Thermostat turned on

The Thermostat turned on trigger fires after a climate entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more] turns on, entering any operational mode (such as Heat, Cool, or Auto). Climate entities include thermostats, air conditioners, heat pumps, and evaporative coolers. The trigger doesn’t care which specific mode the device switches to. It only checks that it transitions from Off to any active mode. Use this trigger when you want to react as soon as the climate entity becomes active, regardless of whether it’s heating, cooling, or in another mode.

Note: The UI labels this trigger as “Thermostat,” but it works with all climate entities.

Using this trigger from the user interface

If you prefer building automations visually, Home Assistant walks you through this trigger step by step. You pick what to watch, tweak a few options, and save. No YAML knowledge required.

To use Thermostat turned on in an automation:

1. Go to Settings > Automations & scenes.
2. Open an existing automation, or select Create automation > Create new automation.
3. In the When section, select Add trigger.
4. Select what you want to monitor. Under By target (see Targets), pick the area your thermostat is in (like your bedroom or living room). You can also select a device, a specific entity, or a label.
5. From the triggers shown for that target, select Thermostat turned on.
6. Under Trigger when (see Behavior), pick Each, First, or All to control how the trigger behaves when multiple thermostats are targeted.
7. Under For at least, set how long the thermostat must stay on before the trigger fires. Leave it at zero to fire immediately.
8. Select Save.

Options in the UI

Using this trigger in YAML

If you work directly in YAML, or you want to know exactly what Home Assistant does under the hood, this section has the technical reference. It lists the field names you use in YAML, their types, and which ones are required.

In YAML, Thermostat turned on is referred to as climate.turned_on. A basic example looks like this:

trigger: climate.turned_on
target:
  entity_id: climate.living_room

This fires every time climate.living_room transitions from off to any active mode.

Options in YAML

YAML sometimes provides additional options for more complex use cases that are not available through the UI.

Targets of the trigger

This trigger requires a target. The target is the object that Home Assistant will watch. You can select a single entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more], a device, an area, a floor, or a label as a target, and Home Assistant will watch every matching climate entity behind that target.

• Entity: one specific climate entity, such as climate.living_room.
• Device: every climate entity that belongs to a device.
• Area: every climate entity in a room or area.
• Floor: every climate entity on a floor.
• Label: every climate entity that shares a label.

You can also select different target types in one trigger. For example, you can add a specific entity and an area as targets in the same trigger to monitor both of them at once.

Behavior with multiple targets

When you target more than one entity (or select an area, floor, or label that contains several), the Trigger when option controls how the trigger responds:

• Each (any in YAML, default): the trigger fires every time any one of the targeted entities transitions. For example, if you monitor three motion sensors in the living room and someone walks past sensor 1, the automation fires. When they walk past sensor 2 a moment later, it fires again. Every individual event counts.
• First (first in YAML): the trigger fires only on the first transition in the targeted group, then waits until all targeted entities have reset before it fires again. For example, if you monitor the same three motion sensors, the automation fires when the first one picks up movement (someone entered the room). The other two firing afterward are ignored, so you get one notification per “someone walked in” event instead of three.
• All (last in YAML): the trigger fires only after the last targeted entity in the group has fired, meaning all of them are now in the expected state. For example, if you monitor the lights in the living room, bedroom, and hallway, the automation fires only once all three have turned off. This is useful for scenarios like “start the robot vacuum only after every light on the floor is off,” so you know the room is truly empty.

Good to know

• This trigger fires when the device transitions from Off to any operational mode (Heat, Cool, Auto, Dry, Fan only, or Heat/Cool).
• The trigger does not fire when switching between active modes. For example, changing from Heat to Cool will not fire this trigger.
• To react to specific mode changes or when the device switches between modes, use Thermostat mode changed instead.

Try it yourself

Ready to test this? Go to Settings > Automations & scenes, create a new automation, and add this trigger. Save the automation, then change the state of the targeted entity to watch the trigger fire on your actual entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup.

Automation: turn on fans when first evaporative cooler starts

When the first of multiple evaporative coolers turns on and stays on for 10 seconds, start a ceiling fan on low speed to help distribute the cooled air more evenly throughout the room. The short delay ensures the cooler has fully started, and firing on the first one prevents multiple fan activations.

• Trigger: Thermostat turned on
  • Target: Multiple evaporative coolers
  • Trigger when: First
  • For at least: 10 seconds
• Action: Turn on fan

alias: "Start fan when first cooler turns on"
triggers:
  - trigger: climate.turned_on
    target:
      entity_id:
        - climate.bedroom_evaporative_cooler
        - climate.living_room_evaporative_cooler
    options:
      behavior: first
      for: "00:00:10"
actions:
  - action: fan.turn_on
    target:
      entity_id: fan.bedroom_ceiling
    data:
      percentage: 30

Still stuck?

The Home Assistant community is quick to help: join Discord for real-time chat, post on the community forum with the trigger you’re using and what you expected to happen, or share on our subreddit /r/homeassistant.

Related triggers

These triggers work well alongside this one:

• Thermostat turned off: Triggers after one or more climate devices turn off.

• Thermostat mode changed: Triggers after the HVAC mode of one or more climate devices changes.