What size gas line does a tankless water heater need?

Often, yes. Rinnai says its tankless water heaters are "designed to provide maximum performance and efficiency using either a 3/4" or 1/2" gas line, as long as sufficient gas flow is available." The catch is the last part of that sentence. Here is Rinnai's own table for natural gas, giving the longest equivalent pipe length each size can serve:

Rinnai bases that table on a supply of 8.0 inches of water column or more and an allowed pressure drop of 3.0 inches. If the pressure at your house is lower, those lengths shrink, so a plumber measures it before trusting them. Rinnai also notes that going tankless does not always mean upsizing the line, because its gas valve compensates for pressure within limits.

The fuel gas code table Phoenix uses is far more conservative. Table 402.4(1), for Schedule 40 steel pipe with natural gas under 2 psi and a 0.3 inch drop, lists these capacities in cubic feet per hour:

Read it this way. A 199,000 BTU heater needs about 199 cubic feet per hour. On that table, 3/4 inch pipe carries 273 at 10 feet but only 188 at 20 feet, so it falls short. One inch pipe carries 243 at 40 feet and 195 at 60 feet. The two tables differ because they assume very different pressure drops. Our page on the longest length method explains how these tables are read and how fittings add length, and the same specific gravity of 0.60 sits behind both.

The heater runs short of gas when demand is highest. Rinnai warns that "an undersized gas line can stress a tankless system and possibly reduce its lifespan," and its units shut down when gas flow is inadequate. In the house that can look like hot water that turns cold during a shower, error codes when two showers run, or a heater that works fine in spring and struggles in the coldest weeks. Our page on a tankless heater going cold mid-shower covers the other causes, so a gas problem is not assumed.

If it does, the shared section has to carry every appliance at once. The code says the flow to plan for is the sum of the maximum input of every appliance on the line, all running at the same time. So add the BTU figures from the rating plates on the tankless unit, the furnace, the range, the dryer, and anything else on that branch. A 199,000 BTU heater added to an older home can double its total load, which is why the meter matters too. The meter and the pipe up to it belong to Southwest Gas, so if the total outgrows it, a bigger meter is a request to the utility. The pipe from the meter to your appliances is yours. Our guide on gas line sizing for a pool heater walks through the same meter check for another big load.

It is on the rating plate on the side of the unit, along with the gas type and the inlet pressure range the maker allows. The installation manual has the sizing table for that exact model. Check that the plate says natural gas, since propane models use different tables and orifices.

Usually, yes. Phoenix's gas clearance table lists an "appliance upgrade with an increase in BTU consumption requiring piping capacity calculations" as permit work, with a city inspection and a gas clearance. Going from a 40,000 BTU tank to a 199,000 BTU tankless is that kind of upgrade. A replacement in kind with no piping changes does not need one. Any new or upsized gas pipe is then pressure tested before the city signs off.

Before you buy, it helps to know the trade-offs in the downsides of tankless water heaters and whether the unit should go inside or outside. HQ Plumbing & Air installs tankless units indoors or outdoors across the Phoenix area, sizes and runs the gas line, and handles the permit, inspection, and clearance. See our water heater services for installs and repairs.