Inputs
Example Data Table
| Scenario | Occupants | Delivery / Cold | Peak Showers | Peak Hour Volume | Suggested Tank | Peak Heater kW |
|---|---|---|---|---|---|---|
| Residential (Hybrid) | 6 | 45 / 15 | 3 × 8 min @ 9 L/min | ~290 L/hr | ~180 L @ 60°C | ~10.0 |
| Hotel (Per-capita) | 40 | 48 / 12 | — | ~1,120 L/hr | ~700 L @ 60°C | ~52.0 |
| Restaurant (Fixture) | 15 | 50 / 15 | — | ~500 L/hr | ~300 L @ 60°C | ~27.0 |
Formula Used
- Daily hot water (per-capita): Vday = N × v
- Fixture peak hour volume: Vpeak = (Vshowers + Vfaucets + Vother) × D
- Per-capita peak hour volume: Vpeak = Vday × f
- Energy (kWh/day): E = (m × Cp × ΔT) / 3600 / η
- Heater capacity for peak (kW): P = (Vpeak × Cp × ΔT) / 3600 / η
- Recovery (L/hr): R = (Pheater × 3600 × η) / (Cp × ΔT)
- Tank mixing conversion: Vdelivered ≈ Vtank × (Ttank − Tcold) / (Tdel − Tcold)
How to Use This Calculator
- Select your unit system and the preferred method.
- Choose a building type, then enter occupants and per-capita assumptions if needed.
- For fixture-driven projects, fill the peak hour shower, faucet, and other-use inputs.
- Set delivery, tank, and cold temperatures to define ΔT and mixing.
- Enter heater power, efficiency, and peak duration to size storage.
- Press Calculate. Download CSV/PDF for records and submittals.
Design Inputs That Drive Demand
Hot water demand starts with occupants, schedules, and end-use patterns. Per-capita demand works well for housing, hotels, and staff facilities. Fixture-based peak modeling suits kitchens, gyms, and mixed-use sites where simultaneous draws dominate. Select a building type preset, then refine per-capita and peak-hour fraction to match project realities.
Peak Hour Modeling for Construction Planning
The peak hour often controls heater and storage sizing. This calculator estimates peak-hour volume from shower events, faucet cycles, and other loads, then applies a diversity factor to represent non-simultaneous use. When uncertainty exists, use a conservative diversity value and add a safety factor to cover future scope changes, shift rotations, or temporary workforce spikes.
Energy and Heater Implications
Daily energy is based on mass, temperature rise, and efficiency. Raising delivery temperature increases energy and may require larger capacity. Efficiency captures conversion and distribution losses, so realistic values prevent undersized systems. The “heater capacity for peak” result indicates the instantaneous power needed if storage were not available, which is helpful when comparing equipment options.
Storage and Mixing Considerations
Storage tanks deliver more usable volume when set above delivery temperature. The mixing ratio converts delivered hot water at the outlet to the equivalent tank volume at the setpoint. Higher tank temperatures improve effective volume but increase scald risk, so confirm tempering valve strategy and compliance with local requirements. Use winter inlet water temperature for worst-case sizing.
Worked Example Snapshot
Example inputs (metric): occupants 6, delivery 45°C, cold 15°C, tank 60°C, showers 3 in peak hour, 8 minutes each at 9 L/min, faucet uses 24 at 20 seconds and 6 L/min, other load 60 L/h, diversity 0.80, efficiency 0.90, peak duration 60 minutes, safety 15%.
Typical outputs: peak hour about 290 L/h, suggested tank about 180 L at 60°C, and daily energy near 9–12 kWh/day depending on daily usage assumptions. Export results to support design notes, procurement, and coordination.
FAQs
1) Which method should I choose?
Use Hybrid for most projects. Choose Per-capita for occupant-driven buildings. Choose Fixture-based for kitchens, gyms, or process-heavy sites where peak simultaneous draws control sizing.
2) What does the diversity factor represent?
It reduces the sum of fixture volumes to reflect non-simultaneous use. Lower values are more conservative. Use higher values only when usage is clearly staggered by schedules or controls.
3) Why must tank temperature be higher than delivery temperature?
The tank is assumed to mix down to the delivery temperature. If tank temperature is not higher, mixing can’t increase delivered volume and the storage conversion becomes invalid.
4) How is the suggested tank volume calculated?
The calculator subtracts heater recovery during the peak duration from delivered demand, adds a safety factor, then converts delivered volume to tank volume using the temperature-based mixing ratio.
5) What cold water temperature should I use?
Use the design winter inlet temperature for sizing. If unavailable, use a conservative low value for your region, because colder inlet water increases required energy and capacity.
6) Can I use the results for final equipment selection?
Use results for preliminary sizing and comparisons. Confirm with manufacturer recovery charts, distribution losses, recirculation impacts, and applicable codes before final procurement and installation.
7) Why does higher delivery temperature increase system size?
Higher delivery temperature raises the temperature rise (ΔT). That increases energy per liter and reduces recovery volume per hour at the same heater power, often pushing storage or capacity upward.