Swamp Cooler Water Supply and Drain Lines: Tubing, Valves, Bleed-Off and Overflow

Plumbing a swamp cooler is simple in principle: a small cold water line feeds a float valve that keeps the pan topped up, and two small drain paths carry away overflow and bleed-off. In practice, most cooler leaks, ceiling stains and roof damage trace back to shortcuts here: saddle valves, sun-rotted tubing, bleed-off dumped on shingles, and shutoffs nobody can reach in December. This page shows how technicians do it so it lasts.

How much water the line has to supply

Size the supply for peak demand, which is evaporation plus bleed-off. Evaporation follows directly from the cooling the cooler does:

The constant comes from 1.08 x CFM x temperature drop (BTU/h of sensible cooling) divided by 1,060 BTU per pound of water evaporated and 8.34 pounds per gallon.

For day-by-day and seasonal totals, use the water usage calculator or read swamp cooler water usage.

Choosing tubing and fittings

Most float valves accept 1/4 in OD tubing with a compression nut and ferrule. Use a brass ferrule for copper and an insert (a small tube stiffener) inside poly tubing so the ferrule does not crush it. Snug the nut until it is firm, then about a half turn more; over-tightening cracks ferrules and is a classic cause of slow leaks.

Tapping the supply: where and how

Pick the source

The usual sources are a cold line in the attic, the water heater cold inlet, a laundry box cold line, or an outdoor hose bib. Always use cold water. Choose a source that lets you place the shutoff valve somewhere warm and reachable, because you will close and drain this line every fall.

Use a tee, not a saddle valve

Self-piercing saddle valves clamp onto a pipe and puncture it. They are quick, but the rubber seal hardens with time and the small needle valve corrodes. Technicians commonly find them dripping in attics years later. A better approach:

1. Shut off the house water and open a low faucet to drain the line.
2. Cut in a tee on the cold line using the method that matches your pipe: sweat, press, push-to-connect or PEX crimp.
3. Add a quarter-turn shutoff on the branch, with a 1/4 in compression outlet, ideally with a drain cap or bleed port.
4. Run the tubing to the cooler with gentle bends, supporting it every few feet and keeping it away from sharp edges and hot flues.
5. Penetrate the roof correctly. Bring the line through the roof jack or a flashed pipe boot, never through a hole sealed only with caulk.

Pressure, backflow and code points

Pressure

Float valves are designed for normal household pressure. If your house pressure is high (many plumbers use 80 psi as the line where a pressure-reducing valve is needed), floats chatter and leak; fix house pressure first.

Backflow

The float valve outlet discharges above the pan, which creates an air gap, but some jurisdictions still require a vacuum breaker or backflow preventer on the cooler supply. Ask your building department.

Shutoff access

Codes and common sense both call for an accessible shutoff. A valve buried behind attic insulation that no one can find defeats the purpose.

Freeze protection

Slope the line so it can drain toward the shutoff or a drain cap, and drain it before the first hard freeze.

Float valve connection and setting

The float valve (fill valve) opens when water drops and closes when the float rises. Connect the tubing, open the supply slowly, and watch the first fill.

1. Fill and stop. Let the pan fill until the float shuts off.
2. Check the level. Water should stop about 1/2 to 1 in below the top of the overflow standpipe and high enough to cover the pump intake.
3. Adjust. Bend the float rod or turn the adjuster (depending on design) and retest with the pump running, since the pump draws the level down.
4. Watch for chatter. A float that buzzes or drips usually has a worn seat or high pressure.

Full float troubleshooting is in float valve adjustment.

Overflow drain routing

The overflow is a standpipe threaded into the bottom of the pan. If the float sticks open, water rises into the standpipe and drains instead of spilling out the louvers. Connect a hose or tubing to the bottom of the overflow fitting and route it to:

• A gutter or downspout, with the tube secured so it cannot pull loose.
• Landscape beds via a line down the wall, discharging away from the foundation.
• A plumbing drain through an air gap (indirect connection) if local code allows.

Do not let overflow or bleed-off run across the roof. Repeated wetting and mineral deposits break down asphalt shingles, stain stucco and corrode metal roofing and flashings.

Bleed-off: setting up the dilution line

As water evaporates, its minerals stay behind. Without bleed-off, the sump concentrates them into scale on pads, pump and pan. Bleed-off continuously removes a small stream of concentrated water so fresh water dilutes the sump. How much depends on hardness:

Two common setups:

• Continuous bleed. A small tee on the pump discharge with an adjustable valve or fixed orifice, running to the overflow fitting or its own drain tube. It wastes water whenever the pump runs, but it is simple and reliable.
• Purge or dump valve. A timer-controlled pump or valve empties the sump on a schedule (for example every 6 to 12 hours of run time) and the float refills it. This often uses less water for the same scale control and keeps the sump fresher.

To set it, measure the bleed-off into a container for one minute and compare with your estimated evaporation. Details and tuning are covered in bleed-off and water quality.

Common plumbing problems and fixes

For leaks that come from the cabinet or pan rather than the plumbing, see swamp cooler leaking. The full installation sequence is in the swamp cooler installation guide.