Furnace Leaking Water? What It Means and What to Do

First: protect the house and the furnace

If water is near the furnace's wiring or spreading across the floor, turn the system off at the thermostat. Soak up the standing water with towels to protect the floor and keep mold from starting. Don't open the furnace cabinet or reach into it.

Is your furnace a high-efficiency model?

Look at the vent pipes. If the furnace vents through white plastic (PVC) pipes, it's a high-efficiency condensing furnace, usually rated 90% efficient or higher. It pulls so much heat out of the exhaust that water condenses inside it, and that water drains away through a small hose or pipe, a trap and sometimes a pump. A working condensing furnace makes water every time it runs. It just shouldn't end up on the floor.

If the furnace vents through a metal flue pipe, it's a standard-efficiency furnace. It isn't designed to make water, so a leak points somewhere else: the air conditioner coil, a humidifier, nearby plumbing or a venting problem.

The most common causes

• A clogged condensate drain or trap. Dirt, sludge and algae build up and the water backs up. This is the most common cause on high-efficiency furnaces.
• A failed condensate pump. Many basement and closet furnaces pump the water to a drain. If the pump is unplugged, broken or its outlet hose is blocked, the reservoir overflows.
• A frozen drain line. On the coldest nights, a drain line that runs outside or through a cold crawlspace can freeze and back the water up.
• A loose or cracked drain connection. Common after a furnace has been worked on or bumped.
• The AC coil above the furnace. In summer, water around the furnace usually comes from the air conditioner's coil, which sits on top of it. A clogged AC drain or a frozen coil from a dirty filter are the usual causes.
• A whole-house humidifier. A cracked housing, loose fitting or blocked drain can drip down onto or beside the furnace.
• Something that isn't the furnace. A water heater, floor drain or nearby plumbing can look like a furnace leak.

Safe checks you can do

• Replace the filter if it's dirty. It won't clear a clogged drain, but a clogged filter can freeze the AC coil in summer.
• Follow the drain line you can see from the furnace to where it ends. Look for kinks, disconnections or ice. Never use an open flame to thaw a frozen line.
• If there's a condensate pump (a small box with a hose, usually next to the furnace), make sure it's plugged in and the hose isn't kinked.
• If water seems to come from a humidifier, close the small water valve that feeds it, if you can do that safely.
• Check that the water isn't coming from the water heater or other plumbing nearby.

Why it shouldn't wait

On many high-efficiency furnaces, a backed-up drain trips a pressure switch or float switch that shuts the heat off, so a drain problem can turn into a no-heat call on a cold night. Water left sitting can also rust parts, damage floors and drywall and lead to mold. If the filter is clean and the drain looks clear but water keeps showing up, it's time for a technician.

How HVAC Doctor can help

Call or text (859) 684-3259. One of the owners, Ahmad or Ammar, answers 24/7, and nights, weekends and holidays cost the same as a weekday. Mention our online special and the diagnostic is $39 (regularly $79), credited to the repair if you go ahead, and you get a firm price before any work starts. Most Lexington calls are same-day. We'll find where the water is coming from, clear and test the drain, and check the pump and safety switches.

Common questions