Estimate shower drain heat recovery energy savings, a low-to-high bill savings range, and simple payback from your household inputs.
Advanced options
Heat assumptions
Efficiency comparison
Energy price and installed cost
Your estimate
Table of contents
How to use our Drain Water Heat Recovery Savings Calculator
- Enter the number of Showers using the recovery unit each day, Average shower time, and Showerhead flow. Count only showers whose drain water actually reaches the unit.
- Enter the Incoming cold-water temperature and choose the Water-heater energy source.
- Open Advanced options to check or edit Drain-water temperature, recovery efficiencies, Water-heater efficiency, Energy price, and Installed recovery-unit cost.
- Click Calculate. Compare Estimated annual utility-bill savings with the lower and higher comparison cases, then check that recovered heat and daily shower water look reasonable for your home.
Definitions
Incoming cold-water temperature: Temperature of water before it reaches the recovery unit and water heater.
Drain-water temperature: Temperature of warm shower water leaving through the drain.
Recovery efficiency: Percent of the available heat difference transferred from drain water to incoming cold water.
Water-heater efficiency: Percent of purchased fuel or electricity that becomes useful water-heating heat in this estimate.
Btu: British thermal unit, a unit of heat. One Btu raises one pound of water by about 1 degree F.
Therm: A natural-gas energy unit equal to 100,000 Btu.[1]
Simple payback: Installed recovery-unit cost divided by estimated yearly utility-bill savings.
Common mistakes and quick fixes
Mistake: Counting every shower in the home under Showers using the recovery unit each day when some drains bypass the unit.
Fix: Count only showers whose drain water passes through the recovery unit.
Mistake: Entering a showerhead rating as Showerhead flow when the actual flow is lower or higher.
Fix: Use the flow printed on the showerhead or measure gallons collected in one minute.
Mistake: Setting Drain-water temperature equal to or below Incoming cold-water temperature.
Fix: Enter measured or reasonable temperatures with drain water warmer than incoming cold water; otherwise the calculator cannot estimate heat recovery.
Mistake: Using an electricity price in Energy price after choosing natural gas or propane.
Fix: Enter $ per kWh for electricity, $ per therm for natural gas, or $ per gallon for propane.
Mistake: Treating Main recovery efficiency as a promised equipment result.
Fix: Use a cautious main value and enter lower and higher values in Low comparison efficiency and High comparison efficiency.
Mistake: Reading Estimated simple payback time as a complete investment analysis.
Fix: Use it only as installed cost divided by Estimated annual utility-bill savings; consider maintenance, incentives, financing, and future rates separately.
Limitations & Key Assumptions / Boundary Conditions
- The estimate covers shower drain water only. It does not include baths, sinks, laundry, dishwashers, or other hot-water uses.
- It uses the entered temperatures all year and multiplies daily use by 365. Real incoming water temperature and shower habits can change by season.
- Actual recovery depends on unit design, flow, plumbing layout, simultaneous cold-water demand, fouling, and whether drain water and incoming water flow at the same time.
- Water-heater efficiency, energy price, and installed cost are user-entered assumptions. Utility bills may include fixed charges, taxes, tiers, and rate changes that are not modeled.
- Simple payback excludes incentives, maintenance, repairs, financing, inflation, equipment life, and the time value of money.
- A zero recovery efficiency produces zero savings. If drain water is not warmer than incoming cold water, the calculation cannot produce a positive heat-recovery estimate.
Methodology
Calculation method
The calculator first finds the shower water that passes through the recovery unit each day. It then estimates heat transferred from warmer drain water to incoming cold water, converts that heat to purchased energy avoided, and multiplies by your energy price.
daily gallons = showers per day x average shower time x showerhead flow
annual recovered heat = daily gallons x 365 x 8.34 x (drain-water temperature - incoming cold-water temperature) x recovery efficiency
The 8.34 factor is pounds of water per gallon. Recovery efficiency is entered as a decimal in the calculation, so 50 percent becomes 0.50.
purchased energy avoided = annual recovered heat / water-heater efficiency / source heat content
Source heat content is 3,412 Btu per kWh for electricity, 100,000 Btu per therm for natural gas, and 91,500 Btu per gallon for propane. A therm equals 100,000 Btu.[1]
annual utility-bill savings = purchased energy avoided x energy price
simple payback time = installed recovery-unit cost / annual utility-bill savings
Efficiency comparison
The low and high savings cards repeat the same calculation using your Low comparison efficiency and High comparison efficiency. If the two values are entered in reverse order, the smaller dollar result is shown as the lower comparison and the larger dollar result as the higher comparison.
Worked example
With 2 showers per day, 8 minutes per shower, 2 gallons per minute, 55 F incoming water, 95 F drain water, and 50 percent recovery efficiency, daily shower water is 32 gallons. The model estimates 1,948,224 Btu of annual recovered heat. With a 90 percent efficient electric water heater and electricity at $0.16 per kWh, that is about 634.44 kWh avoided and $101.51 in yearly bill savings. A $1,000 installed cost gives a simple payback of about 9.85 years.