Swamp Cooler Water Distribution System: Spider, Tubes, Troughs and Rigid-Media Headers
How a swamp cooler spreads water over its pads: spider distributor, tubes, troughs and rigid-media headers, plus how to clear clogs and balance flow.
Short answer
The pump pushes water up a riser to a spider (distributor) that splits it into one tube per pad. Each tube feeds a perforated trough or, on rigid media units, a header that wets the top of the media. Even wetting depends on clear outlets, level troughs and the right flow: enough to keep every pad wet without overflowing.
Key takeaways
- Dry streaks on pads almost always trace to a clogged spider outlet, a kinked tube, a plugged trough hole, or a tilted trough.
- Clear outlets and holes with a wire or drill bit turned by hand, then flush with the pump running.
- Flow should wet every pad top to bottom within a few minutes without spraying, splashing or pouring off the trough ends.
- Rigid media needs a full, even sheet of water across the top; partial wetting leaves permanent scale lines.
- Good bleed-off and regular pan cleaning are what keep distribution parts from scaling shut.
On this page
A swamp cooler's water distribution system carries water from the pump to the top of every pad and spreads it evenly. On side-panel aspen coolers it is a riser tube, a spider (distributor) that splits flow into one tube per pad, and a perforated trough (gutter) along the top of each pad. On rigid-media coolers, a perforated header or tray sheets water across the top of the media. When cooling drops, one pad looks dry, or water sprays out the louvers, this system is the first place to look.
This guide explains each part, how to find and clear clogs, and how to set flow so the pads are fully wet without overflowing. For the pump itself, see swamp cooler pump not working; for how these parts fit into the whole cooler, see swamp cooler parts anatomy.
How water gets from the pan to the pads
- Pump. A small submersible recirculating pump in the pan lifts water through its outlet.
- Riser tube. A flexible tube runs from the pump up to the top center of the cooler. The bleed-off tee, if present, is usually in this line.
- Spider. A plastic manifold with a center inlet and 3 or 4 outlets (one per pad side), often with a flow-adjust feature on some models.
- Distribution tubes. Short tubes from each spider outlet to the corresponding trough.
- Troughs. V or U-shaped channels across the top of each pad frame with a row of small holes or slots that drip water onto the pad.
- Pads and return. Water soaks down through the pad; what does not evaporate drains back into the pan.
On rigid-media coolers such as many Breezair and Phoenix rigid units, the spider and tubes may be replaced by a single pipe loop or a header tray with evenly spaced holes, and a spreader pad or cap on top of the media helps spread water sideways. The principle is the same: get a uniform film of water across the full width of every pad.
The parts, one by one
| Part | Typical material | Common problems | What to do |
|---|---|---|---|
| Riser tube | Vinyl or polyethylene | Kinks, cracks, popped off pump outlet, algae buildup | Reroute, clamp, or replace; flush annually |
| Bleed-off tee | Plastic | Clogged small outlet, disconnected drain line | Clear with wire; confirm drip |
| Spider | Plastic | Scaled outlets, cracked body, lost flow adjuster | Soak and clear, or replace |
| Distribution tubes | Vinyl | Sun-hardened, kinked, slipped out of trough, plugged with debris | Replace when stiff; secure ends |
| Troughs | Plastic or galvanized steel | Clogged drip holes, tilted, sagging, shifted off pad | Clean holes, level, reseat on pad |
| Rigid-media header | PVC pipe or plastic tray | Clogged holes, uneven flow end to end | Flush, clear holes, verify level |
| Spreader pad (rigid units) | Fibrous pad or media cap | Scale, dislodged | Rinse or replace with media service |
Diagnosing uneven wetting
The quickest check is visual. With the cooler on pump-only (or fan off and pump on) for 5 minutes, look at each pad from outside. A good pad is uniformly dark from top to bottom. Patterns tell you a lot:
| What you see | Most likely cause | Fix |
|---|---|---|
| One whole pad dry | Blocked spider outlet, kinked or detached tube | Check tube from spider to trough |
| Vertical dry stripes | Individual trough holes clogged | Clear each hole |
| One end of a pad dry, other end soaked | Trough tilted | Level the trough |
| Bottom of pads dry, tops wet | Weak pump, low flow, or very hot dry day with high airflow | Clean pump screen; check pump output |
| All pads dry | Pump not running, riser off, pan empty | See pump not working |
| Water spraying from louvers | Too much flow or trough off the pad | Reseat trough; reduce flow |
| White crust bands on rigid media | Repeated partial wetting, poor bleed-off | Fix header flow, increase bleed-off |
Dry areas are not just lost cooling. Dry media lets more dust through, and the boundary between wet and dry areas is where minerals concentrate fastest, which is why streaky pads scale up in stripes.
Worked example: what one dry pad costs
A four-sided aspen cooler moves 4,000 CFM, and on a 100 F day with a 66 F wet bulb a fully wet aspen pad at 75 percent efficiency gives a supply temperature of 100 - 0.75 x (100 - 66) = 74.5 F. If one of four pads is dry, about a quarter of the air passes through with little cooling. Mixed supply becomes roughly 0.75 x 74.5 + 0.25 x 100 = 80.9 F. The temperature drop falls from 25.5 F to about 19.1 F, and sensible cooling drops from 1.08 x 4,000 x 25.5 = 110,160 BTU/h to 1.08 x 4,000 x 19.1 = 82,512 BTU/h, about 25 percent less from one clogged outlet. Try your own conditions in the cooling temperature calculator.
Clearing clogs step by step
Safety: Unplug the pump and turn off the blower at the disconnect before reaching into the cabinet. Turn the pump back on only for the flow test steps, keeping hands clear of the blower area.
