How to clean a swamp cooler

Almost everyone who cleans a swamp cooler starts at the pad, because the pad is what they can see and what smells. That is the wrong end.

The pad smells because the water reaching it came out of a tank holding 6.6 to 15.9 gallons — the published capacities of the units on this page — that has been sitting warm and still. Clean the pad and put it back above the same tank and the smell returns within a day.

Work upward through the water path, in the direction water travels: tank, pump, distributor, media. Each stage feeds the next, so cleaning out of order means recontaminating what you just did.

1. Drain the tank completely. Use the drain plug if there is one, and tip the unit if it is light enough — a 25 lb machine you can carry, a full 15.9 gallon tank is over 130 lb of water and you drain it in place.
2. Scrub the tank. A soft brush and warm water with a mild detergent, or a dilute white vinegar solution for mineral film. Get into the corners, which is where sediment collects and where growth starts. Rinse and drain again.
3. Clear the pump. Lift the pump out. It has an inlet screen or cage, and it will be clogged with sediment and scale. Rinse it, brush the screen, and soak the pump body in dilute vinegar if it is visibly furred. A pump running at half flow is the most common cause of a pad that only wets in patches.
4. Flush the distributor. This is the tube or manifold that carries water from the pump to the top of the media. Its outlet holes scale shut, one at a time, and the failure is gradual enough that people never notice. Detach it, flush it through, and clear individual holes with a pin.
5. Remove and rinse the media. For rigid honeycomb, rinse with a hose from the clean side outward, so debris goes out the way it came in rather than being driven deeper. For aspen fiber, do not bother rinsing — it compacts when handled wet. Replace it.
6. Clean the housing and grille. Dust on the intake grille and blades reduces airflow, and lower airflow means less evaporation and less cooling. Same principle as a dusty fan, with water added.
7. Dry everything fully before reassembly. This is the step people skip, and it is the one that determines whether the smell comes back. Leave the parts in air, or in sun if you have it, until they are genuinely dry rather than damp.

Note how few of those are the pad itself. The pad is where the symptom shows; the pump, the distributor and the tank are where the cause tends to live.

One change makes more difference than any cleaning schedule: run the fan with the pump switched off for the last ten to fifteen minutes of every session.

Airflow through wet media with no water feed dries the pad before the machine sits idle. Dry media does not grow anything, and it does not hold the concentrated mineral solution that turns into crust. Most coolers with a pump switch let you do this trivially; some have a dedicated dry-out mode.

Two more worth adopting:

• Drain the tank whenever the unit will be idle for more than a day. Warm standing water is the origin of essentially every swamp cooler smell.
• Do a full clean before storage, not after. A cooler put away dirty spends the off-season as a sealed, damp, dark box. The spring smell is created in October.

Reassemble dry, fill with clean water, and run the pump for two minutes with the fan off. Look at the media face: it should darken evenly, edge to edge, within that time.

If it does not, the distributor is still partly blocked — go back to step four. If it wets evenly and the cooler still underperforms, the media itself is finished and no cleaning will recover it. The pad guide covers what to buy and how to size it.