Methodology

To determine feasibility, FSG conducted detailed analysis of domestic hot water (DHW) usage and wastewater energy availability. This included evaluating logged DHW makeup water data and total water use data from the water utility meter. Key assumptions about energy losses to the ambient environment and uninsulated piping were incorporated to estimate available energy in the wastewater stream. The DHW load profile derived from the logged data was compared with this energy potential to determine the degree to which WWHR could offset the building’s DHW needs.

Data collection for this effort was extensive. Flow rates were measured using a Dynasonics ultrasonic flow meter and temperature with a Hobo strap-on sensor. The total water usage and DHW-specific consumption allowed FSG to establish a baseline load for system sizing. This data was shared with the equipment manufacturer to support a preliminary design and selection of system components.

Calculations to estimate system performance and potential savings included several elements. DHW load was calculated using logged flow and temperature data and validated against ASHRAE standards, confirming consistency with a Medium usage profile and typical peak demand. Available wastewater energy was derived from total consumption data, adjusted for future water efficiency upgrades to avoid overestimating energy recovery potential.

A dynamic system model was developed to simulate hourly operations. The model incorporated assumptions about water use, incoming mains water temperature (based on local NYC data), and wastewater temperatures derived from mixed DHW and cold-water inputs. The model tracked DHW demand, wastewater energy availability, heat extraction, electric energy use (based on equipment COP), and comparative steam energy use. System efficiency was tied to equipment performance curves provided by the manufacturer. From this dynamic model FSG was able to forecast annual fuel savings, utility cost avoidance, total energy savings and the percentage reduction in energy use, as well as greenhouse gas (GHG) emissions reduction. This comprehensive analysis supported both technical feasibility and informed design development.

Solutions

Based on the sizing calculations performed as part of the study, two Piranha T15 heat pumps, a 1000-gallon wastewater holding tank, and 500 gallons of potable storage would be needed to serve the load. A 90kW electric storage tank was included for top-off purposes in time of heavy demand as well to ensure an adequate supply of DHW water when one of the heat pumps is offline due to failure or during regularly scheduled maintenance. Circulation pumps between the potable side of the wastewater heat pumps and DHW storage tanks allow the system to charge the tanks with hot water during periods of reduced DHW demand for use during peak periods.

Based on the electric load analysis which included evaluation of the existing spare capacity of the building infrastructure and the proposed new electric loads of the WWHR heat pumps and 90 kW electric storage water heater, it was determined that an electrical service upgrade was not necessary.

Results

The results of the analysis conducted as part of the feasibility study are as follows.

• Available waste heat energy : 1,890 MMBtu/yr

• DHW load: 1,301 MMBtu/yr

• Energy Recovered from Wastewater: 1,301 MMBtu/yr

• Fraction of Waste Energy Utilized: 68.7%Annual Steam Savings: 1,341 Mlbs

• Post-retrofit Electric Use (for DHW): 103,717 kWh

• % Reduction in DHW Energy Use: 72.8%

• GHG Emissions Savings: 28.5 tCO2e/yr

While the results of the study focus on a specific building, the findings can serve as a blueprint for NYCHA's future efforts to implement similar wastewater heat recovery solutions that can be scaled across its housing portfolio.

Summary and Conclusions

Facility Strategies Group (FSG) conducted a comprehensive feasibility study to help NYCHA explore the potential of using wastewater heat recovery (WWHR) to provide domestic hot water at a public housing development. FSG analyzed water use and wastewater energy availability, assessed space and infrastructure needs, and developed a system design tailored to the site. They also evaluated whether the building's existing electrical system could support the new equipment. The study concluded that using WWHR is technically possible and could lead to meaningful energy savings and emissions reductions. While the upfront costs are high, the study provides a valuable foundation for NYCHA to consider WWHR as a scalable clean energy solution across its housing portfolio.