A high-efficiency condensing furnace is distinguished by:

A condensing furnace earns its efficiency by extracting so much heat from the flue gases that the water vapour in them condenses, releasing latent heat that a conventional furnace sends up the chimney. That single design decision produces the visible identifiers. Flue gas leaves cool enough that metal venting is unnecessary and PVC or CPVC is used instead. It leaves too cool to rise on its own, so the appliance is fan-assisted and can be vented horizontally through a sidewall. And the condensate that forms is acidic, so there is a drain, often with a trap and sometimes a neutralizer or a condensate pump. Seeing plastic venting plus a condensate line together is the reliable field signature; either alone can appear on other equipment.

PVC intake and exhaust piping together with a condensate drain are the field signature of a condensing appliance. Flue gases are cool enough for plastic venting because their heat has been extracted, and the resulting condensate must be drained away.

High-efficiency condensing furnaces recover latent heat by condensing flue gas moisture, which lowers vent temperature enough to permit plastic venting and creates acidic condensate requiring a drain. Conventional furnaces vent hot gases through metal connectors to a chimney.

Plastic Pipe and a Puddle. Cool enough for PVC, wet enough to need a drain — that pair means condensing.

Check the condensate path while you are there — a drain that terminates improperly, a disconnected trap, or a blocked line that shuts the furnace down on a safety switch are common reportable findings.

• •Identifying a furnace by efficiency label instead of by its venting and condensate
• •Mistaking a metal-vented induced-draft furnace for a condensing unit
• •Overlooking condensate drainage defects once the furnace type is identified