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How cool can an evaporative cooler actually get the air today?
Move the two sliders to today's conditions. The figure is roughly what a well-maintained cooler can deliver at the outlet — and it explains why the same machine is brilliant in June and useless for a fortnight in August.
The calculator
An evaporative cooler cannot cool air below its wet-bulb temperature, and in practice it reaches part of the way there. That is the whole physics of it, and it is why humidity matters so much more than the machine does.
Approximate air temperature at the outlet
—That is comfortable air. At this temperature and humidity a cooler in reasonable condition will hold a house perfectly well, and it will do it for a fraction of what refrigerated cooling costs to run.
- Leave a window or two cracked on the far side of the house. An evaporative cooler needs somewhere for the air to leave, and it will not work properly in a sealed house.
- If it is not keeping up despite a figure like this, the problem is airflow, water distribution or pad condition — not the weather.
Workable, but you will notice it. Air at this temperature will take the edge off a room and keep air moving, which is a real part of comfort — but it will not feel like air conditioning, and on a hot afternoon the house will drift up.
- Run it earlier in the day. Getting ahead of the heat matters far more with evaporative cooling than with refrigerated.
- Fresh pads and a clean water distribution line are worth more on a day like this than on a dry one.
- Make sure enough air can leave the house, or the cooler will stall against its own pressure.
This is the fortnight people remember. With this much moisture already in the air, there is very little evaporation left to do, and the cooler is mostly moving warm damp air around the house.
- Nothing is wrong with the machine. This is what the physics allows, and no service visit changes it.
- If these conditions are a few days a year, shutting it down and waiting is a legitimate plan.
- If they are several weeks, that is the honest argument for adding refrigerated cooling — and we will tell you what that involves rather than quoting a box.
How this is worked out: the wet-bulb temperature is estimated from your temperature and humidity using a published approximation, then the outlet air is taken as reaching the chosen share of the way from the outdoor temperature down to the wet bulb — 85% just serviced, 78% typical, 65% with tired pads. Those are ordinary figures for direct evaporative coolers. It is an estimate for orientation, not a measurement of your machine, and it takes no account of altitude, of the moisture the cooler adds indoors, or of how much air your house can actually exhaust.
What a cooler needs, and when
- Spring start-upPads replaced, pan cleaned, float set, pump and bleed line checked, belt and bearings done, water line opened and the winter cover taken off. Before the first hot week, not during it.
- Mid-seasonA look at the pads, because hard water here scales them and a scaled pad both cools less and restricts airflow.
- Late summerThe humid stretch. Nothing to do mechanically — this is when the calculator above earns its keep by telling you it is the weather rather than the machine.
- Autumn shutdownWater off, line drained, pan drained and dried, pads out, and a cover on. A cooler left wet through a Colorado winter rusts from the inside.
- Every yearThe roof penetration and the duct into the house. Leaks around a roof-mounted cooler do their damage slowly and out of sight.
- Whenever it smellsStanding water in the pan or pads that have gone past their life. A service item, and a quick one.
What this calculator is not
- Not a measurement of your coolerIt uses typical saturation efficiencies. What yours achieves depends on pad condition, water distribution, airflow and how the unit was installed.
- Not a comfort predictionOutlet temperature is not room temperature. How cool the house gets depends on the size of the unit, the ducting, how much air can leave and how much heat the house is taking in.
- Not a reason to buy anythingIf it tells you the weather is the problem, the correct response is usually to wait for the weather.