Here’s what AI say’s about it when asked specifically about an under sink water heater. Of course, it’s always best to follow Mfg installation instructions when installing these systems.
do under sink instant water heaters require a TPR valve? - Google Search
Whether an under-sink instant water heater requires a temperature and pressure (TPR) valve depends on its design. Mini-tank (storage) models usually come with or require a combined T&P relief valve, while true instant tankless point-of-use units typically rely on a pressure-only relief valve or internal safety controls. [1, 2, 3]
Mini-Tank (Storage) Under-Sink Heaters
- Small point-of-use storage tanks (like 2.5-gallon units) hold pressurized hot water.
- They require a T&P relief valve to prevent dangerous tank explosions from thermal expansion and over-pressurization.
- Many units ship with an explicit T&P valve that must be installed on the hot or cold port according to the manual.
- Plumbers on PlumbingForums note that local codes may allow the discharge to terminate safely inside the cabinet (e.g., 6 inches above the floor or into a catch) rather than running outside, provided it avoids creating a hazard. [1, 2, 3, 4, 5]
True Tankless (Instant) Under-Sink Heaters
- True on-demand or inline electric water heaters do not store water under a static thermal heat loop.
- They generally do not use a traditional dual TPR valve.
- Most manufacturers and plumbing codes instead call for a pressure-only relief valve on the hot water outlet side, since electronic limit switches manage excessive internal temperatures. [1, 2, 3, 4, 5]