Most of the talk about a wall-hung combi goes to the heat exchanger, the venting, and the condensate line. The gas pipe gets less attention, and it is often the part that decides whether the first cold morning goes smoothly. A combi asks one burner to carry both space heat and domestic hot water, so the gas line has to be checked against the new appliance, not the boiler it replaces.
A current ACHR News story on Navien's NCB700 gives a useful set of numbers to start with. The summary describes the unit as "providing heating and hot water performance in a single wall-hung unit," and lists space-heating inputs of up to 150,000 Btuh and domestic hot water inputs of up to 210,000 Btuh. The full piece is in ACHR News's report on the NCB700, but the inputs are the figures that matter for your gas line.
Turn Btuh Into Cubic Feet
Btuh is a rate of heat energy, and your gas meter counts volume. To bridge the two, divide the input by the heating value of your gas. Many utilities deliver gas near 1,000 Btu per cubic foot, but yours may differ, so look for the figure on your bill or ask the utility. At 1,000 Btu per cubic foot, 210,000 Btuh works out to about 210 cubic feet per hour, and 150,000 Btuh works out to about 150 cubic feet per hour.
The manufacturer lists more than one input, and the manual says which one applies to which mode. Do not assume the space-heating figure is the governing one. Size the pipe to the highest listed input for the appliance, then add the flow for every other gas appliance on the same line, such as the range, the dryer, or a fireplace. That sum is what the run has to carry at full demand without the pressure at the burner dropping.
Check the Run Before the Crew Cuts Pipe
Pipe sizing tables depend on the pipe material, the total length from the meter including fittings and any vertical rise, and the pressure the utility supplies. Use the table your local code adopts, and have the calculation checked by the licensed plumber or gas fitter who will do the work, since many jurisdictions require a licensed installer and a permit for gas piping changes. Ask them to show the run length they used. A line that looks adequate on a drawing can come up short once the real bends and the meter location are counted.
You can also make a field check yourself, at the meter, with the other gas appliances off. Find the test dial on the meter and read its marked volume per revolution, since common sizes are one or two cubic feet. Fire the combi at full demand, for example by running hot water at a steady flow and calling for heat if the system allows, then time one full revolution with a stopwatch. Cubic feet per hour equals the dial volume times 3,600, divided by the seconds per revolution. Compare that result with the input from the manual, converted at your heating value. If the meter flow is well below the appliance requirement, the gas supply is the first suspect, not the burner.
Where the Gas Line Meets the Vent
The gas run and the vent often share a wall or a roof penetration, and that is where a rushed install does the most damage. If the vent route has to pass through the roof rather than the wall, a roofing contractor such as Keys Roofing can handle the penetration flashing so the roof stays sealed around it, while the gas and venting work stays with the licensed trades. Keep the roof work and the gas work on separate invoices and separate permits so each part of the job has a clear owner.
The short version: take the input from the manual, convert it to cubic feet per hour with your utility's heating value, size the line to the full load plus the rest of the house, and verify at the meter before the first cold week. The NCB700 numbers make that calculation concrete, and the meter check turns a paper estimate into a measured one.