How to horizontally vent a gas water heater?

About horizontal venting... you didn't specify is your tankless water heater DV direct vent or PV power vent.

When installing a tankless water heater always refer to the manufacturer's guide and local or national codes and standards. This applies on venting also. All the components for horizontal and vertical venting must be certified and terminated outside.

- Most tankless water heaters are required by code to use Stainless Steel Category III venting line.

- Horizontal venting should be as direct as possible. Avoid dips or sags and support the horizontal vent every four feet and per instructions. Vent diameter must not be reduced.

- Vent connections must be air tight.

- When water heater is vented horizontally, the vent pipe can have the upward or downward slope. Slope horizontal venting 1/4" per foot toward the exhaust terminal if not using the condensate collector, or toward the water heater if used. Condensate collector is necessary if more than one elbows are used in the vertical section of the horizontal run. Condensate must drain away from the water heater not back into any part of the heater.

- The air intake must also angle, recommended is 1/4" per foot, to prevent entry of rain, and at least 12" above grade or snow level.

Check the vent for cracks.

Follow the manufacturer instruction to provide clearances to windows, balconies, porch, deck, to air supply, above sidewalk...