Horizontal vent pipes for appliances, such as a water heater, should go up slightly as they go toward outdoors, as shown below. This prevents CO from leaking if the joints or pipes aren't fitted tightly. The oxidation state of carbon in carbon monoxide is +2 in each of these structures. https://martinrhsmd.post-blogs.com/50430426/the-definitive-guide-to-co-the-best-ranch-homes-for-sale