Shower Drain Heat Recovery Payback Calculator
Understanding shower drain heat recovery (DWHR)
A shower drain heat recovery unit captures heat from warm wastewater flowing down the drain and transfers part of that heat to the incoming cold-water line. Preheating the incoming water reduces how much energy your water heater must add to reach your desired shower temperature. This calculator estimates the annual energy saved, annual cost saved, and the simple payback period based on your shower habits, local energy price, and the unit’s rated effectiveness.
What the calculator is doing
The calculation is built from a few physical relationships:
- Water used per shower depends on flow rate and shower duration.
- Heating energy depends on the temperature rise from incoming cold water to your target shower temperature.
- Recovered energy is a fraction of that heating requirement, based on the DWHR efficiency (effectiveness).
- Water heater efficiency adjusts savings to reflect how much input energy (electricity or fuel) your heater needs to deliver a given amount of heat to the water.
- Annual savings scale with showers per day and days per year.
Formulas (with units)
1) Shower water volume per shower
V = F × t
Where V is gallons per shower, F is flow in gallons/minute (gpm), and t is shower duration in minutes.
2) Temperature rise required (without recovery)
ΔT = Thot − Tcold
3) Heat delivered to the water (BTU per shower)
Using the common approximation that it takes about 8.34 BTU to raise 1 gallon of water by 1°F:
Q = V × ΔT × 8.34
4) Heat recovered (BTU per shower)
Qrec = Q × (η / 100)
5) Convert BTU to kWh
kWh = BTU ÷ 3412
6) Adjust for water heater efficiency
If your heater is ηh% efficient, the input energy avoided is higher than the heat delivered to the water:
kWhsaved = (Qrec ÷ 3412) ÷ (ηh/100)
7) Annual savings and payback
kWhsaved,annual = kWhsaved × showers/day × 365
$saved,annual = kWhsaved,annual × rate
Payback (years) = unit cost ÷ $saved,annual
MathML version
Here Esaved is the estimated input energy saved per shower (kWh). η is DWHR efficiency (%). ηh is water heater efficiency (%).
Interpreting your results
- Energy saved per shower / per year: How much heating energy your water heater does not need to supply because incoming water is preheated.
- Annual cost savings: Energy saved multiplied by your electricity rate (or an equivalent $/kWh rate).
- Simple payback: Unit cost divided by annual savings. This ignores financing costs, maintenance, and changes in energy prices.
If your payback is long, the biggest levers are typically: more showers per day, higher energy price, higher temperature rise (cold inlet water), and higher DWHR efficiency. If payback is already short, adding installation cost can still change the conclusion, so consider the assumptions below.
Worked example
Using the default inputs shown in the form:
- Duration: 10 min
- Flow: 2.0 gpm → V = 20 gallons
- Cold: 50°F, Hot: 105°F → ΔT = 55°F
- DWHR efficiency: 50%
- Water heater efficiency: 90%
- Electricity rate: $0.12/kWh
- Showers/day: 2
Heat delivered to water per shower:
Q = 20 × 55 × 8.34 ≈ 9,174 BTU
Recovered heat per shower:
Qrec ≈ 9,174 × 0.50 ≈ 4,587 BTU
Convert to kWh and adjust for heater efficiency:
kWhsaved ≈ (4,587/3412) / 0.90 ≈ 1.49 kWh per shower
Annual savings:
kWh/year ≈ 1.49 × 2 × 365 ≈ 1,086 kWh
$/year ≈ 1,086 × 0.12 ≈ $130
Simple payback (unit cost $600):
Payback ≈ 600 / 130 ≈ 4.6 years
Your result will differ based on inlet temperature, shower frequency, and local energy pricing.
Comparison: savings vs. showers per day (1–5)
The table below shows how annual savings and payback change when only showers per day changes (all other inputs stay the same). This helps you quickly sanity-check how sensitive payback is to household size and usage.
| Showers/day | Annual energy saved (kWh) | Annual cost saved ($) | Simple payback (years) |
|---|---|---|---|
| 1 | (computed) | (computed) | (computed) |
| 2 | (computed) | (computed) | (computed) |
| 3 | (computed) | (computed) | (computed) |
| 4 | (computed) | (computed) | (computed) |
| 5 | (computed) | (computed) | (computed) |
Assumptions & limitations
- Steady usage: Assumes the same number of showers per day all year (365 days).
- Inlet temperature is constant: In reality, incoming cold-water temperature changes seasonally and by region, which can materially change savings.
- Electricity-rate framing: The calculator uses $/kWh. For gas/propane, you can still use it by converting your fuel cost to an equivalent $/kWh of input energy, but results will be approximate.
- Heater efficiency is simplified: A single efficiency value may not capture standby losses, cycling losses, or real-world performance (especially for storage tanks).
- Installation costs not included: Payback here is based on unit cost only. Labor, plumbing changes, permits, and maintenance can lengthen payback.
- Device performance varies: Actual DWHR effectiveness depends on flow configuration (vertical vs. horizontal units), fouling/scale, and whether hot drain flow overlaps with incoming cold flow (showers typically overlap well).
- Shower-only model: Savings are modeled from shower water use; other hot-water uses (laundry, dishwashing, baths) are not included unless you adjust the inputs to represent them.
| Showers/Day | Annual savings ($) | Payback (years) |
|---|