Start from what you can see
Answer one question at a time and each branch ends in a specific cause and a specific fix. Every step has its own link, so you can send someone straight to an answer.
Before you start: kill power at the disconnect before touching anything inside the cabinet, and treat the roof with respect. Nothing on this page is worth a fall.
What is the cooler actually doing?
Pick what you can observe from inside the house. If more than one fits, start with the airflow.
No air at all
With the switch on, what do you hear at the unit?
Stand where you can hear the cooler itself, not the registers. Kill power at the disconnect before opening anything.
Completely silent
No power is reaching the unit
High confidence · Easy
A silent cooler is an electrical problem, not a cooling one. Coolers are very often on a roof disconnect switch that somebody turned off at the end of last season.
- Check the disconnect switch at the unit itself — it is the one people forget.
- Check the breaker in the panel.
- Check the wall control or thermostat is calling for the blower.
- If all three are on and it is still dead, the problem is in the wiring or the motor, and that is an electrician's job.
A hum, but nothing turns
The motor is getting power but cannot start
High confidence · Moderate
A hum with no rotation almost always means a failed run capacitor, and occasionally a seized bearing. Switch it off promptly — a stalled motor that is still energised overheats quickly.
- Turn it off now, before anything else.
- With power off at the disconnect, check whether the blower turns freely by hand. If it is stiff, the bearings have gone.
- If it spins freely, the capacitor is the likely culprit. Capacitors hold a charge even with power off and must be discharged before handling.
- If you are not comfortable with either, this is a reasonable point to call someone.
The motor runs, but no air moves
The belt is broken or has come off
High confidence · Easy
The motor spins, the blower does not. Open the panel and look — a snapped or thrown belt is visible immediately.
- Kill power at the disconnect first.
- Fit a new belt of the same size. They cost a few dollars and are a wear item.
- Set the tension to about half an inch of deflection at the middle of the span.
- If belts keep failing, check the pulleys are aligned — a misaligned drive chews through them.
More detail: belt
The blower runs normally
The air is going somewhere other than your rooms
Medium confidence · Moderate
If the blower is turning properly, the air is being made and then lost between the unit and the registers.
- Open every damper. Winter dampers left shut are the most common single cause.
- Check the ceiling grilles are open.
- Trace the duct from the unit. A flex duct knocked loose in the attic dumps your entire airflow up there, and it is common after any attic work.
- Clean the blower wheel if it is packed with dust.
More detail: airflow
Air comes out, but it barely cools
With the cooler running, what do the pads look like from outside?
They should be uniformly dark and damp from top to bottom. Give the pump 15 minutes to wet them fully before judging.
Dry patches, streaks, or a dry top edge
The pads are not getting fully wetted
High confidence · Easy
Air that passes through a dry patch evaporates nothing and arrives at your register at close to outdoor temperature, diluting everything the wet part of the pad achieved. This is the most common cause of a cooler that runs but does not cool.
- Confirm the pump is actually running — you should hear it and see water moving.
- Check that water reaches every distribution tube. Clear any that are blocked with scale.
- Check the sump level. A float set too low lets the pump lose its prime.
- Descale the distribution header if the holes are crusted over.
More detail: pad wetting
Wet, but crusty, stiff, or matted
The pads are spent
High confidence · Easy
Minerals stay behind every time water evaporates. Over a season the pads stiffen and close up, losing both wetted surface and open area for air.
- Replace them. Aspen is cheap and annual; rigid media costs more, lasts several seasons, and performs better.
- While the pads are out, scrub the sump and descale the distribution tubes.
- Fit bleed-off or a purge pump so next season does not repeat this.
More detail: scaled pads, bleed off
Uniformly wet and in good condition
How much window area is open in the house?
The rule of thumb is 1.5 to 2 square feet per 1,000 CFM, on the far side of the house from the cooler.
Nothing, or one small window
The house cannot exhaust the air the cooler is pushing in
High confidence · Trivial
An evaporative cooler is a once-through machine. It cannot push air into a sealed box — airflow stalls, the moisture it adds accumulates instead of sweeping through, and cooling collapses. The cooler is fine. The house is the problem.
- Open 1.5 to 2 ft² of window per 1,000 CFM, on the far side of the house.
- Wait ten minutes and feel the difference. It is usually dramatic.
- For a permanent fix, fit an up-duct or a relief damper so it works without leaving windows open.
More detail: relief air
Several windows, well away from the cooler
What is the outdoor dew point right now?
Any weather app reports it. Dew point, not humidity — it is the number that decides whether evaporative cooling can work at all.
Above 60 °F
The weather is outside the machine's envelope
High confidence · Not a fault
Above a 60 °F dew point there is very little wet-bulb depression left to spend, so there is very little cooling available to any evaporative cooler, of any age or price. Nothing is broken. This is monsoon weather, and it passes.
- Nothing to repair.
- On the worst days, consider shutting it off rather than adding humidity for almost no cooling in return.
- Check again in September — the same machine will feel transformed.
More detail: weather
Below 60 °F
How strong is the air at the registers?
Compare it against what you remember from previous seasons. Cold air in small quantities never catches a house up.
Weak, or much weaker than it used to be
The cooling is fine and the delivery is not
Medium confidence · Easy
This is the failure mode where the measurement looks perfect and the house stays hot. The air leaving the pads is properly cold; there is simply not enough of it arriving.
- Check belt tension — about half an inch of deflection. A glazed or loose belt lets the blower turn well below its rated speed.
- Open all dampers and check for disconnected duct.
- Clean the blower wheel. Mineral dust builds up on the blades and destroys its efficiency.
