IAPMO developed the Water Demand Calculator in response to the consistent oversizing of potable water distribution systems in the built environment. Oversized domestic water systems result in larger pipes, larger piping appurtenances, and excessive energy and material use. IAPMO contracted Arup to calculate operational energy savings, embodied carbon savings, and water savings using the WDC for four common building types. The analysis included a single-family residence, a 6-unit multifamily residence, a 45-unit residence, and a high-rise residence; these use cases represent a large majority of housing in the country.
The Water Demand Calculator sizing methodology allows for significant sizing reduction in domestic water systems for residential buildings. The calculator allows for water savings without affecting how the building occupants use water. The water, energy, and carbon savings are all embedded within the efficient design of the domestic water system.
The results of this study indicate that when the WDC is used for domestic water design for residential buildings instead of the UPC or IPC Hunter’s Curve sizing methods, there are resulting operational energy and embodied carbon savings in all four of the use cases, as well as water savings in the non-circulating units.
Water savings were demonstrated through minimized time-to-tap using the Water Demand Calculator sizing. Water savings range from 450 gallons to 71,000 gallons annually depending on the building size. This represents water savings between single-family, 6-unit, and 45-unit buildings from 27% – 34% oversizing using IPC.
A single-family unit prototype showed annual water savings of 450 gallons. Using U.S. Census Data for single residential permits, this water savings across the 975,584 single-unit residences permitted in 2022 has the potential to save approximately 440 million gallons of water annually when compared to the use of IPC for sizing.
High-rise residential buildings show savings in operational carbon between 73 – 84% for booster pumps and embodied carbon savings ranging from 20% to 41%. Using the Water Demand Calculator instead of the UPC or IPC Hunter's method to size domestic water systems in high-rise residential buildings shows savings of operational carbon ranging from 2,000 lbs CO2 to ~24,000 lbs CO2 per high-rise residential building, depending on grid emissions at the project site. Additionally, a reduction in pipe sizing allows for reduced heat loss through pipes.
Notably, there are embodied carbon savings associated with interior water mains and branches, and incoming services appurtenances including ball valves, water meters, and backflow preventers. Using the Water Demand Calculator in lieu of the IPC Fixture Units to size domestic water systems in single-family residences can save up to 32.9 kg CO2e from water service entry appurtenances per single-family home alone.
ARUP has vetted the Water Demand Calculator for its completeness, and it is easy to adopt. A code authority needs to only accept Appendix M of the UPC to allow for the Water Demand Calculator to be used for residential plumbing system sizing. The water, energy, and carbon savings shown in this study can be attained by simply adopting the Water Demand Calculator as an acceptable sizing method. Adopting the Water Demand Calculator does not affect how plumbing fixtures operate, or how they are used.
