Weather is the quickest clue. Trouble that starts on the coldest nights and goes away when it warms up points to freezing. Trouble in mild weather points to a blockage, slope or pump problem.
Why a furnace makes water at all
Condensing gas furnaces, usually rated 90% AFUE or higher, pull so much heat out of the exhaust that water vapor turns to liquid, according to Pacific Northwest National Laboratory. That water has to leave by a gravity drain or a small condensate pump. It is not leftover air-conditioning water, and it's acidic, around pH 3 or lower.
Where lines freeze
Freezing happens where the drain passes through air below 32°F:
- A line that discharges outdoors through a wall. The Department of Energy's furnace guideline says that in cold climates furnace condensate should go to the house drain, never outdoors.
- A furnace or drain in a vented attic, which can freeze even if the end of the line is indoors.
- A run through an unheated crawlspace or garage.
- Low spots where a sagging or nearly flat pipe holds standing water.
If the drain must stay in a cold space, PNNL says it should be insulated or heat-traced. Ask whether the line can be rerouted to an indoor drain before accepting more insulation as the fix.
Why lines back up in mild weather
Without freezing, look at the drain path itself. PNNL's HVAC assessment checklist calls for 3/4-inch PVC that is properly sloped and trapped, or a working condensate pump, plus a level, leak-free pan with a working float and alarm. Common causes:
- Sludge or debris in the trap or line.
- A pump whose float is stuck, whose motor failed, or whose discharge tubing is kinked or blocked. Water around a pump doesn't prove the pump itself failed.
- A drain shared with the air conditioner's coil. A blockage past where the lines join can push water back toward the furnace.
Many systems have a water-level switch that shuts the equipment down when the pan fills. That's protection, not the problem.
The Philadelphia angle
Because furnace condensate is acidic, Philadelphia Plumbing Code P-803.1 bars corrosive liquids from the plumbing system unless they pass through an approved neutralizing device, and the Philadelphia Water Department prohibits corrosive discharges to the sewer. Ask your contractor whether your furnace has a condensate neutralizer and when its media should be replaced.
For air-conditioning condensate, the city's plumbing code Section 314 lists where a drain may empty: a floor drain, floor sink, hub drain, standpipe, utility or laundry sink, or a clothes washer box sized for both flows. In basements where the furnace and washer sit side by side, a shared washer box is common, so ask how the furnace line is tied in too.
What you can safely check
Without opening anything, note where water first appears, whether a visible pan holds water, whether tubing looks disconnected, the outdoor temperature, and any error code shown without removing a panel. Photograph exposed or uninsulated tubing and any ice. Don't open the cabinet, take the trap apart, cut tubing, force a pump float or bypass an overflow switch, and don't touch wet equipment or cords.
What to ask the HVAC contractor
- Where exactly did flow stop: trap, line, pump, or discharge point?
- Is any part of the line exposed to freezing, and can it be rerouted indoors?
- Is the slope correct, with no sags?
- Is there a neutralizer, and is the overflow or float switch working?
- Does the AC coil share this drain?
A plumber may be needed for the receiving drain, and an electrician for new pump power or heat-trace wiring. Our furnace tune-up guide covers what a yearly visit should check, and keeping snow off side-wall vents covers the other cold-weather shutdown cause on these furnaces.
When it's urgent
Call promptly for active overflow, repeated shutdowns, or no heat in freezing weather, and treat water near wiring as a safety issue. A carbon monoxide alarm is different: the CPSC and NFPA say to get outside right away, call the fire department, and stay out until firefighters say it's safe. See when a CO alarm means leave.