How to winterize an evaporative cooler

Almost every complaint about an evaporative cooler in spring — the smell, the pump that will not prime, the pad that wets in a stripe — was created the previous autumn, by putting the machine away wet.

A working evaporative cooler is a tank of standing water feeding a permanently wet pad. That is fine while it is running, because airflow and use keep things moving.

Shut it down without draining and drying, and it becomes a warm-then-cold, dark, sealed, damp environment for two thirds of the year. Three things happen in it:

• Biological growth. Standing water plus a damp organic substrate is the origin of the smell people notice on the first hot day. It cannot be rinsed out once established in the media.
• Mineral scale. Water left to evaporate slowly in a closed tank leaves everything dissolved in it behind, concentrated, onto the pump inlet and the distributor holes.
• Freeze damage. Water trapped in a pump or a distributor line expands when it freezes. A cracked pump housing is the most common winter casualty and it is invisible until you switch on.

All three are prevented by the same hour of work.

1. Run the fan with the pump off for fifteen to twenty minutes. Do this before anything else, while the machine is still assembled. Airflow through wet media dries it far better than air will once it is sitting in a corner.
2. Unplug it and disconnect any continuous water supply. Drain the supply line too if it runs anywhere that freezes.
3. Drain the tank completely. Use the drain plug, and tip the unit if it is light enough. A 6.6 gallon tank you can carry out; a 15.9 gallon tank you drain in place.
4. Remove the media and inspect it. This is when you decide about replacement, with the evidence in front of you and time to order a part. What to look for.
5. Lift out the pump and clear it. Rinse the inlet screen, brush off scale, and — critically — make sure no water remains inside it. Invert it and shake it. This is the freeze-damage step.
6. Flush and clear the distributor, and leave it detached and empty rather than full of water.
7. Scrub the tank with warm water and a mild detergent, or dilute white vinegar for mineral film. Rinse and drain again.
8. Rinse rigid media from the clean side outward, so debris leaves the way it came in. Do not bother with aspen fiber — it compacts when handled wet.
9. Dry everything genuinely dry. In air, in sun if you have it, for as long as it takes. Damp is not dry, and this is the step that decides how June goes.
10. Reassemble loosely, or store components separately, and cover with something breathable.

Water expands as it freezes, and a submersible pump is a sealed plastic housing full of water with an impeller in it. If it is stored with water inside in an unheated garage, expansion can crack the housing.

The failure is invisible. The pump looks fine, sits in the tank all winter, and either fails to prime or leaks internally on the first run of the season — which presents as a pad that wets in a stripe, or one that does not wet at all.

Invert the pump, shake it out, and store it dry and separately. Thirty seconds, and it removes the most likely winter casualty on the machine.

Same logic for the distributor tube and any hose. Anything that can hold a slug of water should be emptied and left open at both ends.

A properly winterized cooler starts up in ten minutes.

1. Refit the media — new if you decided that in autumn, cleaned if not.
2. Refit the pump and distributor, and check the distributor holes are clear before you fill anything.
3. Fill with clean water and run the pump for two minutes with the fan off. Watch the media face: it should darken evenly, edge to edge.
4. If it wets in a stripe, the distributor is still partly blocked. If it does not wet at all, the pump did not survive the winter.

The full cleaning procedure covers both diagnoses in more detail.