What Is Drain and Wastewater Heat Recovery?
Water is used extensively in daily life—whether in homes, businesses, or industrial settings—for activities such as showering, washing dishes, and manufacturing processes. A significant portion of this water is discharged into our sewage system as wastewater, taking valuable heat energy with it. Instead of letting this thermal energy go to waste, drain and wastewater can be harnessed and repurposed to improve overall energy efficiency as part of a waste-to-energy solution, utilizing heat recovery. This recovered energy can serve as a stable thermal source or sink to enable low-carbon heating, cooling, and hot water production.
The process of water heat recovery can be divided into two main types: drain water heat recovery (DWHR) and wastewater heat recovery (WWHR).
- DWHR captures heat from greywater—wastewater from showers, sinks, and dishwashers. Because greywater does not contain significant organic waste or solids, it is relatively straightforward to process. DWHR commonly features individual fixture connections where heat exchangers can be connected to individual fixtures, such as showers, sinks, and dishwashers. This configuration allows for targeted heat recovery from specific sources.
- WWHR captures heat from both greywater and blackwater, the latter containing organic waste from toilets. Given the added complexity of blackwater, WWHR systems require more advanced technologies to extract and reuse thermal energy effectively. WWHR systems commonly utilize centralized heat exchangers installed in the main sewage line, capturing heat from all wastewater sources in the building. This setup is ideal for large buildings, district sites, or complexes with multiple wastewater outlets. This recovered waste heat can be used for preheating domestic hot water, space heating, or even industrial applications.
Implementing DWHR and WWHR systems enhances energy efficiency by tapping into the consistent thermal energy in wastewater, boosting system efficiency and supporting low-emission heating, cooling, and hot water generation. As industries and municipalities focus on optimizing energy use and cutting operational costs, these technologies are gaining traction as practical solutions for improving building performance and long-term resource management.
Key Components
DWHR and WWHR systems both rely on heat exchangers to capture thermal energy from wastewater. In DWHR systems, a copper coil wrapped around a drainpipe transfers heat from warm wastewater to incoming cold water. WWHR systems use more complex heat exchanger designs to achieve similar heat transfer, depending on the specific application, such as incorporating water-to-water heat pumps.
Both systems employ a secondary fluid, such as water or a specialized heat transfer fluid, to absorb the recovered thermal energy. This heated fluid is then distributed to water heaters, boilers, or other equipment to support space heating, cooling, or domestic hot water needs. Some systems also include a storage tank to store the recovered waste heat for later use, ensuring efficient energy utilization even when there is no simultaneous flow of incoming cold water and warm wastewater.
Piping and valves are also essential components in both DWHR and WWHR systems, facilitating the proper flow and control of fluids within the system. By incorporating these components, both types of systems can significantly improve energy efficiency and reduce utility costs, making them valuable additions to commercial, residential, public, agricultural, and industrial settings.
When to Consider This Measure
Buildings with consistently high hot water demand, such as hotels, hospitals, multifamily complexes, fitness centers, and food processing facilities, are ideal candidates for this measure. Since these facilities use significant amounts of hot water, they offer greater opportunities for heat recovery, reducing energy consumption and lowering operational costs.
While cost is another important consideration, including equipment, installation, energy use, and maintenance, the potential energy savings from recovering and reusing hot water can offset upfront expenses over time. In many cases, the high volume of recoverable heat in these settings makes the investment worthwhile. An energy model can be a useful tool in assessing potential payback periods and overall savings from implementing energy-efficient heat recovery technologies.
Another key consideration is the available space for system installation. Due to space constraints and integration challenges, implementing DWHR or WWHR is generally more feasible and less disruptive in new construction projects compared to retrofitting existing buildings.
Sizing Considerations
- Flow Rate Analysis: Determine the average and peak wastewater flow rates to select a heat exchanger that can handle the expected volume.
- Temperature Difference Analysis: Assess the temperature gap between incoming cold water and outgoing wastewater to calculate potential heat recovery.
- Pressure Loss: Ensure municipal supply water can pass through the heat exchanger without excessive pressure drop or velocity issues.
- Drain Size: Comply with plumbing codes, which typically prohibit reducing drain sizes; the heat exchanger drain line must match the required waste pipe diameter.
- Domestic Water Heating Load Calculations: Factor in the total domestic hot water demand, including usage from showers, sinks, dishwashers, and laundry machines.
- System Efficiency: Evaluate the efficiency of the heat exchanger, as higher efficiency units recover more heat and influence sizing requirements.
