How to Drain a Swamp Cooler: Reservoir, Supply Line and Pump, Step by Step
Drain a swamp cooler the right way: shut off water, pull the overflow standpipe, clear the supply line, pump and bleed tube, then dry the pan before winter.
Short answer
To drain a swamp cooler, turn off the water supply and power, unscrew the overflow standpipe so the pan empties down the drain line, disconnect the supply line at the float valve to empty it, clear the pump and bleed tube, then sponge out and dry the pan.
Key takeaways
- Shut off water at the supply valve first, then power, so the float valve cannot refill the pan while you work.
- Most roof coolers drain through the overflow standpipe: unscrew or lift it and the whole reservoir goes down the drain line.
- Draining the pan is not enough before a freeze. The copper or plastic supply line, the float valve body and the bleed tube also hold water.
- Sponge out the last 1/4 inch of water and sludge. Standing water in a galvanized pan all winter is what starts rust-through.
- A full drain takes about 30 to 60 minutes on a roof unit and 10 minutes on a portable.
On this page
Draining a swamp cooler means emptying three separate places that hold water: the reservoir pan, the supply line that feeds the float valve, and the small passages inside the pump, distributor and bleed tube. Most people only do the first. That is fine for a mid-summer flush, but before a freeze the other two are where cracked fittings and split lines come from. This guide covers both jobs: the quick in-season drain and the complete pre-winter drain, for roof downdraft units, side-draft units and coolers with a purge pump.
If you want the whole fall shutdown (pads, belts, motor, covers, dampers), start with the winterizing overview. This page goes deep on the water side only.
Where the water actually sits in a cooler
Before you open anything, know what you are emptying. A typical whole-house cooler holds more water, in more places, than people expect.
| Location | Typical volume | Freeze risk if left full |
|---|---|---|
| Reservoir pan, 4,000 to 7,000 CFM cooler | 3 to 6 gallons at normal float level | Low for the pan itself, high for rust from standing water |
| Reservoir pan, large 18,000 to 21,000 CFM cooler | 6 to 10 gallons | Same as above |
| 1/4 inch copper or poly supply line, 30 ft run on the roof | About 0.04 gallon (roughly 6 oz) | High: small lines freeze first and split |
| Float valve body and compression fitting | A few teaspoons | High: brass seats and plastic bodies crack |
| Pump volute, hose, spider or distributor tubes | A cup or two | Moderate: brittle plastic can split |
| Bleed-off tube and tee | A few ounces | Moderate: ice plugs and cracks the tee |
The supply line is the critical piece. It holds very little water, but it is exposed on the roof, has almost no thermal mass, and is the first thing to freeze on a clear 25 F night. The pan holds the most water, but its risk is corrosion, not ice.
Tools and time
- Wrenches
- Two adjustable wrenches, or 7/16 and 1/2 inch open-end wrenches for typical 1/4 inch compression nuts on the float valve
- Drain tools
- Channel-lock pliers for a stuck standpipe, a large sponge, a bucket, and optionally a wet/dry shop vacuum
- Cleaning
- Plastic putty knife or stiff nylon brush for sludge, rags, a garden hose if you plan to rinse
- Safety
- A sturdy ladder secured at the roof edge, gloves, and a way to lock out the cooler disconnect or breaker
- Time
- 10 to 15 minutes for an in-season drain; 30 to 60 minutes for a complete pre-winter drain on a roof unit
Quick drain: the in-season flush
Use this every few weeks in summer to dump concentrated minerals and sediment. It does not touch the supply line.
- Turn the cooler off at the thermostat or switch. Wait a minute so the pump stops and water drains back off the pads into the pan.
- Kill power at the disconnect. The service disconnect is usually a box or pull-out on or next to the cooler. Never reach into the cabinet with the pump or motor live.
- Remove one side panel. On most downdraft units any panel works; pick the one nearest the drain fitting.
- Pull the overflow standpipe. The standpipe is the short vertical tube threaded into the pan drain. Unscrew it counterclockwise or lift it out if it is a push fit. Water drains down the drain line immediately.
- Scrub while it drains. Push sediment toward the drain with a brush. Check the pump intake screen and rinse it.
- Reinstall the standpipe hand tight. Then turn power back on and let the float valve refill the pan. Watch that it stops about 1 inch below the top of the standpipe; if not, see float valve adjustment.
Pro tip: If the pan drains slowly, the drain line is partly blocked with scale or debris, not the pan. Run a garden hose or a length of 1/4 inch tubing up the drain line from the bottom end before winter, when a slow drain becomes a full pan of ice.
Complete drain before freezing weather
This is the procedure to follow when you shut the cooler down for the season. Do it before the first forecast hard freeze, not after. Our regional timing guide covers when that typically is where you live.
- Close the water supply valve. Find the shutoff that feeds the cooler. It is usually a 1/4 turn or saddle valve on a cold line in the attic, the laundry room, under a sink, or at an outdoor hose bib with a tee. Close it fully. If you cannot find it, trace the 1/4 inch line from the cooler down the roof or wall.
- Open a drain point below the valve if there is one. Many installers fit a small drain cock or boiler drain on the house side of the shutoff. If yours has one, open it and catch the water in a bucket. If not, the line will empty from the top in the next step.
- Turn off power and open the cabinet. Lock out the disconnect or breaker.
- Disconnect the supply line at the float valve. Back off the compression nut where the line enters the float valve. Use one wrench on the valve body and one on the nut so you do not twist the valve through the cabinet wall. Let the line drain into the pan. Coil the end so it points down, or tape a sandwich bag over it to keep insects and dirt out.
