Ductwork for Evaporative Coolers: Drops, Duct Sizing, Diffusers and Insulation
How to size and lay out swamp cooler ductwork: roof jack and drop, main and branch duct sizes by CFM, diffusers, insulation, winter dampers and AC duct reuse.
Short answer
Evaporative cooler ductwork must carry roughly three times the airflow of central AC at low static pressure. Size the main drop for about 1,500 to 1,800 feet per minute and branches for about 800 to 1,200 fpm, insulate everything in the attic, use short straight runs, and add a damper or cover to seal the drop in winter.
Key takeaways
- Duct area (sq ft) = CFM / velocity (fpm). A 4,500 CFM cooler needs about 3 sq ft of main duct at 1,500 fpm, roughly a 21 x 21 in square.
- Residential coolers are built for low external static pressure, so every elbow, long flex run and undersized branch costs real airflow.
- Insulate attic ducts (R-6 to R-8 is common): supply air in the 60s or 70s F gains heat fast in a 130 F attic.
- Existing AC ducts are usually too small for evaporative airflow; dual systems need a dedicated drop or motorized dampers.
- Seal the drop every winter with an insulated cover or damper to stop heat loss and drafts.
On this page
- Why evaporative ducts are bigger than AC ducts
- Sizing the main drop and trunk
- Branch ducts for multi-room systems
- Diffusers and registers
- Insulation and heat gain
- Winter dampers and covers
- Sharing ducts with a furnace or air conditioner
- Troubleshooting duct-related airflow problems
- Frequently asked questions
Swamp cooler ductwork has one job: move a large volume of cool, moist air into the house with as little resistance and heat gain as possible. Because an evaporative cooler moves roughly three times the air of a central AC system, its ducts must be bigger, shorter and better insulated than people expect. Undersized or poorly insulated ductwork is one of the most common reasons a healthy cooler feels weak.
This page covers the drop, main and branch duct sizing with real arithmetic, diffusers, insulation, winterizing, and the question of sharing ducts with an AC or furnace. For the full job sequence see the installation guide.
Why evaporative ducts are bigger than AC ducts
Central AC typically moves about 400 CFM per ton, so a 3 ton system moves about 1,200 CFM of air cooled to the mid-50s F. An evaporative cooler serving the same house might move 4,500 to 6,500 CFM of air in the 60s or low 70s F. It delivers cooling by volume, not by a deep temperature drop. The sensible cooling relationship shows why:
To deliver the same cooling with a smaller temperature difference, CFM must go up. More CFM through the same duct means higher velocity, which means more friction, more noise and less airflow from a blower that is designed for low static pressure. Residential downdraft and side-draft coolers are typically rated at only a few tenths of an inch of water column of external static pressure. Every extra elbow and every undersized branch eats into that.
For background on what CFM means and how it is rated, see CFM explained.
Sizing the main drop and trunk
Duct sizing comes down to one equation:
For residential evaporative systems, a practical target for the main drop or trunk is about 1,500 to 1,800 fpm. Many single-drop installations run faster than that, which works but adds noise and costs some airflow. Multiply square feet by 144 to get square inches.
| Cooler CFM | Area at 1,500 fpm | Square size at 1,500 fpm | Area at 1,800 fpm | Square size at 1,800 fpm |
|---|---|---|---|---|
| 3,000 | 2.0 sq ft (288 sq in) | 17 x 17 in | 1.67 sq ft (240 sq in) | 16 x 16 in |
| 4,500 | 3.0 sq ft (432 sq in) | 21 x 21 in | 2.5 sq ft (360 sq in) | 19 x 19 in |
| 5,500 | 3.67 sq ft (528 sq in) | 23 x 23 in | 3.06 sq ft (440 sq in) | 21 x 21 in |
| 6,500 | 4.33 sq ft (624 sq in) | 25 x 25 in | 3.61 sq ft (520 sq in) | 23 x 23 in |
Worked example: is an existing 18 x 18 in drop adequate?
18 x 18 = 324 sq in, or 324 / 144 = 2.25 sq ft. With a 5,500 CFM cooler, velocity = 5,500 / 2.25 = 2,444 fpm. That is well above the 1,500 to 1,800 fpm target, so expect audible air rush at the diffuser and less airflow than the cooler's rating. With a 4,500 CFM cooler, velocity = 2,000 fpm: workable for a single drop, though still on the fast side.
Rectangular ducts are fine; keep the aspect ratio under about 3:1 for reasonable friction. Use galvanized steel for the drop and trunk. Seal every seam with mastic or UL-listed foil tape, not cloth duct tape.
Branch ducts for multi-room systems
A branched system splits the airflow to registers in several rooms. Size each branch for its share of the CFM at a lower velocity, about 800 to 1,200 fpm, so the registers stay quiet.
| Round duct | Area | Approximate CFM at 1,000 fpm |
|---|---|---|
| 8 in | 0.35 sq ft | 350 |
| 10 in | 0.55 sq ft | 545 |
| 12 in | 0.79 sq ft | 785 |
| 14 in | 1.07 sq ft | 1,070 |
| 16 in | 1.40 sq ft | 1,395 |
Start from the cooler size you chose with the cooler size calculator, then allocate airflow by room size and heat load. A west-facing living room with big windows deserves more than a north bedroom.
