Temperature swings on a Noritz tankless unit often come down to flow rate, cold-water inlet swings, or a scaled heat exchanger, not just the thermostat setting.
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Quick answer
Water that runs too hot, too cold, or fluctuates on a Noritz tankless unit is most often caused by the flow rate falling below the unit's minimum activation threshold, multiple fixtures drawing hot water at once and exceeding the unit's capacity, incoming cold-water temperature swings through the seasons, or scale buildup reducing heat transfer efficiency. A homeowner can safely check the digital temperature setting and note usage patterns; anything involving the burner, gas valve, or heat exchanger should go to a licensed gas fitter.
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A storage tank holds a large reservoir of already-hot water, so temperature stays fairly steady until the tank runs low. A tankless unit like Noritz instead heats water instantly as it flows through, which means output temperature is directly tied to flow rate, incoming water temperature, and how hard the burner is firing at that moment. Open two hot taps at once and the unit has to split its heating capacity across more flow, which can drop the temperature at each fixture even though nothing is actually broken.
This is also why a phenomenon called cold-water sandwich sometimes occurs: a brief slug of lukewarm or cool water arrives between two hot draws, caused by the small window it takes the unit to reignite and ramp back up after a very short pause in flow, such as switching from a sink to a shower quickly.
Confirm the digital temperature setting matches what you expect, since it's easy for it to be bumped or set differently than intended after a repair or a household member adjusting it. Note whether the problem only happens when multiple fixtures run together, which points toward a capacity or sizing issue rather than a fault, and whether it's worse in winter, which points toward incoming water temperature rather than a mechanical problem.
If your household has grown, added a fixture, or installed a second bathroom since the unit was sized, it's worth having a technician confirm the unit still matches your home's peak simultaneous demand.
| Symptom Pattern | Likely Cause | Typical Fix |
|---|---|---|
| Cold water sandwich between draws | Reignition delay | Often normal; recirculation setup can reduce it if installed |
| Temp drops when two fixtures run together | Unit undersized for simultaneous demand | Usage adjustment, or a larger unit if this happens often |
| Worse in winter only | Incoming water temperature swing | May be normal seasonal behaviour; confirm with technician |
| Gradually worse over years | Scale on heat exchanger | Descaling flush |
If the unit is genuinely undersized for a household that has grown since installation, replacement with a higher-capacity model is sometimes the right call; that conversation is best had with a technician who can measure your actual peak demand rather than guessing.
Safety note
If you smell gas, notice soot around the unit's vent termination, hear a rattling or banging you have not heard before combined with a burning smell, or your carbon monoxide alarm sounds, leave the home immediately and call FortisBC at 1-800-663-9911 from outside before doing anything else. Gas connections, the burner assembly, the heat exchanger, ignition components, and venting on a tankless water heater must only be serviced by a Technical Safety BC licensed gas fitter. CANRO holds gas licence LGA0043870.
This is the cold-water sandwich effect, a brief lag while the unit reignites after a short pause in flow, and it's a known characteristic of tankless operation rather than necessarily a fault.
The unit has a maximum heating capacity, and splitting flow between multiple fixtures at once can exceed it, especially in winter when incoming water is colder and needs a bigger temperature rise.
Yes, if the flow rate drops below the unit's minimum activation threshold, it can cause the unit to cycle on and off rather than maintaining a steady flame.
Over years without descaling, scale can meaningfully reduce how efficiently the heat exchanger transfers heat into the water, gradually lowering output temperature or slowing recovery.
Possibly; a technician can calculate your household's peak simultaneous demand and tell you whether your current unit still matches it or whether a larger model makes sense.
Raising the setting too high increases scald risk at the tap and pushes the system closer to the relief valve's trip point, so it's better to diagnose the underlying cause with a technician first.
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