The liquid combustion by-products of condensing appliances shall be collected and discharged in accordance with Section 18-28-801, Plumbing. Condensate piping shall be of Type M copper or Schedule 40 PVC and shall not be smaller than the drain connection on the appliance. Such piping shall maintain a minimum horizontal slope in the direction of discharge of not less than one-eighth unit vertical in 12 units horizontal (1 percent slope).