Temperature swings on a tankless unit often come down to flow rate and multiple fixtures competing for hot water, not just the thermostat setting.
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Quick answer
Temperature problems on a Rinnai tankless water heater, whether too hot, too cold, or fluctuating mid-shower, usually trace to the unit's flow-based heating design interacting with low flow rates, multiple fixtures running at once, a temperature setting that does not match household needs, or scale buildup reducing heat transfer efficiency over time. A licensed technician can check the actual settings, flow rate, and internal condition to identify which factor is driving the problem.
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A tank-style water heater stores a large volume of water at a set temperature, so it is fairly forgiving of small demand changes. A tankless unit like a Rinnai instead heats water on demand as it flows through, which means temperature output is more directly tied to flow rate and how many fixtures are drawing hot water at the same time. This design is efficient, but it also means certain symptoms, like a temporary temperature dip when a second shower or the dishwasher starts up, are a predictable characteristic of tankless systems generally rather than necessarily a fault.
That said, water that is consistently too hot, consistently too cold, or swings unpredictably even under steady single-fixture use is not normal and points to a specific cause that a technician should investigate.
| Symptom | Common Cause |
|---|---|
| Water never gets as hot as expected | Temperature setting too low, or flow rate exceeding the unit's heating capacity |
| Temperature dips when a second fixture turns on | Normal flow-based behaviour, more noticeable with low-flow fixtures or simultaneous heavy use |
| Water runs uncomfortably hot at low flow | Common with very low-flow fixtures like some bathroom faucets; may need a flow adjustment |
| Temperature fluctuates unpredictably during steady use | Could indicate scale buildup, an inlet filter restriction, or an internal fault needing inspection |
| Cold water burst at the start of a shower, called cold water sandwich | A known characteristic of some tankless designs when a fixture is briefly cycled off and back on |
Metro Vancouver's municipal supply is soft water with relatively low mineral content, which generally means less scale buildup inside a tankless heat exchanger compared to hard-water regions, though it is not zero, and units without regular flushing can still accumulate scale over years of use. Fraser Valley properties on well water tend to see this factor more, since well water hardness varies more widely. In older character homes and Vancouver Specials with a tankless retrofit, low-flow fixtures original to the house or added later during a renovation can sometimes sit right at the edge of a unit's minimum activation flow rate, which is worth checking if temperature seems inconsistent at specific fixtures.
Rinnai documents general flow rate and temperature specifications for its tankless models, but exact figures vary by model and installation conditions, so verify against your unit's manual rather than assuming a single number applies across the lineup.
Most temperature complaints on a well-maintained tankless unit are resolved through a setting adjustment, a flush to clear scale, or addressing a flow-rate mismatch at specific fixtures, none of which require replacing the unit. Replacement becomes a more reasonable conversation when the unit is older, has a history of repeated internal faults, or a technician's inspection finds heat exchanger wear that flushing and adjustment cannot resolve. A proper diagnosis should always come before a replacement decision.
Safety note
If you ever smell natural gas near the unit or anywhere in the home, leave the home immediately and call FortisBC at 1-800-663-9911 from outside. Do not attempt to relight a pilot, reset an ignition lockout, or open any gas fitting yourself. Gas connections, venting, and combustion-air work on a tankless water heater must be handled by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870.
Tankless units heat water based on flow rate, so multiple fixtures drawing at once can reduce the temperature rise per fixture, which is a known characteristic of tankless systems rather than always a fault.
It's a brief burst of cold water that can occur at the start of a shower when a tankless unit briefly cycles off and back on; it's a known tankless characteristic in some designs, not necessarily a malfunction.
Basic temperature adjustment is often accessible to homeowners on a remote control or unit interface, but setting water too hot creates scald risk, so keep changes modest and consult your manual.
Yes, scale inside the heat exchanger reduces heat transfer efficiency over time, which can cause inconsistent or reduced temperature output, especially in units that have gone a long time without flushing.
Generally yes, lower mineral content means less scale buildup over time compared to hard-water regions, though periodic flushing is still good practice.
Only after a technician has ruled out settings, flow-rate issues, and scale buildup and finds actual heat exchanger wear or a recurring internal fault that servicing cannot resolve.
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