The Drain Water Heat Recovery (DWHR) Calculator helps estimate the energy savings from using a heat recovery system in residential or commercial plumbing. This system captures heat from wastewater, such as from showers or dishwashing, and transfers it to incoming cold water. By preheating the water, energy consumption is reduced, leading to lower heating costs and increased energy efficiency.
Formula
Energy Saved (kWh/year) is calculated using the formula:
Energy Saved (kWh/year) = n × V × ρ × Cp × (Tdrain – Tcold) × ε × f × 365 / 3600000
Where: n = Number of people in household
V = Average shower water volume per person per day (liters)
ρ = Density of water (1 kg/liter)
Cp = Specific heat capacity of water (4.18 kJ/kg°C)
Tdrain = Temperature of drain water (°C)
Tcold = Temperature of cold water supply (°C)
ε = Efficiency of the heat recovery unit (typically 30-60%)
f = Fraction of hot water that passes through showers
365 = Converts to annual value
3600000 = Converts kJ to kWh
Pre-calculated Energy Savings Table
This table provides estimated energy savings for different household sizes and efficiency levels.
| Household Size | Efficiency (%) | Shower Volume (L/day) | Energy Saved (kWh/year) |
|---|---|---|---|
| 2 | 40 | 80 | 432 |
| 3 | 50 | 120 | 810 |
| 4 | 60 | 160 | 1152 |
| 5 | 50 | 200 | 1350 |
| 6 | 60 | 240 | 1728 |
This table offers a quick reference for users to estimate their energy savings without manual calculations.
Example Calculation
Let’s assume a household with:
- 4 people
- Average shower water usage of 40 liters per person per day
- Drain water temperature (Tdrain) = 35°C
- Cold water supply temperature (Tcold) = 10°C
- Heat recovery unit efficiency (ε) = 50%
- Fraction of hot water used in showers (f) = 0.9
Using the formula:
Energy Saved = 4 × 40 × 1 × 4.18 × (35 – 10) × 0.5 × 0.9 × 365 / 3600000
Energy Saved = 4 × 40 × 4.18 × 25 × 0.5 × 0.9 × 365 / 3600000
= 1152 kWh/year
This calculation shows that the household can save approximately 1152 kWh of energy annually by using a drain water heat recovery system.
Most Common FAQs
The energy savings depend on factors such as household size, shower water usage, and system efficiency. Typically, savings range from a few hundred to over a thousand kWh per year.
Yes, it works with electric, gas, and tankless water heaters by preheating the incoming cold water, reducing the energy needed for heating.
Yes, it can lower energy bills and improve water heating efficiency, making it a cost-effective investment over time.