Swamp Cooler Not Cooling: A Step-by-Step Diagnosis

If your swamp cooler is running but the house is not getting cooler, the cause is almost always water, air, or weather. Either the pads are not soaking wet, the air has nowhere to go once it enters the house, or the outdoor air is too humid to evaporate much water. Work through the checks below in order. The first three take under ten minutes and solve the majority of cases.

Quick diagnosis table

Know what "working" looks like first

A swamp cooler cannot cool below the outdoor wet bulb temperature, and real pads only get part of the way there. Supply air temperature follows this relationship:

To use this, hold a cheap digital thermometer in the supply air at the nearest register for a minute, and look up the current wet bulb with our cooling temperature calculator using your local temperature and humidity. That one comparison tells you whether to look inside the cooler or inside the house.

Diagnostic sequence, easiest first

1. Confirm the pump mode is on. Many controls have separate "fan only" and "cool" settings, and two-speed wall switches often have a pump toggle. Confirm: with the unit set to cool, you should hear water trickling within a minute. Fix: set the control to cool or turn on the pump switch. Run the pump alone for 2 to 5 minutes before starting the fan.
2. Open relief air. A cooler pushes outdoor air into the house; that air must leave through open windows or up-ducts. With the house sealed, pressure builds, airflow drops, and humidity climbs until cooling stops. Confirm: open a window on the far side of the house a few inches. If the register airflow jumps and the room feels better within 15 minutes, this was the problem. Fix: open about 1 to 2 sq ft of window per 1,000 CFM of cooler capacity, in the rooms you want cooled. See venting and relief air for the full method.
3. Check that water reaches the reservoir. Remove one side panel and look at the pan. Confirm: water should sit 1/2 to 1 inch below the overflow standpipe top. An empty or nearly empty pan means the supply valve is closed, the line is kinked or frozen-cracked, the inline filter is clogged, or the float is stuck. Fix: open the valve, replace the cracked 1/4 in line, clean the filter, or adjust the float per our float valve adjustment guide.
4. Look at pad wetness with the pump running. Confirm: every square inch of each pad should be dark and damp to the touch after 5 minutes. Dry vertical stripes point to clogged spider tubes or trough holes; a dry upper edge points to low flow; a uniformly dry pad means the pump is not running. Fix: clear the distributor (a small drill bit or straightened paper clip clears trough holes), flush the spider, and level the cooler so water does not run to one side. If no water flows at all, go to swamp cooler pump not working.
5. Inspect the pads themselves. Confirm: aspen pads that are thin, sagging, matted at the bottom, or crusted white with mineral scale cannot hold water evenly or pass air freely. Rigid media with heavy scale on the air entry face behaves the same way. Fix: replace aspen pads (usually a 30 to 60 minute job); descale or replace rigid media. See how often to replace cooler pads.
6. Check airflow: belt, blower and dampers. Confirm: on belt-drive units, press the belt midway between pulleys; it should deflect about 1/2 inch. A slick, glazed or cracked belt slips and the blower turns slowly even though the motor hums along. Also confirm any ceiling damper or seasonal cover over the duct is open. Fix: adjust or replace the belt per belt and pulley adjustment, and remove winter covers.
7. Check the weather. Confirm: look up the current dew point. Above about 55 F the cooler loses much of its punch, and above about 60 to 65 F it does little more than move air. Fix: there is no repair for humidity. Run high speed, maximize relief air, and close off rooms you do not need. Our monsoon season guide covers tactics.
8. Measure performance against the formula. Confirm: use the worked example method above. Efficiency below about 55% with wet pads usually means air is bypassing the pads (gaps around frames, missing pad retainers, doors left off) or the pads are spent. Fix: seat pads fully in their frames, close gaps, and replace worn media.
9. Evaluate ducts and sizing. Confirm: if supply air is cold but the house stays warm, check for disconnected or crushed ducts in the attic, a duct far longer than the blower can handle, or a cooler that is simply too small. Fix: repair the ducts, or compare your unit's CFM to our cooler size calculator result.

The relief air problem, explained

Relief air causes more "not cooling" complaints than every mechanical fault combined, because it feels counterintuitive to open windows to cool a house. A swamp cooler does not recirculate indoor air like a central air conditioner. It replaces it. A 4,500 CFM cooler pushes 4,500 cubic feet of air into the house every minute, and that air must leave. If it cannot, three things happen:

• Static pressure rises and blower airflow can drop by a third or more.
• The moist supply air stays in the house, so indoor humidity climbs and your skin stops evaporating sweat effectively.
• Cool air short-circuits to whatever leaks exist (often the attic), skipping the rooms you are in.

Open windows on the downwind side of the house and in the rooms where you spend time. The cooled air travels from the register toward the open window, so the window location controls which rooms get cooled.

Water delivery problems in detail

The pads only cool where they are wet. Most water delivery faults build slowly over a season as minerals precipitate out of evaporating water:

Spider / distributor

The plastic hub that splits pump output into tubes running to each pad. Scale or a single aspen sliver can block one tube and leave a whole side dry.

Water distribution trough

The perforated channel on top of each pad frame. Holes clog with scale; clear each one with a small bit.

Pump intake screen

Fills with pad debris and sediment, cutting flow by half or more while the pump still runs.

Float valve

Sticks shut from scale, or the arm drops too low after a season. Too-low water starves the pump intake.

Bleed-off line

If missing or clogged, minerals concentrate, scale builds faster, and pads crust over. See bleed-off and water quality.

When high humidity is the real cause

Evaporative cooling depends on the gap between dry bulb and wet bulb. On a dry 100 F day that gap might be 30 to 35 F; on a monsoon afternoon at 95 F and 40% relative humidity it shrinks to about 19 F, and supply air from aspen pads lands near 81 F. The cooler is working correctly; there is just less to work with. Our climate index shows typical achievable supply temperatures for dozens of US cities, and do swamp coolers work in humidity explains the thresholds. If your home also feels clammy, read swamp cooler making the house too humid.

Sizing and heat load problems

A cooler that has never kept up on the hottest days may simply be too small, or the house may gain heat faster than expected (dark roof, uninsulated attic, west-facing glass). As a rough check, each degree of supply air temperature drop delivers 1.08 x CFM BTU/h of sensible cooling. A 4,000 CFM cooler supplying air 20 F below room temperature removes about 1.08 x 4,000 x 20 = 86,400 BTU/h. If actual delivered airflow is only 2,500 CFM because of a slipping belt and closed windows, that falls to 54,000 BTU/h, which is why airflow problems feel like sizing problems. Rule out airflow faults before buying a bigger unit; then see how to size a swamp cooler.

When to call a pro

• The motor hums but will not start, trips the breaker, or smells hot: see swamp cooler motor not running and call an HVAC or electrical technician if wiring is involved.
• The cooler is on a roof you cannot reach safely with a properly footed extension ladder and three points of contact.
• Ductwork in the attic is disconnected or crushed and needs resealing.
• Everything checks out, efficiency is normal, and the house still will not cool: a pro can measure static pressure and evaluate whether the unit is undersized.

Typical service calls run roughly $100 to $200 for diagnosis in 2026, varying by region and season. Many of the steps above are the same ones a technician performs first, so doing them yourself often saves the visit.