Why summer leaks usually come from the air conditioner side
If water appears around your furnace in warm weather, the furnace itself is often not the source. In many homes with central air, the air conditioner’s evaporator coil sits on top of, or right beside, the furnace and shares its ductwork. As the air conditioner cools your home, it removes moisture from the air. That water collects in a drain pan and flows out through a condensate drain line. If something blocks or damages that path, water can spill out and run down the furnace, which makes it look like a furnace leak.
A few other sources are possible, so it is worth checking each, especially if you do not have central air. Condensing furnaces can also produce condensate, but they do so mainly when heating.
Common causes
Clogged AC condensate drain line
Algae, dust and debris can build up in the line or trap. When it blocks, water backs up and overflows the pan.
Cracked, rusted or tilted drain pan
Pans can crack or corrode over time. A pan that is not level may overflow even if the drain is clear.
Frozen evaporator coil
Restricted airflow, such as from a dirty filter or blocked vents, and some refrigerant problems can cause the coil to ice up. When the ice melts, it can produce more water than the pan can handle. Low refrigerant needs a licensed technician.
Failed condensate pump
If the drain line cannot run downhill to a drain, a pump lifts the water. If the pump fails or loses power, it can overflow.
Humidifier left running or leaking
Some furnaces have a whole-house humidifier. In summer it normally should be off, with its water supply shut. A stuck valve or leaking line can drip water.
Disconnected or damaged drain tubing
Fittings can loosen or tubing can crack and leak before water reaches the drain.
What to do right away
- Turn the air conditioner off at the thermostat so it stops producing condensate.
- Turn the furnace off too, and cut power at the breaker if you can reach it without standing in water. If the panel is in or near the wet area, call an electrician or HVAC company.
- Mop up water and run fans so the area dries quickly. Standing moisture can lead to mold and damage subfloors and drywall.
- Check your filter. A very dirty filter is a common contributor to a frozen coil.
- If you see ice on the refrigerant lines or coil, leave the system off and let it thaw fully.
Clearing the AC condensate drain
Methods vary, so follow your equipment manual. In general, with the power off:
- Find the drain line (often white PVC) near the indoor unit, and any access tee or cap on it.
- Use a wet/dry vacuum on the outdoor end of the drain line to pull out a clog, or flush the line gently with water.
- Some manuals allow a small amount of diluted vinegar in the access tee to help with slime; check yours first, and avoid harsh chemicals that could damage the pan or coil.
- Make sure the line slopes toward its outlet without sags.
If you cannot clear it or find a cracked pan, a technician can repair it. The related process for furnace drains is described in furnace pipe leaking water.
What furnace maintenance involves
Annual professional maintenance typically includes checking the condensate drain and pump, cleaning or inspecting key components, checking safety controls, and inspecting the vent and heat exchanger on gas furnaces. Cooling-season maintenance usually covers the coil, drain and refrigerant system. Homeowners should change filters on the schedule the manufacturer recommends and keep supply and return vents unblocked.
When to call a professional
- The coil keeps freezing, which can suggest a refrigerant or airflow problem.
- You find a cracked pan or cannot clear the drain.
- Water has reached electrical parts, or you see signs of rust or corrosion.
- The leak is inside the furnace cabinet, or you smell gas or have any carbon monoxide concern. In that case leave the house and call emergency services and your gas utility.
If the leak was also running through the AC condensate path, AC duct dripping water and reasons your AC is leaking water cover the cooling side. For a broader risk discussion, see is water leaking from a furnace dangerous.
FAQ
Is it normal for a furnace to leak when the AC is running? Some condensate is normal, but it should drain away. Water on the floor means the drain path has failed.
Can I run the AC if it is leaking? It is better to turn it off until the cause is found. Continuing to run it can increase damage.
How long do furnaces last? It depends on the model and care, and many last more than a decade. A technician can evaluate your system if repairs are adding up.
Could the water actually be from a water heater or other pipe? Yes. If it keeps appearing with the AC off, check nearby plumbing; signs of a leaking pipe can help. See water heater leaking from the bottom.