In a recent code coach case, the owners of a small ADU project were interested in exploring options for water heating. Hoping to avoid natural gas but also concerned about the space requirements of a heat pump water heater, they consulted an Energy Code Coach about an electric instantaneous water heating system option, sometimes called “on demand.”
Interestingly, while the energy code has generally moved away from electric resistance systems, in this case, tankless electric instantaneous is a good fit.
Prescriptively, the 2022 Energy Code allows for instantaneous electric water heating, with two critical criteria: the house must be 500 square feet or less, and it must have “point of use” plumbing distribution (Section 150.2(a)1Div). The house is under 500 square feet, but the point of use distribution requirement for the instantaneous electric water heater led to floor plan changes to keep the water fixtures and water-consuming appliances closer to the water heater.
Reviewing these requirements in advance ensures compliance to take advantage of this small-home option.
Allowed length of pipe depends on the pipe diameter
Table 4.4.5 from the 2022 Reference Appendices is our design guidepost. Generally, the bigger the pipe, the shorter the run that is allowed between the hot water heater and the fixture. If two pipe sizes are used in a single run, half the length of pipe shall be considered for each pipe size.
| Size Nominal (Inch) | Length of Pipe (Feet) |
| 3/8" | 15 |
| 1/2" | 10 |
| 3/4" | 5 |
In an efficient design, a ¾” line leaves the water heater and quickly connects to a manifold, which reduces pipe size for longer runs: ½” to the shower and laundry, 3/8” to sinks.
Point of Use Plumbing in Action
In the small ADU example, the sink and bathroom were close together, but the laundry machine was more than 10’ away. The design team shifted appliances and cupboards to cluster all hot-water fixtures and appliances.
Changes for the 2025 Code Cycle
The 2025 code, effective January 1, 2026, expands electric hot water options for homes 500 square foot or smaller. It prescriptively allows not only instantaneous electric systems but also electric tank systems, as long as point-of-use distribution is used. Gas or propane instantaneous systems are no longer prescriptively permitted, so the performance path must be used for those.
For more information about the 2025 code update and in-person trainings, please check out our events calendar at 3c-ren.org/calenar-of-events-and-trainings
Our on-demand webinars are always available at 3c-ren.org/on-demand-trainings
Don’t wait to reach out to the Code Coach Hotline for more information or to have your Energy Code or CALGreen questions answered! 3c-ren.org/energy-code-coach