Leaking Water
First question decides the call: 90%+ condensing furnace (which is supposed to make water and the leak is a drainage problem), or 80% standard furnace (which shouldn't be making water at all — water is usually tracking from above, or it's a venting problem).
- 90%+ condensate path failure — clogged trap, blocked drain, wrong pitch, failed pump
- On 80% units: water tracking down from AC coil above, humidifier, or plumbing — not the furnace at all
- On 80% units: actual furnace water = venting or combustion problem (serious)
- Heat exchanger or collector failure — only after drain path is fully ruled out
- 1
Identify the furnace type — PVC + visible trap = 90%+, metal B-vent + no condensate = 80%
This one-second ID changes everything that follows.
- 2
Dry the area, run a heat call, and find the highest wet point with a flashlight and mirror
If the top of the cabinet is wet before anything inside is, the source is above — coil, humidifier, plumbing, roof. Don't diagnose a furnace that isn't actually leaking.
- 3
Check pitch — drain lines at 1/4" per foot toward drain, 90+ horizontal vent at 1/4" per foot back toward the furnace
Sags, double traps, and bellies cause a lot of intermittent leaks.
- 4
On 90%+, pull and flush the trap, inspect every port, verify orientation/height, prime before restart
Partially clogged trap is the single most common 90+ leak — and it causes callbacks because it runs fine after reset until condensate backs up again.
- 5
Clear the drain line downstream — flush, wet-vac, check for slime, sags, double traps, freeze risk on outdoor terminations
Trap can be clean and the leak still comes back if the line downstream is blocked or routed wrong.
- 6
Inspect the collector box, inducer housing, and internal drain paths for standing water or rust trails
Standing water inside means condensate is backing up internally — points back to trap/drain/pitch, not 'replace the inducer.'
- 7
If there's a condensate pump, fill the reservoir manually and watch float, discharge, and check valve under load
Stuck float or bad check valve looks exactly like a furnace leak. Pump must be rated for acidic condensate.
- 8
On 80% with confirmed furnace-source water, check venting and flue temp (~300-400°F healthy)
Below 300°F suggests underfiring, oversized venting, or excessive heat extraction — condensation inside the vent corrodes the exchanger. Safety problem, not a drain problem.
If the trap is clean, drain is clear, pitch is right, vent is good, and water is still coming from inside the cabinet at a header or collector — you're past maintenance and into repair-or-replace with a safety conversation. On 80% units with confirmed furnace-source water, treat as a venting/combustion safety issue, not a drain problem.
Water from an 80% furnace itself isn't a nuisance leak — it points to flue gas condensation, which corrodes the heat exchanger and is a combustion/CO concern.