Indirect water tank vs tankless water heater

Domestic hot water can be delivered from storage or made on demand, and the choice between an indirect tank and a tankless water heater is really a choice between those two philosophies. An indirect tank is a storage vessel with no burner of its own: a coil inside the tank carries hot water from the boiler, and the boiler reheats the stored supply between uses. A tankless water heater is the opposite shape of appliance: a burner and heat exchanger that light only when a tap opens and cold water flows through, producing hot water continuously but only at the rate the appliance can manage.

The two names describe different roles in the system, which is worth stating plainly because they are often compared as if they were rival boilers. The indirect tank has no independent heat source; it is an accessory of the boiler. The tankless water heater is a complete, self-contained appliance with its own burner, commonly a condensing unit in its own right, and it does not care how the home is heated. On this site the distinction shows up in the product lines: IBC, for example, documents heating-only boilers that connect directly to an indirect tank, and separately documents its SFT tankless water heater as a standalone condensing appliance, both on its Canadian pages.

Storage buys reserve. An indirect tank sized to the household holds a volume of hot water ready for a full tub, a long shower or two bathrooms running at once, and recovery happens afterwards, at the boiler's pace, rather than during the draw. That reserve is the reason indirect tanks remain the default for larger homes and for households whose peak moments arrive several at a time. Storage also decouples the burner from the tap: the boiler can reheat the tank at a modest firing rate whenever the heating load leaves capacity, which suits a modulating condensing boiler that prefers long, gentle runs.

The costs of storage are space, standby losses and a dependency chain. The tank occupies floor area beside the boiler, keeps its stored water warm between uses, and relies on the boiler for reheating, so a boiler outage pauses tank recovery even though the stored volume keeps serving draws in the meantime. The tank also has its own small maintenance life, and its insulation and controls age with the system.

A condensing tankless water heater spends almost all its life idle and fires only when water flows, which removes the tank's standby losses and its floor footprint entirely. It mounts on a wall, and because it is a sealed condensing appliance, it vents in certified plastic pipe like a boiler. For a household whose hot-water events are short and sequential, one shower after another through the morning, an on-demand unit covers the pattern without ever holding a reservoir warm, which is a genuine operating difference, not a marketing one.

The limit is the same one that constrains combi boilers: instantaneous flow. The unit can deliver only as much hot water per minute as its burner can heat, and the delivery rate falls in winter as the incoming supply gets colder. A large tub fill or two simultaneous showers can exceed the unit's rate, and the water temperature at the fixture starts to fall. Tankless sizing is therefore a flow calculation against the household's largest realistic simultaneous demand, made with the winter supply temperature, not a volume comparison against the tank's gallons.

Both manufacturers and well-meaning sellers quote delivery figures that only apply under stated conditions, and the honest way to compare the two arrangements is to test both against the same household moment: the largest realistic simultaneous draw, on a cold January day. For the tank, the question is whether the stored volume plus the boiler's recovery rate covers the household's morning pattern. For the tankless unit, the question is whether its continuous delivery at the winter supply temperature covers the peak single moment, because on-demand units have no reserve to fall back on.

It is also fair to notice that the two arrangements answer different questions about the heating system. An indirect tank assumes a boiler is already in the building and simply extends it, which is why a home replacing a boiler often keeps the sound indirect tank rather than adding a second appliance. A tankless water heater answers the hot-water question independently, which suits homes whose heating is electric, a heat pump, or a boiler that the owner does not want to enlarge the job around.

The practical test is the draw pattern. Homes with several bathrooms, a large tub or overlapping users will find the tank's reserve the difference between a comfortable morning and a cold one; compact households with sequential draws can be served faithfully by the tankless unit, and gain back the floor space. When both arrangements are plausible, the tie-breakers are the state of the existing boiler and tank, the available space, and whether the owner wants one appliance or two to maintain.

Either way, the gas work is regulated the same way: in BC, installation of the boiler, the tankless heater or the tank's piping requires gas-permit and inspection processes under Technical Safety BC, homeowners in single-family homes may hold the permit in the cases Technical Safety BC describes, and strata owners use a licensed contractor. On this site the consultation service works through draw patterns, space and the existing system before equipment is proposed; the installation and replacement services, delivered with ROMA Heating & Cooling in Greater Vancouver, cover both arrangements, and the instant estimator gives a ballpark range for each path.