- Blow the line out if it has dips. Lines that run up and over a parapet hold water in low spots. A few short puffs of compressed air at under 30 psi, or a long blow from your lungs on a short run, clears it. Do not blow back toward an open valve.
- Empty the pan. Pull the standpipe as above. On units without a drain, use a shop vacuum.
- Clear the pump. Restore power briefly and bump the pump for 3 to 5 seconds so it pushes water out of the distributor tubes, then kill power again. Or unclip the pump, tip it upside down, and let the volute drain. Many owners remove the pump entirely and store it indoors; it is a two-minute job on most units and avoids winter corrosion of the impeller shaft.
- Disconnect and drain the bleed-off tube. Pull the small tube off the bleed tee and let it drain. Leave it disconnected over winter.
- Sponge out the remaining water and sludge. The last 1/4 to 1/2 inch of water sits below the standpipe threads. Sponge it out, scrape out mineral sludge with a plastic knife, and leave the pan as dry as you can.
- Leave the standpipe out or loose. This lets rain or snowmelt drain through the pan instead of pooling.
- Inspect as you go. Look for rust spots in the pan, a cracked float, or a stiff drain line. Note them for spring; see cabinet rust and pan repair if the pan already shows pinholes.
Warning: Draining the cooler does not protect the supply line inside the house or attic. If the line runs through an unheated attic or an exterior wall, it can freeze even with the cooler empty. Close the valve at its source and drain everything downstream of it.
Variations by cooler type
Downdraft roof coolers
These usually have the drain fitting in a corner of the pan with a 3/4 inch hose barb or threaded fitting below. Everything above applies. Close the duct damper or install a cover after draining; see swamp cooler covers.
Side-draft coolers on a wall or ground stand
Side-draft units often sit lower and drain to a nearby gutter or yard. Because the supply line frequently comes straight up the outside wall, it is fully exposed. Drain it carefully and consider insulating or removing the vertical run.
Coolers with a purge or dump pump
Some higher-end units use a purge pump or timed dump valve instead of a continuous bleed. The purge pump has its own small reservoir and outlet hose. Run a manual purge cycle if the control has one, then disconnect and drain the purge hose and tip the pump to empty it. Store a detachable purge pump indoors.
Units with a dump valve and controller
Premium units such as some Breezair and Phoenix models have an electric drain valve and a controller that can drain the pan on shutdown. Use the controller's drain or winter mode first, then still close the supply valve and drain the inlet line by hand. The automatic drain empties the tank, not the supply line.
Portable and window coolers
These are covered separately in winterizing portable and window coolers, because drying the media and the tank matters more than the line.
Where the drain water should go
A cooler pan full of end-of-season water is concentrated tap water: the evaporation that cooled your house left the minerals behind. On hard water it can carry several times the dissolved solids of the tap. It is not hazardous, but it can stain stucco and roofing and build crust on gutters. Route the drain line to a downspout, roof drain or landscape bed, keep it off walls and walkways, and follow local rules if you are tempted to tie it into sewer or storm drains. For hose sizing, slope and freeze-safe routing of the line itself, see water supply and drain lines.
Common draining mistakes
- Draining the pan but leaving the supply line pressurized. This is the most common cause of a split line or cracked float valve found in spring.
- Overtightening the standpipe or float nut. Plastic standpipes crack, and brass compression nuts deform the ferrule. Hand tight plus a quarter turn is plenty.
- Running the pump dry for minutes. A few seconds is fine; longer can overheat the seal and shorten pump life.
- Leaving sludge in the corners. Wet sludge holds moisture against galvanized steel all winter and starts pinhole rust.
- Forgetting the bleed tube. Its small tee is one of the first parts to crack in a freeze.
If you discover damage while draining, our freeze damage repair guide explains how to test and replace each part.
Worked example: how fast a roof line freezes
A 30 foot run of 1/4 inch OD copper tubing has an inside diameter of about 0.19 inch. Its cross-sectional area is about 0.028 sq in, so 360 inches of line hold about 10 cubic inches, or roughly 0.04 gallon of water (about 0.36 lb). Freezing that water releases only about 0.36 x 144 = 52 BTU. On a clear night at 20 F with wind, a bare small line can lose that much heat in well under an hour. That is why a line can split on the first hard freeze while the pan, with 40+ lb of water, only gets a skin of ice.
Frequently asked questions
How often should I drain my swamp cooler in summer?
Most systems benefit from a full drain and rinse every 2 to 4 weeks in season, more often with hard water or a weak bleed-off. A full drain is also the fastest fix for a musty smell.
Where does swamp cooler drain water go?
Typically through a 1/2 or 3/4 inch drain line from the pan to a roof drain, gutter, downspout, landscape bed or an approved sewer connection. Local codes vary, so check before routing cooler drain water anywhere new.
Can I use swamp cooler drain water on plants?
Usually yes for ornamentals, since it is just concentrated tap water. Avoid it on salt-sensitive plants if your water is very hard, and never reuse water from a pan that had chemical treatment or algaecide added.
My swamp cooler has no drain fitting. How do I empty it?
Use a wet/dry shop vacuum or a small transfer pump, then sponge out the rest. On a unit you service often, it is worth adding a standpipe drain fitting so future drains take seconds.
Should I leave the overflow standpipe out over winter?
Many technicians leave it out or loosened so rain and snowmelt drain freely instead of pooling in the pan. Just store it somewhere you will find it in spring and keep the threads clean.
Sources and further reading
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- Evaporative Coolers (fact sheet), Colorado State University Extension
- Evaporative cooler owner's and installation manuals, Phoenix Manufacturing and Champion Cooler