- Clean the pump screen first. A partly clogged pump intake reduces flow to every pad and mimics distribution clogs. Rinse the screen and remove debris from the pan.
- Check the riser. Pull the riser off the spider and run the pump into a bucket for a few seconds. Strong, steady flow means the pump and riser are fine.
- Remove the spider. Unclip it from its bracket and pull the tubes. Look into each outlet.
- Descale. Soak the spider and any scaled tubes in white vinegar or a cooler-safe descaler for 1 to 2 hours. Clear each outlet with a stiff wire or a drill bit of the right size turned by hand, never with a power drill, which can enlarge the outlet and unbalance flow.
- Clean the troughs. Brush out debris and scale. Clear every drip hole with a wire or toothpick. On metal troughs, check for rust holes that leak in the wrong places.
- Flush tubes. Blow through or flush each tube with a garden hose. Replace any that are stiff, cracked or permanently kinked.
- Reassemble and test. Reconnect, run the pump, and watch each trough fill and drip evenly along its length.
Pro tip: Before removing anything, run the pump and hold each trough's tube end over the pan. Compare the streams. A visibly weak stream identifies the clogged outlet before you take the spider apart.
Heavy scale throughout the system means the minerals are winning. Read scale removal and prevention and check your bleed-off rate in bleed-off and water quality.
Adjusting and balancing flow
The target is simple: every pad should be fully wet from top to bottom within a few minutes of the pump starting, with water dripping steadily from the bottom of the pad into the pan, and no spraying, splashing or water pouring off the ends of troughs.
Ways to adjust
- Spider flow adjuster. Some spiders have a flow-limiting cap, a twist adjuster or an extra outlet that returns water to the pan. Open it to reduce flow to the pads; close it to increase.
- Trough level. A trough must be level along its length or water runs to the low end. Use a torpedo level and shim or bend the brackets.
- Trough position. The trough holes must sit directly over the pad's top edge. When aspen pads shrink, they pull away from the trough; reseat the pad or replace it.
- Tube routing. Keep tubes short with no sags where sediment can settle or kinks can form. Secure tube ends in the trough inlet so they cannot pop out.
- Pump choice. A pump that is too small for the cooler leaves pad bottoms dry; one that is too large floods troughs. Replacement pumps are sized for cooler types; see choosing a replacement pump.
Rigid-media flow
Rigid media needs more care than aspen. It needs enough water to wash the media continuously, because a film that partially dries on the media leaves scale lines that permanently block channels. Manufacturer guidance for rigid media generally calls for a full, uniform flow across the whole top edge rather than a trickle. Check that every header hole flows, that the header is level, and that the spreader layer is in place. If you see dry triangles below some header holes, the flow is too low or holes are plugged. Compare media types in aspen vs rigid media.
Prevention and seasonal care
- Spring
- Inspect and replace sun-hardened tubes, soak the spider, clear trough holes, and test wetting before the first hot day. It fits naturally into the spring startup checklist.
- Monthly in season
- Watch the pads for dry streaks and check the bleed-off drip.
- Fall
- Drain the riser and spider so trapped water cannot freeze and crack them. See how to winterize a swamp cooler.
- Spare parts
- A spare spider and a few feet of matching tubing cost little and save a mid-summer roof trip.
Distribution parts are inexpensive: a spider and tube set typically costs about $8 to $30 and troughs about $10 to $40 each in 2026, varying by brand and region. Universal spiders fit many coolers if the number of outlets and tube sizes match; the universal replacement parts guide explains what to measure.
When distribution problems are really something else
Not every dry pad is a distribution problem. Check these before replacing parts:
- Low pan level. If the float is set too low or sticking, the pump starves and sputters. See float valve adjustment.
- Worn-out aspen pads. Thin, compacted aspen does not wick water across its width. No amount of flow fixes it; replace the pads.
- Extreme heat and airflow. On the hottest, driest afternoons, high airflow can evaporate water faster than a marginal pump supplies it, drying pad bottoms. A clean pump screen and correct pump size matter most then.
- Leaks into the cabinet. A cracked trough or tube can dump water in the wrong place, causing both dry pads and leaks. See swamp cooler leaking.
Frequently asked questions
Why is one swamp cooler pad dry while the others are wet?
That pad's tube or spider outlet is blocked, kinked, or popped off, or its trough holes are clogged. Follow the tube from the spider to the trough and check each point with the pump running.
Can I add a valve to reduce water flow to the pads?
Many coolers have a flow adjustment on the spider or riser, and some technicians add a small inline valve. Reduce flow only enough to stop overflow and splashing, and confirm the bottom of every pad still gets wet.
How do I clean a scaled swamp cooler spider?
Remove it, soak it in white vinegar or a cooler-safe descaler for an hour or more, then clear each outlet with a wire or small drill bit turned by hand. Replace it if the body is cracked or the outlets are brittle.
What size are swamp cooler distribution tubes?
They vary by brand, but many residential coolers use small plastic or vinyl tubes of roughly 1/4 to 3/8 inch inside diameter. Measure the old tube and spider outlet before buying replacements.
Why does water spray out of my cooler louvers?
Too much flow, a trough that has shifted off the pad, a tube that slipped out of the trough, or aspen pads that have shrunk away from the trough all let water escape. Correct the trough position first, then reduce flow if needed.
Sources and further reading
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- Evaporative cooler owner's manuals and parts lists, Phoenix Manufacturing and Champion Cooler
- CELdek evaporative cooling pad installation and water distribution guidance, Munters
- Evaporative Coolers (fact sheet), Colorado State University Extension