Tankless water heaters draw far more gas at peak firing than a tank, which means the existing gas line often needs to be resized before one goes in.
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Quick answer
A tankless water heater fires at a much higher BTU input than a standard tank because it heats water on demand rather than keeping a reservoir hot. The existing gas line sized for a tank unit is frequently too small to feed a tankless unit at full output, so a licensed gas fitter has to calculate the total connected load and confirm or upsize the pipe before installation. Skipping this step is one of the most common causes of a tankless unit that underperforms or throws flow-related lockouts.
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A typical residential gas tank water heater has a fairly modest burner, because it only needs to reheat a stored volume of water gradually. A tankless unit works completely differently: it has no reservoir, so it has to heat water instantly as it flows through, which takes a much larger burner and a much higher gas input rating to hit the flow rates homeowners expect for showers, laundry, and dishes running together.
That jump in BTU input is the whole reason gas line sizing becomes a real consideration rather than an afterthought. A pipe diameter that comfortably fed a tank water heater for fifteen years may be undersized the moment you connect a tankless unit rated well above what that pipe run and length can deliver at full demand.
Sizing isn't just about the new appliance in isolation. A licensed gas fitter adds up the total connected load of every gas appliance on that line or fed from that meter — furnace or boiler, range, dryer, fireplace, and the new tankless unit — then works out the pipe diameter needed to deliver that combined load at an acceptable pressure drop over the actual length and number of fittings in the run. Longer runs and more elbows both reduce how much gas a given pipe diameter can deliver, which is why two houses with the same appliances can still need different pipe sizes.
This calculation follows sizing tables and methods set out in the gas code (CSA B149.1), not rules of thumb. It's also why a proper quote for a tankless conversion should include an assessment of the existing gas piping route, not just the appliance itself.
Sometimes the interior gas piping can be upsized to handle a tankless unit, but the gas meter FortisBC has installed at the house can't support the new total load across all appliances. In that case, a gas meter upgrade has to be requested and scheduled with FortisBC before or alongside the tankless installation. This adds lead time to a project, so it's worth flagging early rather than discovering it partway through a renovation timeline.
A lot of the housing stock in this region — Vancouver Specials, older character homes, and many post-war bungalows — was plumbed for gas at a time when a single, modestly sized tank water heater and maybe a furnace were the only loads on the line. When a homeowner wants to add a tankless unit, especially alongside a suite or laneway house with its own hot water demand, the original piping is very often undersized for the new combined load. Longer basement-to-mechanical-room runs in these homes, sometimes with several 90-degree turns, also eat into capacity more than a short, straight run would.
Newer builds and recently renovated infill homes are more often sized generously from the start, but it's still worth confirming rather than assuming, especially if a suite or secondary appliance was added after the original gas plumbing was roughed in.
An undersized gas line starves the tankless unit of fuel at peak demand. Symptoms include the unit failing to reach set temperature when multiple fixtures run at once, temperature fluctuating under load, or the unit locking out on a flow or ignition-related fault under demand it should otherwise handle. These aren't defects in the appliance — they're a mismatch between the appliance's needs and the gas supply feeding it, and no amount of adjusting the unit's settings fixes an undersized pipe.
| Appliance | Typical relative gas demand |
|---|---|
| Standard tank water heater | Lower, steady burner |
| Tankless water heater | Significantly higher peak burner, on demand |
| Combi boiler (space heat + hot water) | Highest, sized for both loads |
Safety note
Gas piping work, including resizing a line for a new appliance, must be performed and pressure-tested by a Technical Safety BC licensed gas fitter under a gas permit. If you ever smell gas, leave the home immediately and call FortisBC's emergency line at 1-800-663-9911 from a safe location.
You generally can't tell by looking at the pipe. A licensed gas fitter needs to calculate the total connected gas load and the pipe's capacity over its actual length and fittings, then compare that against the tankless unit's rated input.
It varies with the length of run and how accessible the piping is. Tankless install costs generally run in the $3,500–$7,500 ballpark, and gas line work, if needed, is part of that overall assessment rather than a separate guess — a technician can only confirm cost after inspecting your specific setup.
Possibly, but 'smaller' tankless units still typically draw more than a tank heater did. The only way to know is to have the total connected load calculated against the existing pipe size and run length.
Only if a gas meter upgrade is required to support the new total load. Interior gas line resizing is handled by your licensed gas fitter, but the meter and service line up to it belong to FortisBC.
Inconsistent temperature when more than one fixture runs, the unit failing to reach set temperature under demand, or intermittent lockouts during high-use periods are common signs worth having checked.
It should be rechecked any time you add another gas appliance to the same line — for example, adding a gas range or a secondary unit for a suite — since that changes the total connected load the pipe has to support.
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