Condensing appliances and plastic venting. Episode eighteen of the Gas Service Animated series.

A condensing appliance refuses to send good heat up the flue. After the primary heat exchanger, the flue gas passes through a secondary heat exchanger that pulls out so much of the remaining heat that the water vapor in the exhaust condenses into liquid. That is where the extra efficiency comes from, and it leaves the flue gas cool enough for plastic pipe to handle. Cool exhaust, a steady trickle of condensate, and high efficiency are all the same story.

Remember the category map. Category four means two things at once. The vent operates under positive static pressure, and the flue gas runs cool enough that condensate forms in the vent. High efficiency appliances usually live in this box. Positive pressure plus condensate is the combination that sets every rule in the rest of this episode.

Section twelve point five sets the material rules. The appliance must be listed for use with plastic venting, and the appliance manufacturer's installation instructions identify the specific plastic pipe allowed. That material carries a label to the product standard the manufacturer names, or is listed and labeled as plastic venting for these appliance categories. Joints go together exactly as the instructions require. And where primer is required, it must be a colored primer, so anyone inspecting the vent can see that each joint was properly prepared before it was cemented.

Positive pressure changes everything. A natural draft vent gently pulls flue gas upward, but a category four appliance pushes it under pressure. Never connect a positive pressure appliance into a natural draft vent, or into a common vent serving natural draft appliances, because pressurized flue gas can be forced out of every joint and draft hood in that system. The exhaust path must be sealed gas tight from the appliance collar all the way to the termination.

All that liquid has to go somewhere. Section twelve point ten requires a means to collect and dispose of the condensate from venting systems serving condensing appliances. Drains are installed following the appliance and vent manufacturers' installation instructions. Low points in the vent get drainage, not puddles, and in cold climates the condensate line needs protection from freezing. Where the condensate may discharge is controlled by local rules, so route it only where the authority having jurisdiction approves.

Some appliances vent with help from a fan. An induced draft fan pulls the flue gas through the appliance, while a forced draft fan pushes it. Mechanical draft systems must be listed draft equipment, installed to both the appliance and the draft system manufacturer's instructions, and their terminations follow the clearance rules from the termination episode. And here is the safety core of section twelve point four: when a mechanical draft system is used, gas flow to the main burners must be prevented whenever the draft system is not performing. No proven draft, no fire.

Plastic vent runs live or die by slope and support. The manufacturer's instructions set the pitch, and for a condensing appliance the horizontal runs slope back toward the appliance, so condensate returns to the built in drain instead of pooling in the pipe. Supports go in at the spacing the instructions call for, because unsupported plastic sags over time. A sag becomes a water trap, and a water trap chokes the vent the appliance depends on.

So on a service call, the whole system gets verified. The vent material matches the appliance nameplate and manual. Every joint shows colored primer where required and sits fully seated in its fitting. The run slopes back to the appliance with no sags, the termination is in a legal location, and the condensate is flowing to an approved drain. And the draft interlock is proven, because when the draft goes away, the burner must drop out.

Category four appliances squeeze heat until the flue runs cool and wet, and every rule follows from that. Listed plastic vent, colored primer, joints fully seated, gas tight the whole way. Slope back, drain the condensate, and prove the draft before the burner fires. Next episode: know your pipe.