- Space Constraints: Consider the physical space available for installation, including the size and layout of the heat exchanger, storage tanks, and associated piping.
- Economic Analysis: Weigh installation and maintenance costs against projected energy savings to determine financial viability.
Installation Configurations
Drain Water Heat Recovery
The figure above illustrates three types of installation configurations for DWHR systems. The left configuration depicts an equal flow setup, where pre-heated water is directed to both the water heater and the fixture. Equal flow configurations are most feasible for single family homes or buildings where the water heater is relatively close to the hot water fixtures.
The middle configuration shows an unequal flow design to the water heater, where pre-heated water is routed directly to the water heater. Unequal flow configurations direct to the water heater in drain water heat recovery systems are ideal for appliances like dishwashers and washing machines, where simultaneous hot wastewater and cold-water flow is inconsistent.
Lastly, the configuration on the right represents an unequal flow design to the fixture, where pre-heated water is supplied only to the plumbing fixture. Unequal flow designs to the fixture are most common in multifamily buildings where the central water heater may be located remotely.
Wastewater Heat Recovery
A common installation configuration for WWHR systems involves integrating a water-to-water heat pump into a building’s plumbing and wastewater systems. In the example configurations above, raw wastewater is collected from a community of buildings, residences, or district sites and is used to operate a water-source heat pump. The heat pump extracts or rejects thermal energy from wastewater to generate clean, low-carbon heating, cooling, and hot water production. These systems are often classified into direct or indirect wastewater heat recovery systems. In both types, designers install additional heat exchangers to further decouple the sewage from the heat pump unit as a safety precaution to mitigate the risk of cross contamination.
The water-to-water heat pump plays a central role in this configuration. The heat pump transfers thermal energy from the wastewater loop to heat a separate water loop connected to the building’s domestic hot water (DHW) system. This process is similar to other heat pump systems but uses a closed loop system. In this setup, the refrigerant absorbs heat from the wastewater and transfers it back to the building’s DHW loop. The heat pump is typically connected to a buffer tank or storage tank that holds the heated water at the desired temperature for use in sinks, showers, and other fixtures within the building.
To maximize efficiency, the system often includes controls and sensors. These tools are used to monitor wastewater temperature, heat pump output, and storage tank levels, ensuring optimal performance of the heat recovery process.
Additionally, advanced systems may use a modulating heat pump that adjusts its capacity based on demand. The installation also typically includes safety measures such as filters to prevent clogging and maintenance valves for routine checks and service. In some cases, recovered heat can also assist with space heating, increasing the system’s versatility and overall energy efficiency.
For the most accurate setup tailored to your specific product, consult the manufacturer’s documentation provided with your equipment. It’s also recommended to work with a qualified professional to determine the optimal configuration for your equipment and project requirements.
Pairing Considerations
The recovered heat from wastewater can do more than just preheat domestic cold water, it can also support building HVAC systems, improving overall energy efficiency. Pairing DWHR or WWHR with complementary technologies further enhances energy savings. When combined with solar water heaters, these systems maximize efficiency by utilizing renewable energy for water heating while simultaneously reclaiming waste heat. All water heating systems benefit from preheated supply water. Tankless systems stand to benefit the most because, by heating water on demand without storage, the reduced temperature lift allows them to reach setpoint more quickly and with less energy input. Therefore, improving performance and potentially extending equipment lifespan. Drain or wastewater heat recovery can help maintain performance and efficiency in these systems during peak use.
WWHR systems can be integrated with heat pumps by using the relatively stable and warm temperature of wastewater as a renewable heat source or sink. A heat exchanger captures thermal energy from wastewater and transfers it to a clean water loop, which then feeds a water-source heat pump. This setup allows the heat pump to efficiently provide space heating, cooling, or domestic hot water, even in colder climates where air-source systems may be less effective. Integration can occur at the building or district scale. This integration offers benefits such as improved energy efficiency, reduced carbon emissions, and support for building electrification and decarbonization goals.
DWHR or WWHR can also be paired with low-flow fixtures. This pairing offers significant benefits including improved energy efficiency, reduced water consumption, and lower utility bills. While low-flow fixtures minimize water use, the heat recovery system captures heat from wastewater to preheat incoming cold water, therefore reducing demand on the water heater. This combination enhances comfort by maintaining consistent water temperatures, reduces strain on plumbing systems. Additionally, it supports sustainability by conserving both water and energy. Over time, these systems can deliver meaningful cost savings and help achieve water and energy sustainability goals.
