Why do tankless installers disagree about needing a bigger gas line?

MGI’s recommendation: Ask each installer to show the calculation. Choose the proposal that documents an adequate fuel supply for the exact installation, rather than deciding by pipe size or which quote sounds more confident.

Rinnai’s RSC manual bases gas-line sizing on the gas type, supplied pressure, pressure drop, and pipe type. It also requires the supply to handle the total gas load at the location. That makes a calculation and field verification more useful than a universal “tankless needs this diameter” rule. Rinnai RSC manual, gas supply and sizing

Give both providers the same exact proposed heater model and address. Request the assumptions behind each answer: the rated gas demand for each appliance, other connected gas appliances, existing pipe material and route, relevant lengths and fittings, supply pressure, and the applicable sizing method. The installer should explain how the address elevation and model’s installation requirements are handled.

An installer recommending no change should provide more than “the existing line looks large enough.” An installer recommending replacement should identify the deficiency the new work corrects. Different assumptions or a different route can produce different proposals, so the disagreement is worth resolving before installation.

Ask who will verify available service capacity. The utility or propane supplier may need to confirm the supply-side facts, while a qualified licensed trade owns the building’s gas-piping work. The quote should identify who coordinates those tasks and any permit or inspection requirements.

The completed job should include the required leak and operating-pressure checks, with a record of performance under the relevant demand. Do not attempt pressure measurements, regulator changes, gas-line work, or installer parameter changes yourself.

A lower price can still be the better choice when its documented design and scope are sound. A larger line can also be necessary. We would not make either conclusion from the quote total alone.

Request a revised side-by-side scope with the calculation attached, exact model, included gas work, responsible providers, and commissioning checks (startup checks that verify installed operation). Professional startup-check explanation. Confirm that the selected contractor handles your tankless model and the specific fuel-supply tasks at your address. If the calculations still conflict, an independent qualified review of the same site facts is the practical next step.