Evaporative cooler controls have evolved slowly over the past century. Early systems in the 1930s used simple wall switches to control the fan and pump. Mechanical thermostats appeared in the early 1960s, followed by digital thermostats in the early 2000s.
Evaporative cooler thermostats control cooling by operating the fan and pump based on indoor temperature. Most designs simply turn the cooler fully on or off when the temperature crosses a setpoint, a control method first introduced with mechanical cooler thermostats in the early 1960s.
Modern controllers now use sensor-based algorithms to stabilize indoor temperature.
See also:
Complete Guide to Evaporative Cooler Control, how modern controllers stabilize temperature and improve evaporative cooling performance.
Below is a timeline of evaporative cooler control systems.
| Period | Control Type | Key Feature |
|---|---|---|
| 1930s–1950s | Toggle switches | Manual control of pump and fan |
| 1950s–1970s | Rotary cooler switch | Combined HIGH / LOW / VENT modes |
| Early 1960s | Mechanical thermostat (SelectaStat) | First automatic temperature control |
| 1990s–2000s | Timer-assisted controls | Pump pre-wetting and cycling |
| Early 2000s | Digital thermostats | LCD display and electronic switching |
| 2020s | Intelligent controllers | Sensor-based cooling stabilization |
1930s – 1950s
Individual Toggle Switches
Early residential evaporative coolers were controlled with three separate wall switches:
- Pump (wet the pads)
- Fan ON/OFF
- Fan speed (HIGH / LOW)
The homeowner manually adjusted these switches depending on weather conditions.
This method worked but required constant attention. During hot afternoons the cooler might run continuously, while cooler mornings and evenings required manual adjustment.
This control method remained common in homes through the 1950s and 1960s.
1950s–1970s
Rotary Combination Switch
By the mid-20th century manufacturers introduced rotary cooler controls that combined several switches into a single knob.
Typical positions included:
OFF
VENT
LOW COOL
HIGH COOL
This simplified operation and became the standard evaporative cooler wall control for decades.
However, it still relied entirely on manual adjustment by the homeowner. Many homes still had these switches well into the 2000s.
Early 1960s
SelectaStat Mechanical Thermostat
One of the earliest thermostats designed specifically for evaporative coolers was the SelectaStat, introduced in the early 1960s.
It used a mechanical temperature sensor and switched the cooler on when the temperature rose above the setpoint and off when it fell below it.
While convenient, this approach introduced a new problem: large temperature swings between “off” and “maximum cooling.”
1970s–1990s
Mechanical Thermostats in Modern Housings
During the following decades, manufacturers such as Emerson and others produced similar mechanical thermostats packaged in modern wall enclosures.
Functionally, they still operated exactly like the SelectaStat:
- sense temperature
- switch cooler on
- switch cooler off
Despite their modern appearance, they still used simple on/off control.
Later thermostats added temperature hysteresis, allowing the temperature to drift a few degrees above and below the setpoint before switching. This prevented rapid cycling but increased temperature swings.
1990s–2000s
Timer-Assisted Controls
Some manufacturers introduced timer-based controllers such as MasterCool units. These attempted to improve performance by periodically cycling the pump or delaying blower operation while the pads became wet.
Although helpful, these devices still relied primarily on simple on/off control.
Early 2000s
Digital Evaporative Cooler Thermostats
Around the early 2000s manufacturers introduced digital cooler controllers such as the Dial 7617 and similar models.
These replaced mechanical temperature sensors with electronic temperature measurement and relay switching.
Features typically included:
- LCD temperature display
- push-button setpoint control
- automatic on/off operation
However, the basic strategy remained unchanged from the 1960s: turn the cooler completely on or completely off.
2010s
Low Voltage Thermostats
More recent controllers such as Champion MasterStat and Dial Digi-Stat added:
- low-voltage controller separate from actual switching
- cooler-mounted outdoor electronics box
- larger displays
These systems often required additional wiring to the outdoor electronics.
Despite the added complexity, the control method still remained the same: binary on/off control of the entire cooler.
2020s
Intelligent Control
Modern controllers such as Roadrunner Comfort represent a new approach. Instead of simply switching the cooler on and off, intelligent controllers:
- monitor indoor temperature continuously
- adjust pump operation dynamically
- stabilize indoor temperature
- reduce temperature swings
This approach treats the evaporative cooler as a system that can be modulated, rather than a device that is simply on or off.
Summary
For more than 60 years, most evaporative cooler thermostats have used the same basic strategy: turning the cooler fully on or fully off at a temperature setpoint.
While the packaging has evolved from toggle switches to digital displays, the underlying control method has changed very little.
Newer intelligent control systems aim to improve comfort and efficiency by managing the cooling process itself rather than simply switching it on and off.