A boiler can heat domestic hot water through a built-in tankless coil or a separate indirect storage tank, and the two behave quite differently.
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Quick answer
A boiler's built-in tankless coil heats domestic hot water on demand by passing cold water through a coil immersed in the boiler's hot water as it flows to the fixture, without storing any hot water in reserve, while an indirect tank is a separate, well-insulated storage tank heated by the boiler that keeps a reserve of hot water ready to use. A tankless coil setup tends to struggle when heating and hot water demand happen at the same time, since both draw on the same boiler output, whereas an indirect tank generally provides steadier, more independent hot water performance because it draws from stored reserve rather than needing the boiler to respond instantly. Homes with a built-in tankless coil experiencing inconsistent hot water, especially during the heating season, are common candidates for a technician to evaluate whether adding an indirect tank would improve performance.
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A tankless coil is built directly into the boiler itself, and domestic hot water is produced by running cold water through this coil, which sits immersed in the boiler's own heated water, transferring heat on demand as the fixture calls for hot water. There is no separate storage of hot water; the moment a tap is turned off, hot water production through the coil stops, and the next draw starts the process again from scratch, depending entirely on the boiler being hot enough at that exact moment.
An indirect tank is a separate, well-insulated storage tank plumbed to the boiler, with the boiler heating water that circulates through a coil or jacket inside the tank to bring the stored water up to temperature, which is then held in reserve and ready for use much like a standard water heater tank. Because the hot water is stored ahead of time rather than produced instantaneously, an indirect tank system is less sensitive to whatever else the boiler happens to be doing at the exact moment hot water is needed.
| Factor | Tankless coil (built into boiler) | Indirect tank |
|---|---|---|
| Storage | None, produced on demand | Reserve of hot water stored ahead of time |
| Performance during simultaneous heating and hot water demand | Can suffer, especially in cold weather | Generally more stable, draws from storage |
| Additional equipment and space needed | None, built into the boiler | Requires space for a separate tank |
| Typical fit | Smaller homes, lower simultaneous demand | Larger households or higher, more variable hot water demand |
| Recovery after heavy use | Limited by boiler's instantaneous output | Depends on tank size and boiler recovery rate together |
During Metro Vancouver's cooler, wetter months, a boiler with a tankless coil is often working to satisfy space heating calls throughout much of the day, and if a hot water draw happens at the same time the boiler is already busy heating the home, the coil can produce noticeably cooler water than it would in warmer months when the boiler sits idle more often between heating cycles. This is one of the most common complaints homeowners raise about tankless coil systems, and it is directly tied to how the coil shares the boiler's output with space heating rather than having its own independent reserve.
An indirect tank largely avoids this issue because the stored hot water reserve is generally sufficient to cover a typical draw even if the boiler happens to be busy with a heating call at that exact moment, with the boiler simply catching the tank back up to temperature once the heating call is satisfied or between calls.
A technician assessing whether to keep a tankless coil, add an indirect tank, or consider a different approach entirely looks at the boiler's actual output capacity, the household's hot water usage patterns, available mechanical room space, and how often hot water and heating demand genuinely overlap in that specific home. This is a household-specific evaluation rather than a one-size-fits-all recommendation, since a smaller household with modest hot water needs may be perfectly well served by an existing tankless coil setup even though a larger household experiencing frequent shortfalls would benefit meaningfully from adding storage.
Safety note
Adding or modifying a boiler's domestic hot water setup, including connecting an indirect tank, involves gas-fired equipment and piping that must be installed by a Technical Safety BC licensed gas fitter. If a gas smell is ever present, leave the home immediately and call FortisBC at 1-800-663-9911.
This is a common symptom of a tankless coil sharing the boiler's output with space heating; when both are calling at once, the coil can produce noticeably cooler water, which an indirect tank generally avoids by drawing from stored reserve instead.
In many cases yes, provided the boiler has adequate capacity and there's suitable space for the tank, though a licensed technician needs to assess the specific system before confirming it's a good fit.
Operating costs depend on usage patterns and boiler efficiency more than the storage method itself; the indirect tank does have standby losses from keeping water warm in reserve, which a tankless coil doesn't have since it produces hot water only on demand.
Not in the same sense of running out of a stored reserve, but its performance can degrade under simultaneous heating and hot water demand, which can feel similar to running short of hot water to the person at the tap.
Sizing depends on household size and peak simultaneous hot water needs; a licensed technician can recommend an appropriate tank size based on your specific household's usage.
Smaller homes with modest, less overlapping heating and hot water demand are sometimes well served by an existing tankless coil, though a technician's assessment of actual usage patterns is the best way to confirm this for a specific household.
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