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post: add HVAC Helper Sensor
Additional changes: - update HVAC overhaul post to point to new post - add automating-hvac-system draft
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‎_drafts/automating-hvac-system.md‎

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---
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title: Automating the HVAC System
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# date: 2025-07-16
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# categories are Family, Photography, Places, Projects, Reviews, Software, Thoughts
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category: Projects
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tags:
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- homeassistant
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- automation
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- smarthome
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- climate
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media_subpath: /assets/img/posts/automating-hvac-system/
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image:
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path: preview.jpg
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lqip: preview.lqip.jpg
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---
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In the [last post]({% post_url 2025-07-16-hvac-helper-sensors %}), we created a bunch of sensors
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and automations to make it possible to control the HVAC system. This time, we're going to build the
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portions of the system which control the thermostat, raising and lowering the temperature as
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desired.
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This is the weirdest one and has a separate script to accompany the automation. To describe the goal,
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we're trying to determine where the the warmest (when AC is running) room is and set the cooling
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to run until _that_ room is at the target temperature. This has the unfortunate side effect of
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cooling the rest of the rooms beyond the target, but I'm fine with that (since my office is the
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hottest room in the house so I get to benefit from it). However, because wee want to make sure that
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we're getting to the actual desired temperature and because of the way thermostats work, we have to
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tell it to cool _beyond_ the point where it thinks it needs to be.
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The result is this... well _long script_. A lot of the length is simply documentation for the inputs
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to the script, but there's more complex stuff in there too
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```yaml
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{% raw %}
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script:
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update_thermostat_temp:
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alias: Climate - Update Thermostat Temperature
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mode: restart
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description: >-
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This script handles checking the current temperatures, the desired temperature,
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and the HVAC mode (all passed into this script) and then instructs the
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HVAC system to turn on/off accordingly by setting the target temperature
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of the thermostat in charge.
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This script expects to work with a single thermostat; if you have multiple
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thermostats, set up an automation for each.
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This script only anticipates working with an enabled HVAC system, meaning
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in "cool" or "heat" mode. Currently, it does not work with "heat_cool".
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If the HVAC system is in "off" mode, then do not call this script. It also
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does not pay any attention to away mode; that should be handled separately,
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perhaps as a condition in the automation.
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fields:
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buffer:
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name: Buffer
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description: >-
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A buffer applied to the temperature when reading which helps prevent
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rapid cycling
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example: "0.5"
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default: 0.5
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selector:
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number:
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min: 0
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max: 2
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step: 0.1
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mode: box
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overshoot:
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name: Overshoot
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description: >-
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This will be added/subtracted to the target temperature in order to
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ensure the thermostat stays engaged until next iteration of this script
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example: "2"
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default: 2
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selector:
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number:
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min: 0
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max: 3
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step: 0.1
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mode: box
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thermostat_temp_sensor:
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name: Thermostat Temperature Sensor
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description: >-
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The temperature the thermostat currently thinks it is. This can often
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be different than the temperature in multiple rooms, yet controls the
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real-world behavior of the system, so we need to include it
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example: "sensor.my_thermostat_temperature"
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required: true
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selector:
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entity:
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filter:
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- device_class: temperature
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domain: sensor
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thermostat:
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name: Thermostat
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description: The thermostat which should be controlled by this script
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example: "climate.my_thermostat"
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required: true
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selector:
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entity:
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filter:
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- domain: climate
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room_temp_sensors:
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name: Room Temperature Sensors
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description: >-
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A collection of room temperatures which should be considered when
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calculating whether to engage or disengage the HVAC system. These are
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likely to be temp sensors in the rooms you want covered.
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example: "['sensor.office_temperature', 'sensor.bedroom_temperature']"
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required: true
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selector:
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entity:
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filter:
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- device_class: temperature
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domain: sensor
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multiple: true
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room_target_temp_entities:
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name: Room Target Temperature Entities
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description: >-
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A collection of entities which contain the target temperatures for
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each of the rooms. This can be a single desired temp (e.g. "cool to 78")
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or it can be a list (e.g. "cool this room to 75, that room to 78")
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example: "['input_number.office_target_temperature', 'input_number.bedroom_target_temperature']"
