Temperature swings on a Takagi tankless unit usually trace back to flow rate, mixed fixtures, or a sensor fault rather than the heater being undersized.
Tell us what you need—we’ll call you back fast.
Quick answer
A Takagi tankless water heater that runs too hot, too cold, or swings between the two is most often reacting to changes in water flow rate, a mix of fixtures pulling from the unit at once, mineral buildup on the heat exchanger, or a flow or temperature sensor drifting out of calibration. Start by checking the set temperature and whether the problem happens at one fixture or the whole house before assuming the unit itself has failed.
Need a licensed gas fitter for your tankless water heater? Call CANRO at (604) 359-1081 — residential natural-gas heating and hot water across Metro Vancouver since 1999.
A tankless water heater has no storage tank holding a steady reserve of hot water. Instead, a flow sensor detects when a fixture opens, the gas valve modulates the burner, and the unit heats water on the way through a heat exchanger to hit the temperature you set at the remote or the unit itself. Because the burner is constantly adjusting to match flow, temperature control depends on clean sensors, a properly sized gas supply, adequate venting, and a heat exchanger that has not been coated in scale.
When something interrupts that balance, the symptom you notice is either water that never gets hot enough, water that spikes uncomfortably hot, or a temperature that hunts up and down while a shower is running. Each pattern points toward a different cause, which is why a homeowner call about a Takagi unit almost always starts with a few questions before a technician opens the cabinet.
The most frequent reason a Takagi unit cannot reach the set temperature is flow that exceeds the unit's rated capacity for the temperature rise required. Metro Vancouver's incoming water is relatively cold for much of the year, especially with water drawn from mountain reservoirs, so the same unit that felt generous in August can feel undersized in January when the incoming water temperature drops and the burner has to work harder for the same output temperature.
Scale buildup on the heat exchanger is the next common culprit, particularly in homes without a water softener or where the unit has gone years without a descaling flush. Even though the Lower Mainland's municipal water is comparatively soft next to well-fed communities in the Fraser Valley, scale still accumulates over years of use, especially at higher set temperatures, and it insulates the heat exchanger enough to cause inconsistent output.
A dirty or failing flow sensor, a temperature sensor drifting from age, or a control board fault can also cause temperature swings that have nothing to do with hot water demand. Mixing valves at fixtures, a failing pressure-balance cartridge in a shower valve, or a recirculation pump running against the tankless unit's minimum flow requirement are other sources that get mistaken for a heater problem.
| Season | Typical incoming water temperature | Effect on output |
|---|---|---|
| Summer | Warmer supply | Unit reaches set temperature easily, even at higher flow |
| Winter | Cold supply, especially after a cold snap | Same flow rate may not reach set temperature; unit may need flow reduced |
| Multiple fixtures open | Combined flow near or above rated capacity | Temperature at each fixture drops or fluctuates |
A technician working on a Takagi unit will typically verify incoming gas pressure and supply line sizing, since an undersized gas line or a pressure drop under full firing rate will show up exactly as a temperature complaint. They will inspect and, if needed, descale the heat exchanger, test the flow sensor and inlet thermistor readings against the unit's expected values, and confirm the venting run meets the manufacturer's length and termination clearance requirements, since improper venting on a retrofit install is a common source of ongoing performance complaints in older Vancouver-area homes.
They will also check for cross-connection issues where a mixing valve or a failed check valve lets cold water bypass the heater, and they will review whether the unit's rated flow matches the household's actual fixture count and simultaneous use pattern. In some cases the honest answer is that the household has outgrown the unit's capacity for winter conditions, and the fix is either accepting a lower simultaneous-use flow rate or discussing a larger or paired system, not a repair.
Sensor and control board issues, scale buildup, and venting corrections are generally straightforward repairs on a unit that is otherwise in good working order. If the heat exchanger itself shows corrosion or repeated scale-related failures, or if the unit is old enough that parts availability is uncertain, replacement becomes the more sensible long-term choice, especially when paired with the opportunity to correctly size the new unit for the home's actual winter flow needs. Ballpark for a tankless installation in Metro Vancouver runs roughly $3,500 to $7,500 or more depending on venting complexity, gas line upsizing, and access, and that figure is a starting point for discussion, not a quote.
Safety note
If you ever smell gas, hear a hissing sound near the unit, or notice soot, a burning smell, or headaches and drowsiness near the appliance, leave the home immediately and call FortisBC at 1-800-663-9911 from a safe location before doing anything else. Any work on gas piping, venting, or combustion components must be carried out by a Technical Safety BC licensed gas fitter.
That cold-water sandwich effect is normal on many tankless units when a fixture is turned on and off quickly, because water sitting in the pipe between uses cools before the burner reignites. It becomes a bigger nuisance in winter and can be reduced with recirculation strategies, but it is not usually a fault on its own.
You can raise the set temperature within the unit's allowed range, but going too high increases scald risk and accelerates scale formation. If turning it up does not solve the problem, the cause is more likely flow rate, scale, or a sensor issue than the temperature setting itself.
Most of Metro Vancouver draws from soft reservoir water, so scale builds more slowly than in well-fed areas of the Fraser Valley, but it still accumulates over years, especially at higher temperature settings, and periodic descaling is still recommended.
Water that is uncomfortably hot at the tap can be a scald risk, and a unit that will not modulate properly can sometimes point to a combustion or venting fault, which is a safety matter. If you notice a burning smell, soot, or shutdowns along with the temperature issue, treat it as a safety call, not just a comfort one.
There is no single number that fits every home, since it depends on set temperature, usage volume, and water chemistry. A licensed technician can check the heat exchanger condition during a service visit and recommend a schedule based on what they actually find.
A recirculation pump can reduce wait time for hot water at distant fixtures, but if it is not matched to the tankless unit's minimum flow requirement it can actually cause the burner to short-cycle or under-fire, which shows up as inconsistent temperature.
Family-run, TSBC-licensed (gas LGA0043870 · boiler LBP0046450), serving Metro Vancouver homes since 1999. Residential, natural gas only.
Call (604) 359-1081 Email info@canroheat.ca“Canro does an amazing job! Super professional and skilled with everything—boilers, furnaces, and AC. Always a pleasure working with them!”
“Five stars for Canro. We needed a replacement of a 30-year-old boiler. A new high-efficiency unit was installed—clean, professional work.”
“I can’t speak highly enough about George, who was recommended for a boiler service call for my parents’ home. George is excellent.”
25+ years keeping Metro Vancouver homes comfortable. Honest advice, upfront pricing, work that lasts.