas piping system must be sized for connected demand, not just the final few feet. Meter capacity, regulator, venting, combustion air, condensate and electrical supply all belong in the proposal.
Electric tankless equipment may require very large electrical service capacity. Evaluate panels and service before assuming it is an easy substitute for an electric tank.
Frequently asked questions
Can I install the largest model and be safe?
Oversizing is not a substitute for checking minimum modulation, gas capacity, venting and flow. Select a model whose operating range fits both low and high demand.
Do low-flow fixtures help?
Yes, within the heater's minimum activation flow. Efficient fixtures reduce simultaneous demand, but extremely low hot flow may not trigger some units.
Do I need two tankless heaters?
Only when the required simultaneous capacity, redundancy or building layout justifies it. A load calculation should support the arrangement.
Design for the busiest ten minutes
West Jefferson Plumbing & Heating sizes tankless systems for how the household actually uses hot water. We serve Circleville and Central Ohio communities throughout Pickaway, Ross, Fairfield and Fayette Counties and coordinate plumbing, gas and electrical needs under one team.
Suggested internal links: Water Heater Installation; Plumbing Services; Article 30 in this package Authoritative sources: DOE tankless transition guide; DOE Midwest weatherization flow example; EPA WaterSense showerhead guidance