This article explains how Flair switches between heating and cooling when Mode is set to "Auto Heat/Cool". This behavior applies to systems with mini splits, or systems with an integrated smart thermostat using Flair as the Set Point Controller.
For systems with integrated smart thermostats using the thermostat as the Set Point Controller, Flair follows the mode and set points read from the thermostat.
For systems with non-integrated thermostats, when Mode is set to Auto Heat/Cool Flair will examine the duct temperature to determine if the system is heating or cooling and operate Smart Vents accordingly.
Pro tip: the auto changeover differential is 3F. This is 2F above and below the set point, plus 1F hysteresis. These values can be adjusted by contacting Support.
In this section:
- Implicit Heat-To and Cool-To Set Points
- Rooms with Holds
- Vent Operation
- Mode Switching
- Auto Heat/Cool Example
- What Values are Adjustable?
When Mode is set to "Auto Heat/Cool", Flair essentially creates a 3F range around the home setpoint for switching modes. Flair will switch to heating when the home temperature drops 3F below the home set point and switch to cooling when the home temperature rises 3F above the home set point.
So for example, let's say the home set point is 72F. Flair will switch to heat mode when the home temperature drops below 69F, and it will switch to cool mode when the home temperature rises above 75F. Read further to understand the details of operation.
Implicit Heat-To and Cool-To Set Points
When Mode is set to "Auto Heat/Cool", Flair will create two implicit set points: a Heat-To set point and a Cool-To set point. By default, these are 2F below and above the home set point, respectively. The 2F is called the Heat Cool Mode Hysteresis.
So, for a home set point of 72F, the Heat-To set point is 70F and the Cool-To set point is 74F.
Note: For systems with integrated smart thermostats where the Set Point Controller is set to thermostat, Flair reads the Heat-To and Cool-To set points from the thermostat.
Rooms with Holds
Each room with a hold will get its own implicit Heat-To and Cool-To set points used for Vent operation. By default these are 2F above and below the room's set point.
In this example, Janie's Bedroom has a hold of 71F and the Hallway has a hold of 72F. Flair will create implicit Heat-To and Cool-To set points for each room as follows:
Janie's Bedroom: Heat-To is 69F, and Cool-To is 73F
Hallway: Heat-To is 70F, and Cool-To is 74F
Like all thermostatic systems, Flair uses an operating range (or hysteresis band) for knowing when to open and close Smart Vents. By default this is 1F around the set point. We call this the Vent Hysteresis. For a set point of 72F, the operating range is 71F to 73F.
The 1F Vent and Thermostat Hysteresis is also applied to Heat To and Cool To set points.
This example shows how Vents operate for a home set point of 72F.
Note: While Vent Hysteresis is not used for mini split set point handling per se, it is used in the calculation for mode switching.
Flair uses the sum of the Vent and Thermostat Hysteresis (default 1F) and the Heat Cool Mode Hysteresis (default 2F) as the mode switching value. By default, the mode switching value is 3F (1F + 2F). Professionally, this is called the auto changeover differential.
Mode switching occurs when the home temperature moves 3F above or below the home set point.
If the home is heating, when the room temperature rises 1F above the Cool-To set point, Flair will switch to cooling. If the home is cooling, when the room temperature drops 1F below the Heat-To set point, Flair will switch to heating.
This example shows when switching occurs based on a 72F home set point.
For rooms with holds, Flair will switch modes based on a consensus of how far each room is from its individual Heat-To and Cool-To set point.
Auto Heat/Cool Example
The graph below represents a home's temperature over two hours. It's Fall, so the temperatures are fluctuating a lot. The blue areas are when the AC is on and orange area is when the heat is on. The home begins cooling, switches to heating, and then switches back to cooling again. Let's see why.
The home set point is 72F, so Flair calculates a Heat-To set point of 70F and a Cool-To set point of 74F (2F around the home set point). The mode switching temperatures are 69F and 75F (3F around the home set point).
The timeline starts while the AC is running (blue). As the home temperature dips 1F below the Heat-To set point, the heat comes on (orange). Then the home temperature begins to rise, and once it rises 1F above the Cool-To set point, the AC comes on (blue).
If the home temperature continues to drop and goes below 69F again, the heat will come on.
What Values are Adjustable?
These values are used for calculating modes switching:
- Heat Cool Mode Hysteresis: the default value is 2F. This determines how Flair calculates the heat to and cool to set points. Closer heating and cooling set pints will maintain tighter temperature control and use more energy. Set points further apart will result in a wider temperature range and use less energy.
- Vent and Thermostat Hysteresis: the default value is 1F. This determines the operating range around a set point for when Smart Vents open and close. A tighter range will maintain tighter temperature control and Smart Vents will open and close more often (this could increase battery usage). A wider range will allow a wider temperature range and Smart Vents will open and close less often.
These values are adjustable by contacting Support: flair.co/contact