A heating plant requires 4,000 MBH (1,000 Btuh) of heating capacity. Select the approximate size of pipe for the main if the plant is a 200\deg F hot water system, 5-psig low-pressure syeam syste, or a 150-psig high-pressure steam system. Using natural gas with heating content of 1,000 Btu/cf, what should be the gas flow rate (cf/hr)? How much combustion air is required with 25% excess air?