Strong, and cold to the hand
The cooler is healthy and outmatched by the house
Medium confidence · Varies
Good effectiveness, good airflow, and the house still climbs through the afternoon. The heat coming in exceeds what the delivered air can carry away.
- Shade the west-facing glass. This is usually the single biggest win.
- Insulate the attic.
- Reduce the load before buying a bigger unit — it is cheaper and permanent.
More detail: undersized
I do not know
Measure before going further
High confidence · Trivial
Without the outdoor conditions there is no way to tell a struggling cooler from unhelpful weather, and the two need completely different responses.
- Run the check on the front page — it works from your location or from two numbers typed by hand.
- It reports your dew point and wet-bulb, and tells you how much cooling is available today.
- Then come back to this question with the answer.
More detail: run the check
Cools well at first, then fades
Over what period does it fade?
The timescale is genuinely diagnostic here — each one points at a different cause.
Within an hour of starting
Across the season — good in spring, poor by August
Mineral scale is accumulating through the season
High confidence · Moderate
Water leaves as vapour and its dissolved minerals do not, so the sump grows steadily more aggressive all summer unless some water is deliberately thrown away. Effectiveness decays with it.
- Replace the pads and descale the distribution tubes and sump.
- Fit a bleed-off line, which continuously wastes a small stream to dilute the sump.
- Or fit a purge pump, which dumps the sump periodically and wastes less water overall.
More detail: scaled pads, bleed off
Every afternoon, recovering overnight
Water problems — leaking, overflowing, or none
Which water problem is it?
If water is reaching the ceiling, deal with that before anything else on this site.
No water reaching the pads
Water is not getting to the pads
High confidence · Easy
Work upstream from the pads until you find where the water stops.
- Is the supply valve open? It gets closed for winter and forgotten.
- Is the sump filling, and does the float shut off about an inch below the overflow?
- Does the pump run? A whine without water usually means a low sump or a blocked inlet screen.
- Are the distribution tubes clear? Scale closes the holes.
More detail: water
Overflowing, or running down the roof
The float valve is not shutting off
High confidence · Easy
The float should close cleanly about an inch below the overflow standpipe. When it does not, the unit runs water continuously onto the roof.
- Adjust the float arm so it closes at the right level.
- If it will not hold, replace the valve. They are inexpensive and they do wear out.
- Do not leave this running. Continuous overflow onto a roof finds its way inside eventually, and that repair is not the cooler's.
More detail: water
A stain on the ceiling below the unit
Water is getting into the building
High confidence · Moderate
Either the sump has corroded through, or overflow water is tracking back along the duct. Both need attention now rather than at the end of the season.
- Shut the water supply off at the valve.
- Let it dry out, then run it briefly and watch where the water actually goes.
- Patch or replace the sump, or correct the overflow routing, before running it again.
- If the ceiling is sagging or stained widely, get the structure looked at.
More detail: water
It smells
What does it smell like?
A burning or hot-electrical smell is a different category from the others — treat it as urgent.
Musty, swampy, or stagnant
Biological growth in the sump
High confidence · Easy
The classic swamp smell is growth in water that has been sitting warm and still, usually combined with pads at the end of their life. A unit left full over winter is almost always the source.
- Drain and scrub the sump.
- Replace the pads.
- Keep bleed-off running so the water does not stagnate.
- Drain it at the end of the season rather than leaving it full.
Like wet wood, and it started with new pads
That is normal, and it passes
High confidence · Not a fault
Fresh aspen pads smell faintly of wet wood for the first few days of use. It is the pads themselves, not a fault, and it fades.
- Give it a week.
- If it turns musty rather than fading, the sump is the source instead.
Burning, or hot electrical
Stop and inspect before running it again
High confidence · Moderate
A burning or hot-electrical smell is not a cooling problem. It means something is overheating, and continuing to run it is how a fault becomes a fire.
- Switch it off at the disconnect now.
- With power off, look for scorching at the motor, the capacitor, and the wiring terminals.
- A stalled motor, a failing capacitor, and a loose connection all produce this.
- If nothing is obviously wrong, have it inspected rather than guessing.
It makes a noise
What kind of noise?
Kill power at the disconnect before opening the cabinet to look at any of these.
Squealing, especially on startup
The belt is slipping
High confidence · Easy
A squeal at startup is the belt slipping on the pulley before the blower comes up to speed. It also means you are not getting rated airflow.
- With power off, press the belt at the middle of the span — about half an inch of deflection is right.
- Look for glazing, cracks, and black dust from wear. Replace it if you see any.
- Check pulley alignment while you are in there.
More detail: belt
Rattling or vibration
Something is loose or out of balance
Medium confidence · Easy
Rattles are usually mechanical and cheap to fix. Vibration that has come on gradually is more often the blower wheel losing balance as mineral deposits build unevenly on the blades.
- Check the access panels and their latches.
- Look for debris in the blower wheel.
- Clean the wheel — uneven mineral buildup unbalances it.
- Check the motor mounts are tight.
Grinding or rumbling
Bearings are failing
Medium confidence · Moderate
Grinding or rumbling from the motor or blower shaft is wear, and it only goes one direction. This is worth planning a repair around rather than waiting for the failure.
- With power off, turn the blower by hand. Roughness or side-to-side play confirms it.
- Some motors have oil ports and want a few drops annually; many are sealed.
- Budget for a motor or bearing replacement rather than hoping it settles.
The house feels humid and sticky
When does the house feel humid?
An evaporative cooler adds moisture by design. The question is whether that moisture is being swept out or piling up.
It gets worse the longer the cooler runs
It is humid outside as well
I am not sure
Still stuck?
Take a supply-air measurement before going further — it separates "the cooler is broken" from "the house is sealed" from "the air is too humid," which is most of the diagnostic work. Run the check.