What actually goes wrong

A swamp cooler not cooling is rarely a broken swamp cooler. These are ordered by how often each one turns out to be the answer, not by how dramatic it sounds — and the first two account for most complaints without a technician being involved.

1. Cause 1 · most common

   Not enough relief air

   The house is sealed up, so the air the cooler is pushing in has nowhere to go. Airflow stalls, the moisture the cooler adds accumulates instead of sweeping through, indoor humidity climbs, and cooling collapses — while the cooler itself is working perfectly.

   Symptom

   Weak air from the registers, rooms feel clammy rather than cool, and it gets worse the longer the cooler runs.

   Mechanism

   A once-through machine cannot push air into a closed box. Mass in must equal mass out.

   Confirm it

   Open several windows on the far side of the house. If performance jumps within a few minutes, this was your problem.

   Fix

   Keep 1.5 to 2 ft² of open window or relief opening per 1,000 CFM, distributed away from the cooler. A dedicated up-duct or relief damper does it permanently.

   Difficulty

   Trivial — open a window.

2. Cause 2

   Pads are not getting fully wetted

   Dry streaks and channels let air slip through the pad without evaporating anything. That air arrives at the register at close to outdoor temperature and dilutes everything the wet portion of the pad achieved.

   Symptom

   Some cooling, but far less than it used to give. Measured effectiveness lands in the 0.40–0.65 band.

   Mechanism

   Clogged distribution tubes or spider, a weak or dead pump, a stuck float, a low sump level, or scale on the distribution header.

   Confirm it

   With the cooler running, look at the pads from outside. They should be uniformly dark and damp top to bottom. Dry patches or streaks are the diagnosis.

   Fix

   Clear the distribution tubes, check that water reaches every one, verify the pump moves water, and confirm the float keeps the sump at the right level.

   Difficulty

   Easy — hand tools, half an hour.

3. Cause 3

   Scaled, compressed, or spent pads

   Hard water leaves minerals behind every time water evaporates, and in the Southwest it does so relentlessly. Pads stiffen, the passages close up, and effectiveness decays steadily across the season.

   Symptom

   Performance that was fine in May and disappointing by August.

   Mechanism

   Mineral deposits reduce both the wetted surface area and the open area for airflow. Aspen pads also compress and slump under their own wet weight.

   Confirm it

   Pull a pad. Crusty, white, stiff, or matted means spent. Good pads are pliable and open.

   Fix

   Replace them. Aspen is cheap and annual; rigid media lasts several seasons and performs better.

   Difficulty

   Easy — but you are on a roof, so use proper ladder practice.

4. Cause 4

   No bleed-off or purge

   Not a failure you notice directly — it is the cause of cause 3. Water leaves as vapour and minerals do not, so sump concentration climbs all season unless some water is deliberately thrown away.

   Symptom

   Rapid scaling of pads, pump, and sump. Chalky white deposits everywhere.

   Mechanism

   Evaporation concentrates dissolved solids. Without dilution the sump becomes increasingly aggressive scale-forming water.

   Confirm it

   Look for a bleed line running to a drain. If there is none, and no purge pump, you have none.

   Fix

   A bleed-off line continuously wastes a small stream. A purge pump dumps the sump periodically instead, wasting less water. Manual draining works if you are disciplined about it.

   Difficulty

   Moderate — plumbing a bleed line, or fitting a purge kit.

5. Cause 5

   Slipping or glazed blower belt

   The first failure on this list where effectiveness can measure perfectly while the house stays hot. The air leaving the unit is properly cold — there is simply not enough of it.

   Symptom

   Cold air at the register, weak flow, house never catches up. Sometimes a squeal on startup.

   Mechanism

   A loose or glazed belt lets the blower turn slower than rated, so delivered CFM falls well below spec while ε is unaffected.

   Confirm it

   With the power off, press the belt — about half an inch of deflection is right. Look for glazing, cracks, and dust from wear.

   Fix

   Tension it, or replace it. Belts are a few dollars and a wear item.

   Difficulty

   Easy — but kill the power at the disconnect first.

6. Cause 6

   Blocked ducts, closed dampers, dirty blower wheel

   The same failure as cause 5 by a different route: the cooling is fine and the delivery is not.

   Symptom

   Poor flow at some or all registers. Often one room is much worse than the rest.

   Mechanism

   Winter dampers left shut, disconnected or crushed flex duct, leaks into the attic, or a squirrel-cage wheel packed with dust and mineral dust.

   Confirm it

   Walk every register with your hand. Then check the damper positions and inspect accessible duct runs.

   Fix

   Open the dampers, reconnect and seal ducts, clean the blower wheel.

   Difficulty

   Easy to moderate, depending on where the ductwork runs.

7. Cause 7

   Undersized unit, or a load it was never meant to carry

   The machine is healthy and simply outmatched by the house.

   Symptom

   Good effectiveness, adequate airflow, and the house still climbs through the afternoon.

   Mechanism

   West-facing glass, an uninsulated attic, or added square footage. Heat gain simply exceeds what the delivered air can absorb.

   Confirm it

   Cooling holds in the morning and loses ground after about two in the afternoon, with everything else measuring correctly.

   Fix

   Shade the west glass, insulate the attic, and reduce the load before buying a larger unit. Load reduction is usually cheaper and always permanent.

   Difficulty

   Varies — from hanging blinds to a re-roof.

8. Cause 8

   The weather is outside the machine's envelope

   Monsoon season. Nothing is wrong, and nothing can be done about it.

   Symptom

   Cooling that was excellent in June is useless in late July, and returns in September.

   Mechanism

   High wet-bulb leaves almost no depression to spend. Above a 60 °F dew point the machine has very little to work with and is adding humidity to a house that is already humid.

   Confirm it

   Run the check on the front page. If your measured effectiveness is fine and the supply temperature is still high, the air is the problem.

   Fix

   None available. Shut it down on the worst days rather than adding moisture for no cooling.

   Difficulty

   Not applicable — this is weather, not a fault.

9. Cause 9

   Water supply and sump problems

   Usually obvious once you look, and grouped here because the symptoms overlap.

   Symptom

   Overflow, a puddle under the unit, a stain on the ceiling, a pump that whines without moving water, or no water at all.

   Mechanism

   A float valve out of adjustment, a corroded sump, a pump cavitating on a low water level, or a kinked, frozen, or shut-off supply line.

   Confirm it

   Watch the sump fill. The float should close cleanly about an inch below the overflow. Then look for where water is going that it should not.

   Fix

   Adjust or replace the float, patch or replace the sump, clear the supply line.

   Difficulty

   Easy to moderate.

Questions people actually ask