Quick answer
A tankless water heater that goes hot, then cold, then hot again is usually a 'cold water sandwich': turn a tap off and back on within minutes and you get the hot water left in the pipe, then the cold slug that went through the unit as it re-ignited. It is a design quirk, not a fault. Water that stays cold points instead to low flow, scale or a crossover valve.
If your tankless water heater goes hot, then cold, then hot again in the first minute of a shower, you are most likely feeling something every on-demand heater does by design. The trick is telling that harmless pattern apart from the faults that look similar — because one costs nothing and the others get worse the longer they run.
The cold water sandwich, explained
A tankless unit only fires when it senses water flowing through it. When someone shuts a tap, the burner stops, and the water still inside the heat exchanger and the pipes stays hot for a while. Turn a tap on again a couple of minutes later and that stored hot water comes out first. Behind it is a short plug of cold water that moved through the unit during the few seconds it took the fan to purge, the igniter to spark and the burner to reach output — and then the fresh hot water arrives.
You notice it most in the morning: one person showers, the next starts three minutes later and gets a warm start, a cold jolt, then steady hot. The length of the cold slug depends on the distance from the heater. In a two-storey home in Headon Forest with the mechanical room at the back of the basement and the ensuite at the front of the second floor, that can be a 15–20 metre pipe run, so the sandwich lasts longer than it would in a bungalow.
How to get rid of the cold water sandwich
- Buffer tank: Navien's NPE-A2 series carries a small built-in buffer tank and an internal recirculation pump precisely to smooth this out. If you are choosing a new unit and the sandwich bothers you, pick a model with one.
- Recirculation: a recirculation pump on a timer or motion sensor keeps the loop warm during the hours you use hot water. Rinnai's RUR series has the pump built in; other units can be paired with an external pump and either a dedicated return line or a crossover valve under the farthest fixture.
- Small electric mini-tank: a 2–4 gallon point-of-use tank on the hot line after the tankless absorbs the cold plug. It is the lowest-cost retrofit and works well on a single problem bathroom.
- Habit change: leave a short gap between showers, or run the shower until it is steady before stepping in. Free, and enough for many households.
When hot-then-cold is actually a fault
If the water goes cold and stays cold, or the unit drops out partway through a long shower, it is not the sandwich. Work through these in order:
- Flow too low to keep the burner on. Every tankless has a minimum activation flow, typically around 0.4–0.5 gallons a minute. A low-flow showerhead, a half-closed shut-off valve or a partly blocked inlet filter screen can drop a warm-mixed shower below that threshold, so the burner cuts out and you get cold water. Cleaning the inlet screen takes minutes.
- Scale on the heat exchanger. Halton and Hamilton draw moderately hard lake water, and scale builds inside the exchanger until the unit can no longer hold its set temperature at full flow; it starts to hunt, and many models throw an error code. An annual descale flush ($150–$300) is both the fix and the prevention. Our guide to hard water and tankless heaters in Hamilton and Halton covers the chemistry.
- Crossover in a mixing valve. A worn single-handle shower cartridge lets cold water bleed into the hot line. The symptom is lukewarm water at every tap while the heater shows no fault at all. Shut the cold supply to the heater and open a hot tap: if water keeps flowing, you have a crossover.
- Undersized unit or gas supply. When two showers and a dishwasher run together in February, the unit cannot lift enough water to temperature and the shower cools. If the gas line was never upsized from the old tank, the burner can also starve when the furnace fires at the same time.
- Set point too low. A unit set to 49°C leaves almost no margin for mixing; nudging it to 52–55°C at the controller gives the shower valve something to work with. A home with a thermostatic mixing valve is capped at the valve, not the unit.
Does hot-then-cold mean the tankless is failing?
Rarely. A unit that sandwiches every morning but otherwise holds temperature is behaving normally, and units from Navien, Rinnai and Bosch routinely reach 15–20 years with annual maintenance. The pattern to worry about is a change: a heater that never dropped out and now does, or cold gaps getting longer each month. That usually tracks scale build-up or a dying flow sensor, and both are ordinary tankless repairs — most run $150–$800 — rather than a reason to replace.
If you are in Burlington, Oakville or Hamilton and want the sandwich gone or the dropouts diagnosed, ZK Mechanical services all three tankless brands we install. Book through our tankless water heater page or contact the shop and mention which fixture and what time of day — it points us at the cause before we arrive.