Troubleshooting guide

Why Is My Air Conditioner Leaking Water Indoors?

Indoor AC water usually comes from a blocked drain, a full pan, a failed condensate pump or thawing ice. How to separate drainage from cooling faults.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

A cooling coil removes moisture from the air, and that water has to leave through a drain. Two questions come first: is the water normal condensate that cannot get away, or is it melting ice? The cooling performance is a separate question, although a frozen coil can cause both.

Water near electrical equipment is a hazard. If water has reached the electrical parts, switch the system off at the breaker before anything else. Repairs inside the unit belong with qualified technicians.

Symptom	Likely cause	What to check
Water around an indoor unit, system cooling normally	Blocked or kinked condensate drain line, algae or debris in the drain	Drain line and cleanout. Many systems have a float switch that stops the unit when the pan fills
Water in the secondary or overflow pan	The primary drain is blocked, or the primary pan has failed	Primary drain first. A pan that is rusted or cracked needs replacing
Pump running, but water still comes out	Condensate pump failed, clogged, or the discharge line is blocked or too long a lift	Pump operation, float, check valve and discharge line
Water appears when the system is off, after weak cooling	Ice thawing from a frozen coil	Frozen coil guide: airflow and charge. The water volume is large because ice stores a lot of water
Water drips from the unit casing or ducts	Condensation on uninsulated surfaces, a leaky cabinet, or missing duct insulation	Insulation condition and air leaks, especially in a hot attic
Water at the unit only on very humid days	High condensate rate, with a drain that is marginal or has the wrong slope	Drain slope, trap design and pan level

How much water is normal?

The coil condenses water from the latent part of the load. A 3-ton (36,000 Btu/h) unit with 25% of its capacity as latent cooling removes 9,000 Btu/h of latent heat. With about 1,060 Btu per pound of water condensed:

9,000 ÷ 1,060 = 8.5 lb/h = 1.0 gal/h, about 24 gal/day

A drain that is slightly restricted may cope on a mild day and overflow on a humid one, when the rate is higher. The numbers also show why a pan that holds only a cup fills within minutes when the drain stops.

Keep water and cooling separate

If the system cools well and water leaks, treat it as a drainage problem: line, trap, pan and pump.
If the system cools poorly and water leaks, check for a frozen coil first, because the same airflow or charge fault causes both.
A float switch that shuts the unit off protects the building, but it also means a drain problem can look like a cooling failure.

Write a useful problem statement

Where the water appears, how much, and whether it appears with the system running or after it stops.
Whether the unit cools normally, and whether it is iced.
Drain type (gravity or pump), any float switch, and the last time the drain was cleaned.

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Questions

Can I pour bleach down the drain?

Follow the equipment instructions. Some makers allow a dilute cleaning solution, others advise against it because it can damage the coil or pan. Vinegar or manufacturer-approved products are sometimes recommended.

Why does the drain need a trap?

On many systems, the trap keeps air from being pulled back up the drain when the pressure at the coil is negative. The right trap design depends on whether the coil is before or after the fan.

Is a float switch required?

Local codes often require a secondary protection device, such as a secondary pan or a float switch, especially where a leak can damage the building.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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