Worked example: splitting a 5,000 CFM cooler four ways
Living and kitchen get 2,000 CFM; master bedroom 1,200 CFM; two small bedrooms 900 CFM each (total 5,000). At about 1,000 fpm: living and kitchen need 2.0 sq ft, so two 14 in branches (2 x 1,070 = 2,140 CFM); master needs about 1.2 sq ft, a 14 in branch running slightly above 1,000 fpm or a 16 in branch; each small bedroom needs 0.9 sq ft, a 12 in branch at about 1,150 fpm. All fall inside the 800 to 1,200 fpm range.
Layout rules that save airflow
- Keep branches short and straight. Each 90 degree elbow adds resistance equal to many feet of straight duct.
- Use rigid metal where possible. If you use flex, pull it taut, support it every 4 to 5 ft, and avoid sags and kinks.
- Take branches off the plenum with proper take-off fittings, not crushed collars.
- Add balancing dampers at each branch take-off so you can tune airflow after start-up.
Diffusers and registers
The ceiling diffuser sets how air spreads in the room. Evaporative diffusers typically have adjustable louvers in 2, 3 or 4 directions so you can throw air toward living spaces and away from walls. Size diffusers so their free area is at least equal to the duct they serve; a restrictive grille undoes careful duct sizing. A closable diffuser makes winter sealing easy.
Pro tip: Point the louvers toward the rooms where you will open relief windows. Air takes the path of least resistance; aligning supply and exit sweeps cool air through the occupied zone instead of short-circuiting.
Insulation and heat gain
Summer attics in the Southwest often exceed 130 F. Supply air coming off good media might be 65 to 72 F. An uninsulated metal drop or trunk in that attic can add several degrees before the air reaches the room, which is a direct loss of cooling capacity.
Wrap all attic ductwork with duct insulation; R-6 to R-8 is common and check local energy code for the minimum. Seal the vapor jacket seams with tape so moist supply air cannot soak the insulation. Check that the roof jack connection is sealed too; a gap there pulls hot attic air or sucks air out into the attic.
Winter dampers and covers
An open drop is a big hole in your ceiling insulation. In heating season, warm house air rises up the drop and leaks out through the cooler, and cold air falls back down. Seal it with one or more of:
- An insulated ceiling diffuser cover or closable diffuser on the inside.
- A barometric or manual damper in the drop.
- A fitted, weatherproof cover on the cooler outside.
The full seasonal routine is covered in how to winterize a swamp cooler.
Sharing ducts with a furnace or air conditioner
Using existing heating and AC ducts is tempting, but rarely works as-is.
| Approach | How it works | Trade-offs |
|---|---|---|
| Separate drop | Cooler has its own roof jack and diffuser; AC or furnace uses its own ducts | Simplest and most common; one extra ceiling opening to seal in winter |
| Shared trunk with motorized dampers | Cooler connects to the supply trunk; dampers isolate the furnace or AC coil when the cooler runs | AC-sized registers limit airflow; dampers must seal well or moist air reaches the coil and furnace |
| Hybrid or two-stage system | Evaporative pre-cooling or staged control with a matched design | Requires an HVAC designer; best done as a planned system |
Warning: Never let evaporative supply air flow backward through a furnace heat exchanger or AC coil. The moisture can corrode the heat exchanger and grow mold on the coil. Shared systems need tight, interlocked dampers.
Troubleshooting duct-related airflow problems
- Check relief first. Weak airflow at registers is more often a closed-house pressure problem than a duct problem. Open more windows and see if flow improves. See venting and relief air.
- Look for disconnected or crushed runs. In the attic, check the roof jack seal, take-offs and flex runs for gaps, kinks and sags.
- Measure supply temperature at the cooler and at the register. More than a few degrees of rise indicates missing insulation or a leak pulling attic air.
- Calculate velocity. Divide cooler CFM by drop area. Well above 2,000 fpm in a single drop means the duct is the bottleneck.
- Check the cooler itself. A slipping belt or dirty blower wheel mimics a duct problem. See swamp cooler not cooling.
Frequently asked questions
Can I use flex duct for a swamp cooler?
Flex duct works for short branch runs if it is pulled taut and properly supported, but it has more friction than metal duct. Keep runs short, avoid sags and tight bends, and use rigid metal for the main drop.
Why is my swamp cooler duct sweating?
Supply air from an evaporative cooler is near saturation, so ducts can collect moisture where they pass through cooler spaces or where the drop is uninsulated. Insulating and sealing the duct and keeping relief air open usually solves it.
How many registers does a swamp cooler need?
A single central diffuser is common in small single-story homes. Larger or chopped-up floor plans benefit from a branched system with a register in each main room, each sized for its share of the CFM.
Should swamp cooler ducts have return air?
No. Evaporative cooling is a once-through system: outdoor air comes in and leaves through windows or relief dampers. Recirculating humid indoor air back to the pads would reduce cooling.
What is a roof jack on a swamp cooler?
The roof jack is the sheet metal transition that connects the cooler's discharge through the roof deck to the duct below. It must be flashed into the roofing and sealed to the duct to prevent leaks.
Sources and further reading
- ASHRAE Handbook: Fundamentals (duct design chapter), ASHRAE
- ASHRAE Handbook: HVAC Applications (evaporative cooling chapter), ASHRAE
- Evaporative Coolers, U.S. Department of Energy, Energy Saver
- Evaporative Coolers (fact sheet), Colorado State University Extension