Why is my high-efficiency furnace leaking water?
Updated August 2026
Water pooling under a furnace almost always means a clogged condensate drain line or trap on a high-efficiency (condensing) furnace, backing up the acidic water it normally sheds. If your furnace vents through a white PVC pipe, it is a condensing unit and the drain is the first suspect. A standard-efficiency furnace with a metal flue should not make condensate at all, so on those the water is coming from somewhere else. Match the symptom to the list below before you start mopping.
Safety first: condensate from a high-efficiency furnace is mildly acidic, so wipe up pooled water and dry the cabinet base to keep it from rusting the burners and heat exchanger from below. A slow leak left for weeks warps subfloor, feeds mold, and can short the control board. Keep water away from the electrical box and the igniter, and if the leak is soaking drywall or the platform, shut the furnace off at its switch until it is repaired.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Clogged condensate drain line or trap | Most common | Clear the P-trap and tubing with a wet/dry vac or a vinegar flush so the condensate drains freely again. | Free to $250 |
| Failed condensate pump | Common | The pump that lifts condensate to a drain quit; test the float and replace the pump if it is dead. | $50–150 part, $150–300 installed |
| Cracked drain trap or loose tubing | Occasional | Reseat or replace the split trap and reconnect the drain hose to the collector box. | $20–150 |
| Clogged AC coil drain above the furnace | Occasional | If the evaporator coil sits above the furnace, its clogged drain drips down in cooling season; clear that line. | $75–250 |
| Flue or humidifier leak (standard-efficiency) | Rare | On an 80% furnace, water usually comes from a corroded flue or a leaking humidifier line, not a condensate drain. | $100–400 |
Check these first
- Look at the vent pipe: white PVC means a condensing furnace with a condensate drain, metal means a standard-efficiency unit that should not make condensate.
- Follow the clear drain hose to the P-trap and look for standing water or green algae sludge blocking it.
- If there is a small condensate pump, pour a cup of water in and confirm it kicks on and empties.
- Check whether the AC coil sits on top of the furnace, since in summer its clogged drain can drip down and look like a furnace leak.
- Note the season: a leak only in heating season points at furnace condensate, a leak only in summer points at the AC coil above it.
Prevent it next time
A condensing furnace can shed several gallons of water a day in a cold snap, so keeping that path clear is the whole game. Flush the condensate line and trap with a cup of vinegar at the start of every heating season, the same visit you run the pre-winter furnace check. Fold it into the annual burner service so a tech also inspects the trap, collector box, and pump. If the cabinet has been sitting in water for a while, weigh the furnace's remaining life before pouring money in, and see when replacement makes sense.
When to call a pro
Call a pro when the drain is clear but water keeps appearing, when you find rust or water inside the burner compartment, or when the leak traces to the heat exchanger or collector box rather than the drain. A drain clearing runs $75–250, a condensate pump $150–300 installed, and a collector-box or trap repair $200–500. A cracked heat exchanger weeping water is a replace-the-furnace conversation, not a patch.
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Stop chasing symptoms, free for 30 daysFrequently asked questions
Why is my furnace leaking water only in winter?
That points straight at the condensate drain on a high-efficiency furnace. A condensing unit pulls so much heat out of the exhaust that the water vapor cools and condenses inside, draining several gallons a day when it runs hard. If the trap or line is clogged with algae, that water backs up and spills onto the floor. A leak that shows up only in heating season, from a furnace with a white PVC vent, is a condensate problem nine times out of ten.
Can a leaking furnace be dangerous?
The water itself is mildly acidic and will rust the cabinet, feed mold, and short the control board or igniter if it reaches the electrical box, so it is worth fixing fast. The larger concern is what a leak can signal: a cracked heat exchanger or a corroded collector box on an older furnace can weep water and also leak combustion gases. Keep a carbon-monoxide detector on every level, and have a tech confirm the source if the drain checks out clear.
How do I unclog a furnace condensate line myself?
Turn the furnace off first. Find the P-trap on the clear drain hose, pull it or open its cleanout, and flush it with warm water to knock the algae sludge loose. Then attach a wet/dry vac to the drain's exit end and pull the clog through; a cup of distilled vinegar poured in afterward slows regrowth. Reconnect everything and pour water in to confirm it drains. If it clogs again within weeks, the line likely lacks slope and needs a pro to re-pitch.
Does a standard-efficiency furnace leak water too?
It should not make condensate, so water under an 80% AFUE furnace with a metal flue comes from somewhere else. The usual sources are a whole-house humidifier leaking its supply or drain line, a clogged AC coil drain when the coil sits above the furnace in summer, or flue-gas condensation from an oversized or poorly vented flue. Trace the water back uphill to its origin, because on a non-condensing furnace the drain trap is not the answer.