Why condensing furnaces vent in PVC
An 80% AFUE non-condensing furnace exhausts combustion gases at roughly 350–500°F — hot enough to need metal (galvanized steel B-vent) and to rise naturally up a chimney by draft. A 95% AFUE condensing furnace extracts most of that heat and vents gases at closer to 100–130°F — cool enough for PVC, but too cool to rise on their own.
Instead of a chimney, a condensing furnace uses a sealed PVC vent (typically 2–3 inch Schedule 40) pushed by the furnace's draft inducer blower. The vent exits horizontally through a side wall rather than vertically through the roof, which has clearance requirements from windows, doors, mechanical intakes, and neighboring properties.
Where the condensate goes
A condensing furnace produces roughly a gallon of liquid condensate per 100,000 BTU-hours of input. On a cold day the furnace may generate several gallons. That water is slightly acidic (pH ~3–5) from dissolved combustion products and must drain to an approved location.
Typical acceptable terminations: a nearby floor drain, a condensate pump that lifts it to a laundry standpipe or utility sink, or a dedicated condensate drain line routed to the home's main waste plumbing with proper trapping. Some jurisdictions require a neutralizer cartridge before the condensate enters the drain system; we install one where it's required and often where it isn't, because it extends the life of iron waste pipes.
Combustion-air routing — direct-vent (sealed) vs. single-pipe
A condensing furnace can be installed two ways:
- Direct-vent (two-pipe) — one PVC pipe brings combustion air in from outside, another pipe exhausts gases out. The furnace is sealed from house air entirely. Preferred for tight new homes where pulling combustion air from the room would depressurize the house.
- Single-pipe — one PVC exhausts gases; combustion air comes from the surrounding room (garage, utility closet, basement). Simpler install but only acceptable where the room has adequate make-up air by code.
When a condensing retrofit gets complicated
The complexity shows up in retrofits where the existing furnace was 80% venting through a shared masonry chimney, often with the water heater. The old B-vent liner relied on heat from both appliances to stay drafting; removing the furnace's contribution can orphan the water heater and cause its gases to spill back into the home.
The install options are: reline the chimney for the water heater alone (usually a stainless-steel liner), run a new side-wall PVC for the furnace while keeping the water heater on the original chimney, or replace the water heater with a direct-vent unit at the same time. We walk through this on quote; it's a case where 'just replace the furnace' isn't actually just that.
Common questions
Can my old chimney vent a new condensing furnace?
Not as-is. The condensing furnace vents cooler, wetter gases that will damage a masonry or metal chimney over time. If the existing chimney is being kept for another appliance (water heater, fireplace), the furnace typically gets its own PVC side-wall vent run.
What are the clearances for the side-wall vent?
Codes (and manufacturers) specify minimum distances from windows, doors, soffit vents, air intakes, and gas meters. For most 2–3 inch PVC vents the critical ones are 12 inches above grade (clear of snow), 4 feet from any air intake, and 3 feet from mechanical equipment inlets. Your installer verifies the specific manufacturer spec against local code.
Why do some houses stay at 80% AFUE instead of upgrading to 95%?
Usually one of three reasons: no practical path for a side-wall PVC vent (interior equipment with no exterior wall nearby), no acceptable condensate termination without major plumbing work, or a shared-chimney retrofit situation where the water heater needs to be addressed at the same time. Any of those can push the overall project cost high enough that 80% equipment makes more sense.
Does the condensate pump need its own circuit?
It needs a 120V outlet nearby. Most installers add a dedicated outlet at the furnace during install if one isn't already there, since the pump needs reliable power — a tripped breaker on a condensing furnace means condensate backs up and the furnace locks out on a float-switch safety.