Rinnai Takes Too Long for Hot Water: Why
On this page (6 sections)
The Short Answer: Most of the wait for hot water from a Rinnai is the cold water already sitting in the pipe between the heater and the faucet. A tankless heater adds only a few seconds to start the burner. Long pipe runs and low-flow fixtures make the wait longer. Recirculation, either built into the heater or added to the plumbing, is the real fix for distant bathrooms.
Time it at two taps
Use a phone stopwatch at the hot tap closest to the heater and at the slowest one.
- Close tap fast, far tap slow: pipe volume. The heater is working normally.
- Both slow to get hot: look at the heater (scale, flow problems) or a crossover in the plumbing.
- Hot arrives, then a burst of cold, then hot again: a short pause while the burner restarts after a recent draw. This “cold water sandwich” is a known trait of tankless heaters.
The arithmetic of pipe delay
Half-inch copper holds about 0.012 gallons per foot. A 50-foot run holds about 0.6 gallons of cold water. At a 1.5 gallon-per-minute shower head, it takes about 24 seconds just to push that water out, before any heater delay. Three-quarter-inch pipe holds roughly twice as much per foot, so the wait doubles. A low-flow faucet at 0.5 gallons per minute stretches the same run to over a minute.
A tank heater in the same spot would have the same pipe delay. The difference people notice is often that a new tankless was installed in a different location from the old tank.
Low-flow fixtures and minimum flow
A Rinnai needs a minimum flow to start heating; Rinnai lists 0.4 gallons per minute for its RSC SENSEI models. A bathroom faucet barely opened, or a shower that mixes in a lot of cold, may not reach it. The heater then never fires, and the water stays cold no matter how long you wait. Open the tap fully to hot for the first few seconds, then adjust.
Recirculation options
- Built-in pump models (Rinnai RUR, RSC and similar) can keep the loop warm. Rinnai’s manual (opens in a new tab) describes a Smart-Circ mode that learns household patterns, plus timed and on-demand options. These work with a dedicated return line or, on some models, a crossover valve under the farthest sink.
- External recirculation can be added to a non-pump Rinnai with a pump, controls, and a return line or crossover, installed by a plumber to match the heater’s instructions.
- On-demand (button or motion sensor) recirculation uses less energy than running a loop all day.
Other things that slow it down
A clogged inlet water filter, scale, or partly closed service valves reduce flow through the heater and can make it slower to deliver full temperature. If the water arrives hot but never quite reaches the setting, see Rinnai not getting hot enough.
Related: tankless takes a long time to heat up, Navien DHW wait, and Rinnai constantly running.
Frequently Asked Questions
Q: Will raising the temperature make hot water arrive faster? A: No. The setting changes how hot the water is, not how fast it travels through the pipes.
Q: Is a 30 to 60 second wait at a far bathroom normal? A: In a house with long runs and no recirculation, yes. If the wait was much shorter with the same piping before, look for a flow problem or a crossover.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- Rinnai: RSC SENSEI installation and operation manual (100000773), recirculation settings (opens in a new tab)Recirculation modes and minimum activation flow for Rinnai RSC SENSEI models with a built-in pump.
- Rinnai: official document library (opens in a new tab)Select your exact model and owner manual before interpreting a fault or reset procedure.
Editorial standards and corrections · Prepare a useful service request