A gas tankless water heater heats water only as it flows through the unit, using a burner, heat exchanger, and flow sensor working together instead of storing hot water in a tank.
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Quick answer
A gas tankless water heater has no storage tank at all; instead, a flow sensor detects when a hot tap opens, a burner ignites, and cold water passing through a heat exchanger is raised to the target temperature on its way to the fixture. This on-demand approach means there is no standing reserve of hot water and no standby heat loss, but it also means the unit's output is limited to its rated flow rate at any given moment, which is why sizing the right unit for a household's actual hot water habits matters more with tankless than it does with a tank.
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| Component | Role |
|---|---|
| Flow sensor | Detects water movement and triggers the ignition sequence |
| Igniter and burner assembly | Provides and sustains the flame that heats the exchanger |
| Heat exchanger | Transfers heat from combustion to the water flowing through it |
| Control board | Modulates burner output to hold target temperature as demand changes |
| Venting system | Carries combustion exhaust safely outside and supplies combustion air |
| Temperature sensors | Monitor incoming and outgoing water temperature for the control board |
A storage tank keeps a fixed volume of water hot around the clock, so hot water is available instantly but limited once the tank empties under heavy draw. A tankless unit has no such reserve, so hot water is available continuously as long as the flow rate stays within the unit's rated capacity, but a brief delay exists before hot water actually reaches the fixture, since the unit has to sense flow, ignite, and heat the water passing through it in that moment.
This is also why simultaneous high-demand use matters more with tankless equipment. A single unit sized for typical household use can be pushed past its capacity by two showers and a washing machine running together, which shows up as reduced water temperature rather than a total loss of hot water, unlike a tank running dry.
Because a tankless unit's burner cycles on and off far more often than a tank's, and often at a different input rate, its venting requirements are frequently different from what an existing tank used, particularly in older Vancouver-special homes originally built around a natural-draft chimney flue. Many tankless installations use dedicated sidewall venting suited to the unit's specifications, with attention paid to termination clearances from windows, doors, and property lines, which matters more in this damp coastal climate where condensate and moisture management around the vent terminal need to be handled correctly.
Metro Vancouver's comparatively soft municipal water reduces how often descaling is typically needed compared with harder-water regions, but it does not eliminate the need for it, and homes on Fraser Valley well water should expect a technician to recommend a more attentive schedule.
A homeowner can reasonably notice and report things like a longer-than-usual wait for hot water, a burning or unusual smell near the unit, an error indicator lighting up on the control panel, or reduced flow at a fixture. Opening the unit's casing, testing gas pressure, or accessing the heat exchanger for descaling is licensed gas fitter work, both because of the gas connections involved and because incorrect handling can affect the venting and combustion safety of the appliance.
Safety note
This equipment burns natural gas and is vented to the outside, so any work on the gas line, burner, venting, or combustion air supply must be done by a Technical Safety BC licensed gas fitter. If you ever smell gas, hear a hissing sound near a gas fitting, or suspect a venting problem, leave the home immediately and call FortisBC at 1-800-663-9911 before doing anything else.
Not in the way a tank does, but if demand exceeds the unit's rated flow capacity, water temperature can drop rather than the supply stopping outright.
The unit has to detect flow and complete its ignition sequence before it starts heating water, which takes a few seconds longer than water already sitting hot in a tank.
Often not directly, since tankless units frequently require different venting suited to their specific input rate and cycling pattern; a technician needs to assess this.
It varies by water source and usage, but the region's comparatively soft municipal water generally means less frequent descaling than in harder-water areas, while well water homes need closer attention.
Igniter or flame sensor issues are common documented causes, though venting obstructions and gas supply problems can also prevent ignition; a technician's diagnostic identifies the actual cause.
It avoids standby heat loss from a tank sitting full and hot, which is a genuine efficiency advantage, though actual household energy use depends on many factors specific to the home.
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