Your Thermostat Is Lying to You.
Meet Järki Kettu.

That is the intentional Coast Zone. Traditional thermostats keep the number static by rapidly short-cycling, which conditions only the air and abuses your compressor. Järki Kettu executes deep thermal soaks that condition your home's structural mass (walls, floors, drywall). Once saturated, the house coasts naturally for hours without needing the HVAC equipment to restart.

In practice, no. Steady-state, continuous compressor runtimes operate at much higher mechanical efficiency (COP) than frequent stop-and-start cycles. Furthermore, long runs eliminate the repeated high-amperage inrush currents that occur every time a compressor starts. For heat pumps, the built-in Setpoint Walker specifically prevents the system from triggering power-hungry auxiliary resistive heating elements (typically 5kW–15kW).

No! Järki Kettu Core is written in pure, native Home Assistant YAML and Jinja2 templating. It runs entirely on native Home Assistant primitives with zero external Python runners or cloud dependencies. The only optional HACS component is Thomas Lovén's card_mod if you choose to deploy the transparent dashboard UI card.

Järki Kettu features built-in Graceful Degradation. Because Järki Kettu communicates via standard setpoints and modes rather than overriding low-level equipment wiring, your physical thermostat always retains its own internal safety logic. If Home Assistant reboots or a sensor drops offline, the automations safely idle and your wall thermostat simply continues running in its last commanded state, governed by its own onboard sensors and equipment delays.

Edge Case Note: Because Järki Kettu commands an off state during Coast Zones to prevent short-cycling, if your Home Assistant host suffers an unrecovered power outage while coasting, the thermostat will remain in off until Home Assistant boots back up, you change the mode at the physical wall, or the thermostat's native hardware freeze protection engages. Always ensure your wall thermostat's minimum safety/freeze limits remain active in its installer settings.

Any thermostat that integrates into Home Assistant as a standard climate entity and allows setting HVAC modes (heat/cool/off) and target temperatures will work. Heat pumps benefit the most from Järki Kettu's thermodynamics—the integrated Setpoint Walker steps target temperatures gradually to ensure the compressor does the heavy lifting while locking out expensive auxiliary heat strips. (Note: While any climate entity allowing mode and temperature control can receive commands, testing has been focused on central ducted 24V thermostat platforms like Venstar and Honeywell, rather than mini-splits).

Nest and Ecobee utilize rigid, hard-coded deadbands designed to comfort consumer psychology rather than respect HVAC engineering specs. After suffering through Ecobee cloud outages, Honeywell T9 software limitations, and an outdoor unit that survived being crushed by a falling tree, Talvi Fox codified these thermodynamic principles to protect expensive hardware and deliver genuine whole-home comfort.