Swamp cooler overflowing: a stuck float and a pan that will not stop filling

A swamp cooler pan that keeps filling until it runs out the overflow has a float valve problem, not a pump or pad problem. The valve that is supposed to shut the water off at the right level is either set too high or held open by scale on its seat. It is one of the cheapest repairs on the roof: a $150 to $350 service call with a float adjustment inside it, or $100 to $250 for a new float valve if the seat is gone. What it is not is something to leave running, because the water has to go somewhere and on most Reno roofs that somewhere is your roof decking, your stucco, or your ceiling.

Check this first (two minutes, water on)

1. Shut the supply off at the hose bib. Do this before anything else. Everything below can wait an hour; soaked decking cannot.
2. Pull a side panel and read the water line. The pan should fill to roughly an inch below the top of the overflow standpipe and stop. Water at or above the standpipe means the float is not closing in time.
3. Turn the water back on and lift the float arm by hand. If the flow stops cleanly, the valve works and the float is simply set too high, which is a bend or a screw adjustment. If water keeps running with the arm lifted, the valve seat is worn or scaled and the whole valve gets replaced.
4. Look at the float itself. A float ball or cup that has taken on water, or one crusted with mineral, rides low and tells the valve to keep filling even when the pan is full. That float is finished regardless of what the seat does.
5. Check the standpipe is clear. A blocked standpipe turns a small overfill into the pan spilling over its own edge, which is how water gets under the cooler instead of down the drain line.

What is usually wrong

Every number is the same range published on the pricing page, and the pump and float valve page explains how each part is sized and what a bleed-off does.

Why Reno floats stick

The water is soft on average and unsteady in practice. The Truckee Meadows Water Authority treats soft Truckee River water at its Chalk Bluff and Glendale plants, and most areas read low: Central Southeast Reno at 1.9 to 3.6 grains per gallon over the last four quarters, Old Sparks at 3.9 to 5.4. But the number does not hold. Southwest Reno read 13.9 grains in the first quarter of 2026 and 2.6 in the second; West Reno ranged from 4.7 to 10.4. TMWA says the groundwater wells it runs from mid-April through October can and do change the water chemistry somewhat, so a cooler cannot be assumed to be drinking soft water. A float valve is a rubber seat and a small brass orifice sitting in that water all summer, and mineral on either one stops it sealing. A pan does not average its supply, it concentrates whatever arrives. On a private well in the Carson Valley or at the edges of Fernley it happens faster.

A freeze cracks what a season scales. Reno averages 111.6 nights a year at or below 32 degrees, and NOAA’s normals give the airport a 50 percent chance of the first fall freeze by October 19 and a 90 percent chance by November 2. A float left full of water through that first hard night can crack its body, and the cooler that “worked fine last year” fills without stopping at the May start-up. The fall winterization at $100 to $200 is what prevents it.

Nobody looks at a cooler that is cooling. An overflowing pan still makes cold air, so the failure runs for weeks before anyone notices the wet stucco. The spring start-up sets the float and clears the standpipe once a year, which is when this gets caught.

The bleed-off is not a leak

Plenty of overflow calls turn out to be a bleed-off line doing its job. A bleed-off is a deliberate small stream of pan water dumped to the roof drain so evaporating water cannot concentrate its minerals in the pan. On a supply that swings as wide as Reno’s does it is the single best thing you can add to a cooler, and the trade is a thin trickle you would not hear from the ground. A stream you can hear is a float, not a bleed-off, and a bleed-off turned wide open by a previous tech wastes water without helping. Either gets set correctly on the visit.

When it is not worth fixing

Overflow is cheap to fix and expensive to ignore. A season of water running over the pan lip rusts a galvanized pan from the top down, and a pan that is thin or weeping is the end of the cooler, not a part. On the decades-old units still running on the post-war ranches in Sparks and the mid-century southwest, a float call is often when the pan gets its honest assessment. If that is the answer you get, the installation page has the numbers, and the repair cost calculator will price both paths before you decide.

What to expect on the call

Say whether the water stopped when you lifted the arm, because that one answer decides adjustment from replacement and it changes the quote. Send a photo of the pan with the panel off, one of the float and standpipe, and one of the outside of the cooler where the water is running down. Floats for the common sizes ride on the truck, so most overflow calls across Reno, Sparks, and Carson City finish on the first visit.