Atmospheric direct-vent and power-vent. Select either an adapter of three-inch or an adapter of four inches depending on what water heater model you decide. Gas Hot Water Heater Troubleshooting Repair Light The Pilot Gas Water Heater Components Gas Water Heater Water Heater Water Heater Repair Special stainless steel pipe. Venting tankless water heater through chimney . Special stainless steel pipe. The vent collar of the water heater must be fastened directly to an unobstructed vent pipe. All emission of liquid or gas water heaters which are tankless needs vent pipes made of stainless steel and approved by UL. The standard galvanized vent from your old storage tank heater would quickly rust away if your tankless heater vented through it. Always increase the size of the flue pipe as you go to the chimney and make sure that the flue always pitches upward to avoid trapping exhaust gases. Find Trusted Plumbers Get Free Quotes. The vent pipe for the gas tankless units...