A combi boiler replaces a separate space-heating boiler and water heater with one appliance, which changes both the floor space you need and how the home is vented.
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Quick answer
A gas combi boiler combines space heating and domestic hot water production in a single wall-hung unit, needing one gas connection and one vent termination, while a separate boiler plus a standalone gas water heater or indirect tank needs floor space for two appliances and, in many cases, two vent penetrations. Combi boilers suit smaller Metro Vancouver homes, condos converted to house-style mechanical rooms, and character houses with limited utility space, while a two-appliance setup can suit homes with higher simultaneous hot water demand or where a large storage buffer is preferred. The right choice depends on household hot water use, existing venting, and available space, which a licensed technician should assess in person.
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A combi boiler is a single wall-hung unit, typically mounted against a wall with a small clearance envelope around it, and it needs no separate floor-standing tank at all. A traditional setup with a space-heating boiler and a separate gas storage water heater, or a boiler paired with an indirect tank, needs floor space for the tank in addition to the boiler itself, plus the piping and access clearance around both. In older Vancouver-special basements or character homes with a compact mechanical room, that floor space difference is often the deciding factor, especially when a homeowner wants to reclaim area for storage or a laundry upgrade.
The trade-off is that a combi boiler has no reserve of stored hot water, so its instantaneous output has a real limit, while a tank-based setup has a buffer of already-hot water it can draw from quickly even if the burner is momentarily behind demand.
| Setup | Venting | Gas connection |
|---|---|---|
| Combi boiler | Usually one sealed-combustion vent termination through a wall or roof | One gas line sized for the combi's combined load |
| Separate boiler + gas water heater | Often two vent terminations, sometimes sharing a common vent if code and the appliances allow it | Two gas connections, each sized for its appliance |
| Boiler + indirect tank | One vent termination, since only the boiler burns gas | One gas connection sized for the boiler |
An indirect tank does not burn gas itself; it is heated by hot water circulated from the boiler, so it does not add a second vent penetration, which narrows the venting gap between a combi and a boiler-plus-indirect setup considerably. The bigger venting difference shows up when comparing a combi to a boiler paired with its own standalone gas-fired water heater.
Converting from a separate boiler and tank to a combi, or the reverse, is a bigger project than a like-for-like replacement, since it usually means new venting, new gas line sizing, and different piping layouts. It is worth discussing during any replacement conversation rather than assuming the existing configuration is the only option, but it should be planned as its own project with a full site assessment rather than decided from a general comparison alone.
Safety note
If you ever smell gas near a boiler, water heater, or vent, do not investigate further: leave the home first, then call FortisBC at 1-800-663-9911. Combustion appliances should only be serviced by a Technical Safety BC licensed gas fitter.
In most cases yes, since it replaces two appliances with one and eliminates a standalone storage tank, though the exact space saved depends on the existing layout.
Sometimes, but combi boilers often have different venting requirements than an older boiler and separate water heater, so this needs to be assessed on site rather than assumed.
A properly sized combi can handle multiple simultaneous draws, but very high simultaneous demand is where a tank-based reserve, such as an indirect tank, has an advantage.
No, an indirect tank is heated by water from the boiler and does not burn gas itself, so it does not add a second vent termination.
Costs vary by scope of venting and piping changes needed; as a ballpark, not a quote, boiler installs commonly run $6,500 to $14,000 or more depending on the setup, and a technician can scope your specific project.
Yes, but it is a significant retrofit involving new piping, venting, and possibly gas line changes, not a simple swap.
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