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If a thimble is not used, all combustible material must be cut away to provide a 6 inch clearance. Any material used to close the opening must be noncombustible. Heater Parts from ACF Greenhouses INSTALLATION Horizontal Two Pipe Venting 1. When horizontal vents pass through a combustible wall (up to 8 inches thick), the vent passage must be constructed and insulated as shown in Figure 10. 2. The termination of horizontally vented system must extend 12 inches beyond the exterior surface of an ext