Heat Exchanger Effectiveness Rating
The effectiveness of a heat exchanger is a key metric in determining its performance. The measurement compares actual heat transfer to the maximum possible heat transfer between fluids. A higher effectiveness rating indicates a more efficient heat exchanger, leading to greater energy savings and improved system performance. Factors influencing effectiveness include the heat exchanger’s design, flow rate, and temperature differential between incoming and outgoing fluids.
What Are the Benefits?
- By recovering heat energy from wastewater, less energy is required to heat incoming water, reducing overall energy consumption.
- WWHR systems can be paired with HVAC systems by using wastewater as a stable thermal source or sink. This improves efficiency and enabling low-carbon heating, cooling, and hot water production.
- By lowering energy consumption, these systems contribute to reduced greenhouse gas emissions and overall environmental impact.
- Early coordination of DWHR or WWHR systems in new construction enables optimal design integration, reduces costs, and maximizes energy performance. This leads to improved cost-effectiveness over time. By capturing and reusing waste heat, these systems enhance a building’s hot water availability.
- By reducing the workload on water heating equipment, heat recovery systems help extend the life of water heaters and improve system reliability.
- Reusing heat from wastewater instead of discharging it into water bodies helps maintain aquatic ecosystems.
- Lowering peak energy demands can reduce the need for carbon-intensive backup power sources.
- Reduces water waste by decreasing hot water wait times—minimizing the amount of water users run down the drain while waiting for hot water, especially in multifamily and commercial settings.
What Are the Challenges/Constraints?
- WWHR systems, especially in large buildings or district energy setups, can have significant capital costs for equipment, design, and installation—even if the long-term payback is strong.
- Retrofitting into existing buildings can be technically challenging and costly due to space constraints or access to drain lines. For this reason, these systems are generally easier to incorporate in new construction.
- While DWHR systems typically require minimal maintenance, routine inspections are recommended to ensure long-term efficiency and identify any buildup or performance issues early.
- Their design is more complex than a standard water heating system, requiring specialized expertise for proper design and installation.
- Fluctuating wastewater temperatures can reduce system efficiency, as less energy can be recovered compared to applications with consistent hot water usage.
- Health and safety concerns (e.g., cross-contamination risks) may be misunderstood or overestimated by authorities or designers.
- Concerns about fouling, maintenance needs, or reduced performance over time, especially with grease or solids in wastewater streams.
Qualifications for CEDA Inducements
For a project to be eligible for inducements, it must meet the following requirements:
- Be located within the SCE, SoCal Gas, PG&E or SDG&E territory
- Enrolled in CEDA
- Use a DWHR system with a minimum rated effectiveness of 50 percent
- For a central water heating system serving multiple dwellings or commercial buildings, the DWHR or WWHR systems must:
- Recover heat from half the showers located above the first floor
- At least transfer that heat either back to all the respective showers and/or the water heater
- For a central water heating system serving an industrial process, the WWHR system must:
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- Recover heat from half of the industrial processes
- Transfer that heat either back to all the respective processes or the water heater
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- The heat recovery system shall comply with all applicable laws and regulations
- Participate in on-site verification and possible data logging of the system
- Be a non-residential building, or a high-rise multifamily building (four or more habitable floors above grade)
Notes:
- Project may be selected by PG&E for a future case study
- Measure requirements are subject to change; this guide reflects information available as of April 2025 — for the most current measure requirements, contact CEDA@willdan.com
Contact us today to enroll and build resiliency into your project.
Resources:
- Connor, N. (2019, June 4). What is heat exchanger – definition. Thermal Engineering. What is heat exchanger – definition – thermal-engineering.org
- Department for Energy Security and Net Zero. (n.d.). Wastewater heat recovery systems (instantaneous). Energy Technology List. Wastewater Heat Recovery Systems (Instantaneous)- etl.energysecurity.gov.uk
- Energy Solutions Center. (n.d.). Drain water heat recovery. Energy Solutions Center. Drain Water Heat Recovery – Gerald Van Decker, Renewability – energysolutionscenter.org
- Leotsakos, C. (2024, March 27). The basics of wastewater heat recovery. Sewer Thermal Energy Network. The Basics of Wastewater Heat Recovery – www.sewerheat.org
- SHARC Energy. (n.d.). PIRANHA wastewater heat recovery system. The PIRANHA Series Heating & Cooling Using Wastewater – sharcenergy.com
- S. Department of Energy. (2025, March 26). Drain-water heat recovery. Energy Saver. Drain-Water Heat Recovery- energy.gov