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required: true
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selector:
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entity:
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filter:
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- domain: input_number
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multiple: true
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current_hvac_mode:
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name: Current HVAC Mode
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description: >-
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The mode which the HVAC system is currently in. Accepted values are
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"cool" and "heat".
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example: cool
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required: true
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selector:
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select:
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options:
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- cool
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- heat
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variables:
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buffer: "{{ buffer | default(0.5) | float }}"
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overshoot: "{{ overshoot | default(2) | float }}"
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# add the thermostat temp sensor to the list of room temp sensors
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all_temp_sensors: >
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{{ room_temp_sensors + [thermostat_temp_sensor] }}
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# Gather the temperatures for all the sensors
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all_current_temps: >
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{{ all_temp_sensors | map('states') | select('match', '^[0-9.]+$') | map('int') | list }}
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# Gather all the target temperatures
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all_target_temps: >
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{{ room_target_temp_entities | map('states') | select('match', '^[0-9.]+$') | map('int') | list }}
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# We'll also need the thermostat current temp for future use
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current_thermostat_temp: "{{ states(thermostat_temp_sensor) | int }}"
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# Determine the temp to use as the current temp; max temp if cooling, min temp
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# for heating. More description: if we're heating, we want to make the
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# coldest room the current temp so that can be heated to the desired temp.
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# For cooling, we want to cool the hottest room to the target temp
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current_aggregate_temp: >-
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{% if current_hvac_mode == "cool" %}
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{{ all_current_temps | max }}
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{% else %}
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{{ all_current_temps | min }}
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{% endif %}
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# Determine the temp to use as the target temp; min temp if cooling, max temp
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# for heating.
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current_target_temp: >-
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{% if current_hvac_mode == "cool" %}
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{{ all_target_temps | min }}
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{% else %}
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{{ all_target_temps | max }}
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{% endif %}
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sequence:
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- alias: Decide whether we should engage the HVAC and set the target temp
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if:
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- alias: Test to see if we're in cooling mode
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condition: template
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value_template: >-
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{{ current_hvac_mode == "cool" }}
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then:
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- alias: Set the variables for cooling
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variables:
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should_engage: >-
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{{ current_aggregate_temp > current_target_temp + buffer }}
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target_engaged_temp: >-
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{{ current_target_temp - overshoot }}
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target_disengaged_temp: >-
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{{ current_thermostat_temp + overshoot }}
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else:
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- alias: Set the variables for heating
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variables:
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should_engage: >-
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{{ current_aggregate_temp < current_target_temp - buffer }}
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target_engaged_temp: >-
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{{ current_target_temp + overshoot }}
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target_disengaged_temp: >-
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{{ current_thermostat_temp - overshoot }}
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- alias: Choose the target temp as appropriate based on mode and temp
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if:
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- alias: Test to see if we need to engage the thermostat
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condition: template
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value_template: >-
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{{ should_engage }}
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then:
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- alias: Use the engage values for the target
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variables:
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actual_target_temp: "{{ target_engaged_temp }}"
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else:
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- alias: Use the disengaged value for the target
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variables:
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actual_target_temp: "{{ target_disengaged_temp }}"
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- alias: Check to see if the HVAC system is disabled; don't continue if it's not
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condition: state
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entity_id: binary_sensor.disable_upstairs_climate_operation
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state: "off"
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- alias: Check to see if the thermostat is on; don't continue if it's not
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condition: template
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value_template: >-
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{{ is_state(thermostat, "heat") or is_state(thermostat, "cool") }}
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- alias: Update the thermostat's temperature
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action: climate.set_temperature
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target:
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entity_id: >-
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{{ thermostat }}
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data:
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temperature: "{{ actual_target_temp }}"
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{% endraw %}
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```

‎_posts/2025-07-02-hvac-overhaul-in-home-assistant.md‎

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plan to do these in later posts; setting up what sensors we need, what helpers, and how to link
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everything together to make a fully integrated climate system.... 85% of which we could probably
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do natively in Flair and Ecobee apps, but where's the fun in that?
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The way I see it, there's four major phases to
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this project, and I'll cover each in its own post:
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1. **[Helpers and automation prep]({% post_url 2025-07-16-hvac-helper-sensors %})** in which we prep all the sensors we'll need to do the subsequent steps
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2. **HVAC Automation** which takes the sensors and directly controls the thermostat temperature/mode
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3. **Flair Automation** which adds onto the HVAC automation by controlling where the air flows
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4. **Additional Stuff** such as guest mode, away mode, etc., things which I want but aren't needed for daily operation (meaning I will likely ignore them forever)
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So hang on and I'll publish posts as each step gets sorted!

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