Both black iron and CSST can feed a tankless water heater correctly, and the right choice usually comes down to routing, existing piping, and the specific install.
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Quick answer
Black iron pipe and corrugated stainless steel tubing (CSST) can both correctly supply a tankless water heater when sized and installed to code, and neither is universally better; black iron is rigid, has a long track record, and needs threaded fittings at every direction change, while CSST is flexible, needs fewer fittings, but requires electrical bonding. The right choice depends on the specific routing, existing piping in the home, and total gas demand.
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Tankless water heaters have a higher instantaneous gas input demand than most storage tank water heaters, because they're heating water on the fly rather than maintaining a reservoir. That higher demand is often exactly what forces a homeowner to reconsider gas line sizing during a tankless conversion, even if the rest of the house's gas piping has been fine for years.
Whichever material is used, the line to a tankless unit needs to be sized for that appliance's full input demand plus whatever else shares the line, verified against the pipe's capacity tables and the distance from the meter, not assumed adequate just because the old tank heater worked fine on the existing pipe.
Black iron threaded pipe is the traditional material, rigid, and has decades of installation history in Metro Vancouver homes. It's resistant to punctures and doesn't require electrical bonding the way CSST does. Its main practical downside is that every change of direction needs a threaded fitting, which means more joints, each one a potential leak point that needs testing, and more labour time on a run with several turns.
In a straightforward, mostly straight run from the meter or an existing branch to the new tankless location, black iron is often the simpler and more economical choice, especially where the existing house piping is already black iron and a fitter is extending it.
CSST's flexibility lets it be fished through joist bays and wall cavities with far fewer fittings, which matters a great deal in a retrofit where the tankless unit is going somewhere the original gas piping never reached, such as an exterior wall for a new direct-vent tankless location. Fewer fittings can mean a faster, cleaner install in a finished space where opening up drywall for a rigid run would be disruptive.
The tradeoff is that CSST must be properly bonded to the home's electrical grounding system, and its thinner wall makes routing and support details more important than with rigid pipe. In a home already fitted with CSST elsewhere, extending with the same material for consistency is often practical.
| Factor | Black Iron | CSST |
|---|---|---|
| Routing through finished walls | Harder, needs more access | Easier, fishes through cavities |
| Number of fittings on a bent run | More, one per direction change | Fewer, flexes around turns |
| Electrical bonding required | No | Yes, per code |
| Puncture resistance | Higher, rigid wall | Lower, thinner corrugated wall |
| Typical use case | Straight runs, matching existing black iron system | Retrofits reaching new locations with tight access |
In practice, the choice usually comes down to the specific route from the gas source to the new tankless location, what piping already exists in the home, and how much disruption opening walls for a rigid run would cause versus fishing flexible tubing. Either material, correctly sized and installed by a licensed gas fitter, will supply a tankless unit safely and reliably.
If a fitter recommends CSST for your job, confirm bonding is included in the quote. If black iron is recommended for a route with several turns, ask how many fittings are involved and whether that changes the labour estimate, since it's a legitimate factor in project cost even without giving you a specific dollar figure here.
Safety note
If you ever smell gas, hear a hissing sound near a gas line or appliance, or suspect a leak, leave the home immediately and call from outside. Do not flip switches, light anything, or use a phone inside the house. Call FortisBC at 1-800-663-9911 to report a suspected gas leak or carbon monoxide concern before doing anything else. Gas piping, venting, and combustion appliance work in BC must be done by a Technical Safety BC licensed gas fitter; CANRO holds gas licence LGA0043870 and boiler licence LBP0046450.
It depends on the route. CSST can reduce labour on a run with many turns or tight access, but it adds the cost of bonding. A straight, accessible run often favours black iron. Costs vary by job and should come from a site-specific quote.
Yes, transitions between the two materials are common in retrofits, using appropriate fittings at the transition point, as long as sizing and bonding requirements are met throughout.
Often yes, because tankless units have higher instantaneous input demand. Whether your existing line is adequate depends on its size, length, and what else it supplies, which is exactly what a load calculation determines.
Both materials have long service lives when installed and, in CSST's case, bonded correctly. CSST's specific vulnerability is to electrical arcing, which bonding addresses, not general wear from gas flow.
Your licensed gas fitter, based on the routing, existing piping, and total load calculation for your specific home, recommends the material and confirms it during the quote and permit process.
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