Swamp Cooler Bleed-Off and Water Quality: How Much to Bleed and Why

Bleed-off is the small stream of pan water a swamp cooler sends down a drain on purpose. Its only job is to carry minerals out of the cooler. Evaporation takes away pure water and leaves calcium, magnesium and other dissolved solids behind, so without bleed-off the water in the pan becomes steadily harder until the minerals drop out as scale on your pads, pump and pan. Set it correctly and pads last longer, pumps stop clogging, and cooling stays strong all season.

How minerals build up in the pan

Think of the pan as a pot that is always boiling down and always being topped up. The float valve replaces exactly the water that evaporates, plus any bleed. If none leaves except by evaporation, every mineral that enters stays. The ratio between the mineral level in the pan and the level in your supply water is called the cycles of concentration (often shortened to cycles).

Rearranged, cycles = (evaporation + bleed) / bleed. At 2 cycles, you bleed as much as you evaporate. At 3 cycles, you bleed half the evaporation. At 5 cycles, a quarter. Higher cycles save water but push the pan closer to the point where calcium carbonate starts to precipitate.

Calcium carbonate is the main scale former, and how high you can push cycles depends on how much hardness and alkalinity you start with, plus pH and temperature. Commercial evaporative cooling and cooling tower practice works from a full water analysis. For homes, a simple rule is enough: the harder the supply, the lower the cycles you can tolerate and the more you have to bleed.

How much to bleed: a decision table

These are starting points we derived from the cycles formula and typical scaling behavior, not manufacturer specifications. If your manual gives a bleed rate, use it. Then observe: if white crust still forms quickly on the air-entering face of the pads, increase the bleed. If pads stay clean all season, you can try trimming it.

Ways to bleed off

A bleed-off tee is the standard residential choice. Smart cooler controls on some newer units include an automatic purge cycle, which is worth using if you have it.

How to install or set a bleed-off line

Time

20 to 45 minutes

Difficulty

Easy to moderate (roof work)

Tools and parts

Bleed-off kit or tee matched to your pump hose size, 1/4 inch tubing, hose clamps, a measuring cup or 1 quart container, a watch or phone timer

1. Kill power. Thermostat off, disconnect off.
2. Cut the pump hose and insert the tee. Place it on the discharge side between the pump and the distributor, secure with clamps.
3. Route the bleed tube to the drain. Run it to the overflow standpipe (many kits drop it into the top of the standpipe) or to a separate drain line. Keep it sloped downward with no kinks.
4. Run pump only and measure. Catch the bleed in a measuring cup for 60 seconds. Multiply cups per minute by 3.75 to get gallons per hour (1 cup per minute = 60 cups = 3.75 gal/h).
5. Adjust. Open or close the bleed valve, or change the orifice, to hit your target from the table above.
6. Recheck after a week. Small bleed fittings clog. Make it part of your weekly look-and-listen check.

Where the bleed water goes matters. It must reach a proper drain or approved landscape outlet, not run across the roof. Roof runoff stains, corrodes flashing and can void roofing warranties. See water supply and drain lines.

Signs your bleed-off is wrong

Too little bleed

White crust on the air-entering face of pads within weeks, scale at the waterline, pump screen clogging, float valve sticking, and aspen pads that harden and lose cooling. See how to clean a swamp cooler.

Too much bleed

The float valve runs constantly, water use is far above expectations, and the pan level sits low. On some coolers a very high bleed can starve the pads if the pump discharge pressure drops.

Bleed when pump is off

Water running from the overflow while the cooler is idle means the float is set too high or leaking, not a bleed-off problem. See float valve adjustment.

Water quality beyond hardness

Softened water

Ion-exchange softeners replace calcium and magnesium with sodium. That prevents hard scale, but sodium still concentrates in the pan, and several cooler manufacturers caution that softened water can increase corrosion of galvanized steel. If your house is softened, many installers tap the cooler supply from a line before the softener. Check your manual.

Reverse osmosis and well water

RO water is very low in minerals, so scale is not an issue, but the supply rate is usually too slow and costly for a whole-house cooler. Well water varies enormously; some wells carry iron, which leaves orange stains, or high silica, which forms a glassy scale vinegar cannot remove. Get a basic water test if you are on a well.

Chemical treatments

Scale-inhibiting and odor treatments sold for coolers can help in some water conditions, but they do not replace bleed-off. Use only products your cooler manufacturer allows, and never add anything that should not be breathed, since the blower carries pan water vapor and droplets indoors.

Reading your water quality report

Municipal utilities publish an annual water quality report (often called a Consumer Confidence Report). Look for "hardness" (as calcium carbonate, in mg/L or grains per gallon) and "total dissolved solids" (TDS, in mg/L). One grain per gallon equals about 17.1 mg/L. Many utilities report a range because the source changes by season; use the high end for cooler planning, since summer sources are often harder. If you are on a well, an inexpensive TDS meter and a hardness test strip give a usable estimate, and the same meter dipped into your pan water tells you your actual cycles of concentration: pan TDS divided by supply TDS.

What bleed-off costs you

At 2 to 4 gallons per hour for 10 hours a day, bleed adds about 600 to 1,200 gallons per month to a cooler's water use. At typical municipal water rates, that is usually a few dollars per month, though tiered rates in some cities raise it. Compare that with a set of pads or a pump replaced early. For full water budgets, see swamp cooler water usage, and for how pad life ties to water quality, see how often to replace cooler pads.