Advanced Tankless Sizing Calculator
Enter fixture loads, temperature rise, model capacity, efficiency, altitude, and safety margin. The calculator estimates practical heater count for planning.
Capacity Chart
Fixture Load Breakdown
| Fixture | Quantity | Flow rate GPM | Connected load GPM |
|---|---|---|---|
| Showers | 2 | 2.00 | 4.00 |
| Bathtubs | 1 | 4.00 | 4.00 |
| Bathroom sinks | 3 | 0.80 | 2.40 |
| Kitchen faucets | 1 | 1.50 | 1.50 |
| Dishwashers | 1 | 1.20 | 1.20 |
| Clothes washers | 1 | 2.00 | 2.00 |
| Custom fixtures | 0 | 1.00 | 0.00 |
| Total connected demand | 15.10 GPM | ||
Example Data Table
| Project type | Peak fixtures | Incoming water | Outlet water | Likely sizing note |
|---|---|---|---|---|
| Small home | 2 showers and 1 faucet | 55 °F | 120 °F | Often one high capacity unit |
| Large residence | 3 showers, laundry, kitchen | 45 °F | 120 °F | May need linked units |
| Light commercial space | Multiple sinks and wash equipment | 50 °F | 125 °F | Check code and fixture schedules |
Formula Used
Temperature rise: Rise = Outlet temperature - Incoming temperature
Connected demand: Total GPM = Σ Fixture quantity × Fixture flow rate
Design demand: Design GPM = Total GPM × Simultaneous factor × (1 + Safety margin)
Altitude factor: Altitude factor = 1 - Derate rate × Thousands of feet above 2,000 ft
Output capacity: Output BTU/h = Input BTU/h × Efficiency × Altitude factor
Energy based flow: Capacity GPM = Output BTU/h ÷ (500 × Temperature rise)
Usable single unit capacity: Usable GPM = Min(Model max GPM, Energy based GPM)
Recommended units: Units = Ceiling(Design GPM ÷ Usable single unit capacity)
How To Use This Calculator
- Enter each fixture quantity and expected hot water flow rate.
- Add local incoming water temperature and desired outlet temperature.
- Enter the model maximum flow rating from the selected heater data.
- Enter heater input, efficiency, altitude, and derate assumptions.
- Choose a simultaneous use factor based on building use.
- Add a safety margin for cold days and future fixture changes.
- Press calculate and review unit count, capacity gap, and gas load.
- Use the CSV or PDF button to save the result for project records.
Construction Sizing Guide
Tankless Sizing For Building Work
A tankless heater is sized by flow and temperature rise. Flow is the hot water rate needed at one time. Temperature rise is the difference between incoming water and selected outlet temperature. Both values change by climate, fixture type, and owner expectations. A small error can reduce comfort during peak use.
Why Simultaneous Demand Matters
Construction plans rarely use every fixture at once. Still, busy homes, small rentals, salons, and site facilities can have heavy overlap. This calculator lets you enter fixture counts, fixture flow, and a simultaneous use factor. That factor turns connected load into a practical design load. A safety margin then adds extra room for cold days and future fixtures.
How Capacity Is Checked
The tool compares design demand with heater capacity at the entered temperature rise. It also checks the heating energy available from the entered input rating, efficiency, and altitude derate. The lower value becomes the usable capacity. This is important because catalog maximum flow often assumes a small rise. Real winter water may require more energy per gallon.
Planning Gas And Installation Needs
The recommended unit count is based on usable capacity. The gas load estimate shows total input rating for those units. This is not a final gas pipe design. Pipe length, pressure, regulator capacity, fuel type, venting, combustion air, and local code still matter. Use the result as a planning guide before detailed mechanical design.
Good Field Practice
Confirm local groundwater temperature before ordering equipment. Check shower head flow rates and fixture schedules. Use conservative values for luxury bathrooms and high occupancy spaces. Review manufacturer tables for the exact model. Also allow service clearance, condensate handling, freeze protection, electrical supply, and drain routing. A well sized system should deliver steady hot water without overspending on unneeded capacity.
Using The Result
If the result shows a small shortage, raise the model input, reduce fixture flow, or increase the unit count. If the result shows a large surplus, compare installed cost with operating needs. Balanced sizing supports comfort, code review, and clear communication between owners, designers, contractors, installers, inspectors, suppliers, managers, estimators, buyers, and teams during early construction planning.
FAQs
1. What does this tankless sizing calculator estimate?
It estimates connected fixture demand, design flow, temperature rise, usable heater capacity, recommended unit count, and total gas input. It is intended for early planning, not final engineering approval.
2. Why does incoming water temperature matter?
Colder inlet water needs more heat to reach the outlet setpoint. A heater that works well in warm weather may deliver lower flow during winter if the temperature rise becomes large.
3. What is simultaneous use factor?
It is the estimated percentage of connected fixtures used at the same time. Lower values fit normal homes. Higher values fit busy homes, rentals, salons, locker rooms, and light commercial spaces.
4. Should I use the catalog maximum GPM?
Use it only with the temperature rise table for the exact model. Maximum GPM often assumes a low rise. This calculator also checks energy capacity against your entered rise.
5. Why is altitude included?
Some gas appliances lose available output at higher elevations. The derate field reduces usable heat capacity above the selected altitude threshold, helping avoid oversized performance expectations.
6. Does this design the gas pipe size?
No. It estimates total gas input only. Pipe sizing also needs fuel type, pressure, length, fittings, regulator capacity, appliance load, and local code requirements.
7. Can I use this for commercial buildings?
Yes, for early estimates. Commercial projects need detailed fixture schedules, code review, redundancy planning, maintenance access, vent design, combustion air checks, and professional review.
8. Why add a safety margin?
A safety margin helps cover colder water, fixture changes, owner expectations, pressure variation, and real field conditions. It reduces the chance of undersizing the hot water system.