An 80% furnace and a 96% furnace burn gas the same way. The 96% pulls more heat out of the exhaust, and that changes the vent, the drain and sometimes the water heater.
The 80% furnace and its chimney
The exhaust leaves hot, often above 300°F, and rises on its own through a metal vent. In most Denver houses that vent goes into a brick chimney shared with the water heater. The furnace takes its combustion air from the basement.
If you keep an 80% furnace, you are counting on the chimney you have. Have the liner, the cap and the joint where the vent meets the brick checked.
The 96% furnace and its two pipes
A condensing furnace has a second heat exchanger that cools the exhaust to around 100 to 120°F. Water condenses out of it, so the vent is plastic and the furnace needs a drain.
- Two PVC pipes. One brings outdoor air to the burners and the other carries exhaust out, usually through a side wall, sometimes through the roof.
- Clearances. The pipe ends sit at least a foot above the ground and the usual snow, and away from windows, doors and the gas meter.
- Condensate. A few gallons of mildly acidic water on a cold day, piped to a floor drain or a small pump.
What happens to the water heater
When the furnace leaves the chimney, the water heater vents alone into a flue built for two. Its small, hot exhaust can't warm that much cold masonry, so it drafts poorly and can spill fumes at the draft hood. The fix is usually a 3- or 4-inch metal liner. It belongs in the furnace quote.
Send photos of where the vent leaves the furnace and where it leaves the house, or schedule an estimate.
Common questions
Can a 96% furnace use my chimney?
Not as its vent. The exhaust is cool and wet and has to run in the plastic pipe the manual lists. Some manuals let that pipe run up an empty chimney, as long as nothing else vents into it.