I will be dismantling chase where vent pipe for gas water heater is located. WH will be replaced with tankless system that vents horizontally through wall. How do I vent existing water heater until New system is installed. (2-3 days)
The proper way of venting natural draft water heater (assuming that this is what you have) is with a chimney / through the roof. If you are removing that chimney there is nothing else I can recommend… 2-3 days without hot water or postponing dismantling of the chimney until the last possible moment.