Furnace leaking water: where it comes from and what to do

Water under a furnace has exactly four sources. Which one depends mostly on whether your exhaust pipe is metal or plastic.

Condensate drain (high-efficiency, PVC exhaust)

Condensing furnaces make water constantly; it leaves through a small trap and hose. Algae clogs the trap, water backs up and out. Find the trap at the unit's base, pull it, flush it clean. Ten-minute fix, no parts.

Condensate pump

If the drain runs to a small box that pumps water up and away, that pump's float sticks or the pump dies. Pumps are ~$50 and swap with two hose connections and a plug.

The AC coil above the furnace

If it leaks in summer or the shared coil drain pan rusted through, water drips down onto the furnace. The furnace is the victim, not the cause — this is an AC-side repair.

Flue condensation (metal exhaust, 80% furnaces)

A conventional furnace venting into a too-large or cold chimney condenses exhaust that runs back down. Needs a technician to assess the flue sizing or add a liner; ignoring it rusts the furnace from the top down.

Is the furnace flashing a light?

If the burners will not light, the control board is almost certainly flashing a diagnostic code behind the little window. Count the flashes and look the code up by brand here — walking into the phone call knowing the code saves diagnostic time.

Which code your brand shows for this

Water and fault codes meet at the pressure switch. On a condensing furnace, the same blocked condensate path that puts water on the floor also stops the pressure switch from proving draft — which is why a leak and a no-heat code so often arrive together. Every control family has a code for the switch that will not close:

If the furnace is leaking and flashing one of these, clear the condensate trap before anyone quotes a pressure switch. The switch is usually reporting the blockage, not failing.

Fix it yourself, or call?

Homeowner territory: pulling and flushing the condensate trap, checking a condensate pump's float and outlet hose, and wiping everything dry so you can watch where fresh water appears first. It is all water-side plumbing; none of it touches gas.

Call a technician for flue condensation on a metal-vented 80 percent furnace — that is a venting design problem, not a part — for any water that has reached the control board area, and for pressure-switch codes that persist after a clean trap. If wires are sitting in water, cut power at the furnace switch before touching anything.

What the fix typically costs

A pressure switch runs $150–400 in the repair cost guide, which carries the explicit note: check for a blocked flue and drain first. Water that reaches the electronics is the expensive branch — boards run $400–900 installed per the control board cost guide, which adds that a new board installed under the same drip dies the same death. The trap flush itself costs nothing, which is why it comes first.

Error codes behind this symptom

If your furnace is flashing a code while doing this, these code families are the usual suspects: pressure switch codes. Look your exact code up from the brand index, and see the furnace repair cost guide for what the fix typically runs.

Sources

• Carrier 58MCB Service Manual, 327884-101 Rev. B — status codes
• Goodman Manufacturing — Diagnostic Code Identification System (model-specific fault code lookup)
• International Comfort Products *9MPT and *9MPV installation instructions — Gas Valve Diagnostic Codes (third-party archive of the ICP document)
• Lennox Residential Communicating Systems Alert Code Guide, document 100017, March 2023 — Table 2, Alert Codes and Troubleshooting
• Rheem Professional Technical Support — Standard Gas Furnace Diagnostic Flash Codes
• Johnson Controls Unitary Products technical service letter ST-045-09 — York UTEC furnace boards, with the previous White-Rodgers Emerson code table reproduced for comparison