The size of the gas line required is based on the consumption of the appliance, the total consumption of gas in the house, the size of the gas line, the type of gas (Natural Gas or Propane) and the distance from the meter. There is a calculation that can be done (it is listed in the installation manuals of the tankless unit you would like to use). The new tankless gas water heaters can operate on 1/2" gas lines* (based on specific requirements), mainly because they can operate on much lower gas pressure than typical appliances. We have found that most houses can get a tankless water heater installed without upgradeing the gas line.