How to Size a Tankless Water Heater
Tankless water heaters are sized by the hot-water flow you need at one time and how many degrees the unit must heat the incoming water. Use this guide to estimate both numbers, understand the tradeoffs, and compare models at the conditions your home will actually face.
- Add your simultaneous hot-water flow
- Calculate the required temperature rise
- Verify the model’s rated GPM at that rise
Find Your Tankless Size in 3 Easy Questions
No measurements are needed for this quick estimate. Choose what best describes your home and the result will update automatically.
Look for a unit rated for at least 2.5 GPM
This is a light whole-home demand for one shower at a time.
When comparing products, confirm this flow at a 70°F temperature rise.
Shop 5.0 GPM Tankless Water HeatersQuick-estimate assumptions: 2.5 GPM per shower, a 120°F heater setting, and typical fixture flow. This is a shopping starting point, not an installation specification. A qualified installer should confirm actual fixture flow, winter inlet temperature, utilities, and product performance.
The Two Numbers That Determine Tankless Size
Choose simultaneous uses
List the hot-water fixtures you realistically expect to run together during the busiest part of the day—not every fixture in the home.
Add the flow rates
Add those fixtures in gallons per minute (GPM). Use measured or manufacturer-listed flow rates whenever possible.
Calculate temperature rise
Subtract the coldest expected incoming-water temperature from the desired heater setpoint, then find a model that delivers your required GPM at that rise.
A unit’s advertised “maximum GPM” is usually measured at a relatively small temperature rise. As incoming water gets colder or the setpoint gets hotter, the same heater produces fewer gallons per minute. Always compare the manufacturer’s flow chart at your calculated rise.
Step 1: Estimate Your Peak Hot-Water Flow
Build a realistic peak-use scenario. For example, if two showers often run while the kitchen faucet is in use, add those three flows. Do not automatically add a dishwasher or clothes washer if it normally runs at another time.
| Fixture or appliance | Planning range | How to improve the estimate |
|---|---|---|
| Water-saving shower | 1.5–2.0 GPM | Read the showerhead label or measure at the tub spout/shower. |
| Standard shower | 2.0–2.5 GPM | Federal showerhead flow is generally limited to 2.5 GPM; some local standards are lower. |
| Bathroom faucet | 0.5–1.5 GPM | Check the aerator marking; only part of mixed faucet flow is hot water. |
| Kitchen faucet | 1.5–2.2 GPM | Check the aerator or product specification. |
| Dishwasher | About 1.0–2.0 GPM while filling | Use the appliance literature; demand is intermittent, not continuous. |
| Clothes washer | About 1.5–3.0 GPM while filling | Confirm the selected cycle and how much of the fill is hot water. |
| Large tub filler | 4.0 GPM or more | Measure the actual fill rate; high-flow and whirlpool tubs can dominate sizing. |
Step 2: Calculate the Required Temperature Rise
Use the coldest incoming-water temperature you expect during the year. Groundwater can be much colder than outdoor air, and it changes by region and season. Measuring cold tap water during winter gives a better estimate than relying only on a broad climate map.
| Incoming water | 120°F setpoint | Required rise | What it means |
|---|---|---|---|
| 35°F | 120°F | 85°F | Very cold inlet; available GPM will be substantially lower. |
| 45°F | 120°F | 75°F | Cold-climate winter condition. |
| 55°F | 120°F | 65°F | Common example used for model comparison. |
| 65°F | 120°F | 55°F | Warmer inlet permits more flow from the same unit. |
| 75°F | 120°F | 45°F | Warm-climate condition; advertised maximum GPM may be more attainable. |
Many homes use a 120°F setpoint, but follow the equipment instructions and local requirements. Higher settings increase scald risk and reduce the GPM a tankless heater can deliver at a given inlet temperature.
Step 3: Put the Two Numbers Together
Example peak-use scenario
- Primary shower: 2.0 GPM
- Second shower: 2.0 GPM
- Kitchen faucet: 1.5 GPM
Total simultaneous flow: 2.0 + 2.0 + 1.5 = 5.5 GPM
Winter inlet water: 50°F
Desired outlet: 120°F
Required performance
120°F − 50°F = 70°F rise
Compare models that can deliver at least 5.5 GPM at a 70°F rise—not simply models advertised as “up to 5.5 GPM.”
If no single unit meets that point, reduce simultaneous demand or consider a higher-output or multi-unit design.
For a gas-fired unit, a useful engineering check is BTU/h ≈ GPM × temperature rise × 500 before efficiency losses. The example represents about 5.5 × 70 × 500 = 192,500 BTU/h of heat transferred to the water. Product input ratings and delivered performance are not the same, so use the manufacturer’s published curve for the final decision.
Capacity Is Only Half of the Selection
Gas supply & venting
Whole-home gas units can have much higher input than a tank-style heater. An installer must verify fuel type, meter capacity, pipe size, pressure, vent category, combustion air, clearances, and termination location.
Electrical service
Whole-home electric tankless heaters may require multiple high-amperage, double-pole circuits. Confirm panel capacity, service size, voltage, conductor size, and breaker spaces before selecting a unit.
Water quality
Hard water and scale can reduce heat-transfer performance. Review the manufacturer’s water-quality limits, treatment guidance, isolation-valve requirements, and descaling schedule.
Altitude & cold weather
Altitude can reduce combustion capacity, and outdoor installations require the correct climate rating, freeze protection, power, and piping protection. Apply manufacturer derating instructions.
Minimum activation flow
A tankless unit needs a minimum water flow to activate and remain firing. Very small draws may not start some models, so check minimum flow and minimum firing rate.
Recirculation & distance
Tankless does not mean instant hot water at a distant fixture. Long pipe runs still cause a wait. Built-in or external recirculation can improve delivery but must be designed and controlled correctly.
Before You Choose a Model
| Verify | What to record | Why it matters |
|---|---|---|
| Peak simultaneous demand | _____ GPM | Sets the flow target. |
| Coldest inlet temperature | _____ °F | Determines worst-case heating load. |
| Desired outlet temperature | _____ °F | Completes the temperature-rise calculation. |
| Required temperature rise | _____ °F | The exact column or curve to check in product data. |
| Model output at that rise | _____ GPM | Must meet or exceed the flow target. |
| Available utility | Natural gas / propane / electric | Determines eligible models and infrastructure. |
| Installation conditions | Indoor/outdoor, altitude, venting, panel, water quality | Can change model choice, output, and installed cost. |
Tankless Water Heater Terms to Know
- GPM
- Gallons of water delivered per minute.
- Temperature rise
- Outlet temperature minus incoming-water temperature.
- BTU/h
- Heating input or output expressed in British thermal units per hour.
- UEF
- Uniform Energy Factor, a standardized water-heater efficiency metric.
- Condensing
- A high-efficiency gas design that captures additional heat and produces condensate.
- Activation flow
- The minimum water flow needed for the unit to begin heating.
- Turndown ratio
- The range between a gas unit’s maximum and minimum firing rates.
- Recirculation
- A system that moves hot water through the piping to reduce wait time at fixtures.
Need Help Sizing Your Tankless Water Heater?
Have your simultaneous fixture list, winter inlet-water temperature, fuel type, and installation location ready. An HVACDirect.com equipment specialist can help you compare models at the temperature rise your home requires.
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