Thousands of Colorado residents found themselves locked out of their smart thermostats during sweltering temperatures last week in an effort to prevent power demand from overwhelming the grid.

About 22,000 Xcel customers lost control of their smart thermostats for hours on August 30th, Denver7 News reports. That led to backlash on social media as some people said the temperatures inside their homes reached as high as 88 degrees Fahrenheit. Outdoor temperatures climbed into the 90s that day across parts of Colorado as much of the western US grappled with sweltering heat.

All of the customers affected had enrolled in an energy-saving program, called AC Rewards, that’s meant to ease the strain on the power grid during heatwaves. Xcel can adjust those customers’ smart thermostats when demand gets so high that there might not be enough supply to meet it. Xcel offers a one-time $100 credit on electricity bills upon signing up, and $25 a year afterward for participation.

When the utility adjusts a customer’s thermostat, the customer typically has the option to opt-out. But, “On rare occasions, system emergencies may cause a control event that cannot be overridden,” the company says on its website.

Last week was the first time Xcel barred customers from overriding their adjustments in the six years since the program started, according to Denver7. High temperatures, soaring power demand for air conditioners, and an unexpected outage all contributed to last week’s energy emergency, Xcel vice president Emmett Romine told Denver7. Xcel did not immediately respond to a press inquiry from The Verge.

Extreme heat is stressing out power grids across much of the Western US during a prolonged heatwave that started last week and is expected to linger well into this week. California’s grid operator has urged residents to conserve energy over the next several days to avoid blackouts.

Americans are facing growing power interruptions compared to the past, in part because of more extreme weather. 2020 was a record year for power outages in the US. Punishing summer heat in Texas last year triggered a similar backlash from residents when utilities set customers’ smart thermostats to higher temperatures to limit power demand.

Such programs are part of a strategy called demand response that’s supposed to help power grids become more resilient to extreme weather events that are becoming more frequent and severe as global temperatures rise. Energy providers are always managing a precarious balance between supply and demand that can quickly lead to power outages during demand peaks. Demand response can smooth out those peaks, which typically spike when heatwaves drive up demand for electricity to power air conditioning. So while some customers might see their thermostats set to higher temperatures than they anticipated, that’s supposed to help power stay on for their households and their neighbors.

There is a risk with that strategy, however, if smart thermostats keep homes at dangerously high temperatures. Prolonged exposure to high temperatures — especially at night when people are at home sleeping — can lead to heat-related illness and even death. Heat spells kill more people in the US than any other weather-related disaster.