Quick answer
A tankless water heater shuts off mid-shower for one of two reasons. Either hot-water flow fell below the minimum the unit needs to stay lit, about 2 litres per minute, which happens with low-flow showerheads and a shower valve mixing in a lot of cold. Or the unit locked out on flame loss or overheating. An error code on the display means lockout; no code means flow.
When a tankless water heater shuts off mid-shower, start by looking at the display, because the two causes leave different evidence. A unit that dropped out on low flow shows no fault and fires again when you open the tap wider. A unit that locked out shows an error code and stays cold until it is reset.
No error code: flow fell below the minimum activation rate
A tankless burner only runs while a flow sensor sees enough water moving through the heat exchanger, about 2 litres per minute on most residential units. That figure is hot-water flow through the heater, not total flow at the showerhead. The shower valve makes up its stream from hot and cold, and the hotter the heater's setpoint, the smaller the hot share of the mix. Turn the handle toward cool on a restricted showerhead and the hot share slips under the minimum, the burner shuts down, and the shower goes cold.
- A low-flow or water-saving showerhead, especially one partly blocked with scale.
- A thermostatic or pressure-balancing shower valve throttled toward cool.
- A setpoint of 55 to 60 °C on the heater, which shrinks the hot share of every mix.
- A clogged inlet filter screen on the cold connection under the unit.
- Late summer, when incoming cold water is at its warmest and the shower needs even less hot.
- A second tap closing elsewhere, on a unit that was only just above its minimum.
The fix that costs nothing: set the heater to 45–49 °C so the shower runs mostly on hot, and rinse the inlet filter following the manual. If the shower stays lit after that, nothing is broken.
Error code showing: flame loss or overheat lockout
A code means a safety stopped the burner. These are the causes in the order a technician checks them.
- Scaled heat exchanger: mineral scale insulates the water passages, the outlet temperature spikes and the overheat limit trips. Hard water in Halton and Hamilton makes this the first suspect on a unit that has never been flushed; the signs are in how to know if a tankless needs descaling.
- Dirty flame rod: the rod proves the flame to the control board. Coated with oxide, it loses the signal mid-burn and the board closes the gas valve.
- Blocked intake or exhaust: lint, leaves, a nest, snow or ice at the termination, or a vent run that has sagged and holds condensate.
- Undersized gas supply: the burner can need up to 199,000 BTU an hour at full fire. On a marginal line, pressure sags when the furnace or range comes on and the flame drops out.
- A failing fan, sensor or control board: less common, and found by measurement.
The error code on the display identifies which
Write the code down before resetting, because it disappears with the power. Flame-loss codes include Navien E012 and Rinnai 12. Overheat and scale codes include Navien E016 and Rinnai LC. Blocked-vent codes include Navien E110 and Rinnai 10.
Code tables differ between model lines, so the list in your manual or inside the front cover is the one that counts. A lockout that returns after one reset is a service call, and repeated resets on an overheating heat exchanger shorten its life.
How a shut-off differs from the cold-water sandwich
The cold-water sandwich is a short slug of cool water between two stretches of hot. It happens when a tap is closed and reopened: the burner stops, takes a few seconds to relight, and the unheated water that passed through in between arrives at the shower as a brief chill. The heater never stops for long, no code appears, and the hot returns without anyone touching anything.
A shut-off is different: the water goes cold and stays cold until flow increases or the unit is reset. The sandwich and other temperature swings are covered in why a tankless goes hot then cold.
What the fix costs, and when to call
- Diagnostic visit: $90–$150, deducted from the repair if you book it.
- Descale flush of the heat exchanger: $150–$300.
- Gas line upsizing, where the supply is too small: $400–$1,200.
- Flame rod cleaning, sensors and other parts: quoted in writing on the visit.
Call instead of resetting if the same code returns after one reset, if you smell gas or exhaust near the unit, if water is dripping from the cabinet, or if the unit bangs when it lights. ZK Mechanical services Navien, Rinnai and Bosch tankless units. See tankless water heater repair, read how hard water affects tankless units in Hamilton and Halton, or call (647) 801-1252 or book a visit online.