Calculator Inputs
Example Data Table
| Scenario | Draws/Day | Wait (sec) | Flow (gpm) | Water Rate ($/1000 gal) | Pump Hours/Day | Net Annual Savings (USD) | Simple Payback (Years) |
|---|---|---|---|---|---|---|---|
| Small apartment, short runs | 18 | 15 | 1.2 | 5.00 | 1.0 | ~35 | Long |
| Family home, moderate waits | 35 | 25 | 1.5 | 6.50 | 3.0 | ~110 | 8–12 |
| Large home, long runs | 55 | 40 | 1.8 | 8.00 | 4.0 | ~240 | 4–7 |
Formula Used
How to Use This Calculator
- Enter equipment and installation costs, then add any rebates.
- Estimate daily hot-water draws, wait time, and flow during waiting.
- Set water and energy rates that match your utility bills.
- Choose pump runtime based on your control method (timer or on-demand).
- If you have a standby-loss estimate, enter extra loop loss per day.
- Click Calculate ROI to view results above the form, then export.
Why hot water recirculation affects operating costs
Homes with long pipe runs often waste 10–45 seconds per draw while waiting for hot water. At 1.5 gpm, a 25‑second wait discards about 0.63 gallons each use. When this happens 30 times daily, annual wasted water can exceed 6,800 gallons, creating avoidable water and sewer charges. Recirculation improves comfort, but savings depend on runtime controls and insulation quality. Short, scheduled circulation often captures most convenience while limiting standby heat loss, making ROI more predictable across seasons. Track both water and energy rates from recent bills.
Water savings potential from reduced draw-down
Annual water saved is calculated as (wait_seconds ÷ 60) × flow_gpm × draws_per_day × 365. Using 40 seconds, 1.8 gpm, and 55 draws/day produces roughly 24,090 gallons saved yearly. With a combined utility rate of $8 per 1,000 gallons, that is about $193 per year in water savings alone.
Energy impacts and heater efficiency considerations
The hot portion of wasted water also carries heating cost. Delivered heat is hot_gallons × 8.34 × ΔT (BTU). With inlet 60°F, setpoint 120°F, and 35% hot fraction, the avoided heating load can be meaningful. Efficiency matters: a 95% heater requires less fuel input than an 80% unit for the same delivered BTU.
Controls, pump runtime, and electricity trade-offs
Pump electricity is (watts × hours/day × 365) ÷ 1000 × rate. A 35 W pump running 3 hours/day uses about 38 kWh/year; at $0.18/kWh, that is roughly $6.80 annually. Timer or demand controls reduce runtime, while always‑on schedules can add loop heat loss, entered as kWh/day or therms/day.
Financial outputs: payback, NPV, and IRR
Net annual savings equals water savings plus heating savings minus pump cost, loop loss cost, and maintenance. Upfront capital is equipment plus installation minus rebates. Simple payback is capital ÷ net savings when net savings is positive. NPV discounts yearly savings over system life, and IRR estimates the effective annual return from the cash‑flow stream.
FAQs
1) What does “hot fraction” mean?
Hot fraction estimates how much of the waiting water is already heated from a prior draw. Higher values increase heating savings; lower values shift benefits toward water savings only.
2) How should I estimate wait time?
Time the seconds from turning on the furthest fixture to reaching your preferred temperature. Repeat a few times at different hours and use the average for a more stable estimate.
3) Does recirculation always reduce energy use?
Not always. Pump power and loop heat loss can offset heating savings, especially with long runtimes. Timers, demand controls, and pipe insulation generally improve the net outcome.
4) What discount rate should I use?
Many households use 4%–10% depending on risk tolerance and alternative investments. A higher discount rate lowers NPV and makes long paybacks look less attractive.
5) How do I model on‑demand versus timer control?
Adjust pump runtime hours/day. On‑demand systems often average well under 1 hour/day, while timers can be 1–4 hours/day. Keep other inputs constant to compare scenarios.
6) Should I include sewer charges in the water rate?
Yes if sewer fees scale with metered water use. Combine water and sewer into a single $/1000 gallons value for a more complete savings estimate.