Swamp Cooler Leaking: How to Find the Source and Stop It

A leaking swamp cooler is usually a water level problem, not a broken cooler. The float valve lets in too much water, the overflow cannot carry it away, or water escapes past the pads into the cabinet or duct. Less often the pan has rusted through or a supply fitting has failed. Start with where the water appears, because the location points straight at the cause.

Quick diagnosis table

Diagnostic sequence, easiest first

1. Check the water level. Confirm: shut off power, wait 2 minutes for pads to drain back, and remove a side panel. Water should sit about 1/2 to 1 inch below the top of the overflow standpipe. If it is at or above the standpipe top, or water is still entering, the float is the problem. Fix: bend the float arm or turn the adjusting screw so the valve closes lower. If it will not shut off at all, the seat is worn or scaled and the float valve should be replaced (a cheap part, usually 15 minutes). Full procedure in float valve adjustment.
2. Check the overflow and drain. Confirm: the standpipe should be threaded tightly into the pan fitting with its washer in place, and a drain hose should run from underneath to a roof drain, downspout or approved location. A cracked fitting, missing washer, or a hose that pops off lets water run down the roof. Fix: tighten or replace the fitting and secure the hose with a clamp. See water supply and drain lines.
3. Watch the pads with the pump running. Confirm: with the side panels on, look at the louvers from outside. Water spraying or sheeting out means a distributor tube has slipped out of its trough, a trough is cracked or tilted, or a pad has slumped and left a gap. Fix: reseat tubes and pads, replace pad retainers, and if pump flow is overwhelming the troughs, partially restrict it (some pumps have a flow adjustment or you can add a valve on the spider feed).
4. Check that the cooler is level. Confirm: place a level on the pan edge front to back and side to side. A cooler tilted toward the duct opening will push water toward the blower and into the duct. Fix: shim the stand or adjust the legs. A slight tilt toward the drain is acceptable; never toward the duct.
5. Inspect the supply line and fittings. Confirm: trace the 1/4 in copper or plastic line from the house to the float. Look for wet spots at the saddle or angle stop valve, compression nuts, and where the line passes through the cabinet. Plastic lines on roofs crack from UV and freezing. Fix: snug compression nuts a quarter turn, replace cracked tubing, and replace old self-piercing saddle valves with a proper angle stop if possible.
6. Inspect the pan for rust. Confirm: drain the pan and look for rust blisters, pinholes and failed seams, especially around the overflow fitting and corners. Fix: wire brush, dry thoroughly, and apply a cooler pan coating rated for submerged service. Many pans benefit from a coat every few seasons as prevention. If the pan is rusted through in several places or the bottom flexes, plan for a replacement cooler or a new pan if your model offers one.
7. Trace leaks below the cooler. Confirm: if water appears on the ceiling but the cooler looks dry, check the roof jack (the curb or transition where the duct passes through the roof), the duct connection to the cooler, and the ceiling register box. Fix: reseal flashing and seams with roof-compatible sealant, and in winter close the damper or install a cover to stop warm house air condensing in a cold duct.

Water in the ducts and registers

Water dripping from a register is the leak that does the most damage, because by the time you see it, water has been sitting in the duct and possibly the ceiling. It almost always comes from one of three things:

• Pad overspray. High airflow can pull droplets off pads that are over-wet. Overflowing troughs or torn pads make it worse. Rigid media handles high face velocity better than aspen, but only when installed with the flutes in the correct orientation.
• Too much water in the pan. The blower inlet sits close to the pan on many downdraft units. A high water level plus air turbulence can splash water into the blower.
• An out-of-level cooler. Water migrates toward the low side. If that side is the duct, it finds its way in.

How much water should be leaving the cooler?

Not all water leaving the cooler is a leak. The bleed-off line deliberately sends a small stream to drain to control mineral buildup. It helps to know the normal numbers so you do not chase a non-problem.

Our water usage calculator estimates evaporation and bleed-off for your cooler, and swamp cooler water usage explains what affects it.

Fixing a rusted pan the right way

Pan rust is the leak that sneaks up on older coolers, usually starting where water sits longest: corners, around the overflow fitting, and under the pump. A careful repair can add several seasons to a pan with isolated pinholes.

1. Drain and dry completely. Coatings fail on damp metal. Let the pan dry in the sun for at least a few hours.
2. Remove loose rust. Use a wire brush or wire wheel on a drill until you reach firm metal around each pinhole. Vacuum the dust.
3. Patch holes. Small pinholes can be filled by the coating itself; larger holes need a patch of fiberglass mesh or a metal patch bedded in the coating, per the product directions.
4. Coat the whole pan. Apply a cooler pan coating or asphalt-based sealer rated for continuous water immersion, extending up the pan walls and around the overflow fitting.
5. Cure fully before refilling. Cure times vary from hours to a couple of days. Refilling early is the most common cause of a patch failing.

If the pan bottom flexes when you press it, rust has thinned the metal across a wide area and patching will not hold for long. At that point compare the cost of a replacement pan (where available) with a new cooler; see cost of a swamp cooler.

Portable and window cooler leaks

Portable evaporative coolers leak for simpler reasons: an overfilled tank, a drain plug or gasket not seated, a cracked tank, or a unit sitting on an uneven floor so the pads drip outside the drip tray. Window units leak when they are tilted toward the room. A window cooler should tilt slightly outward so any excess water drains outside; see window cooler installation.

Preventing leaks

• Recheck the float level at every spring startup, because floats drift and scale changes their closing point.
• Coat the pan every few seasons before rust starts, and replace sacrificial anodes if your cooler has them.
• Drain the pan and disconnect the supply line before freezing weather; cracked lines are a common spring leak. See how to winterize a swamp cooler.
• Replace pads before they collapse and leave gaps.

When to call a pro

• Water is getting into the ceiling, attic insulation or walls.
• The roof jack or roof membrane around the cooler is leaking; roof penetrations should be sealed by someone who knows your roofing type.
• The pan is rusted through and you are weighing a pan replacement against a new cooler.
• You cannot safely reach a roof-mounted unit.