This article explains how Flair switches between heating and cooling when Mode is set to "Auto Heat/Cool" for systems with mini splits and integrated smart thermostats. For systems with non-integrated thermostats, 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:
- Overview
- Implicit Heat-To and Cool-To Set Points
- Rooms with Holds
- Understanding Hysteresis
- Mode Switching
- Auto Heat/Cool Example
- What Values are Adjustable?
Overview
When you set Mode to "Auto Heat/Cool", Flair essentially creates a 3F range around the set point for switching modes. This is the number of degrees past the set point the home needs to get before switching from one mode to another.
So for example, let's say the home set point is 72F. Flair will not switch to heat mode until the home temperature drops below 69F, and it will not switch to cool mode until 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 above and below the home set point. 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.
Rooms with Holds
Each room with a hold will get its own implicit heat-to and cool-to set points. 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
Understanding Hysteresis
Like all thermostatic systems, Flair uses a hysteresis band (or range) around the set point. The default hysteresis band is 1F above and below the set point. So for a set point of 72F, the range is 71F to 73F.
For systems using Smart Vents, Flair will operate Vents when the room temperature rises or drops 1F from the set point. When Mode is set to "Auto Heat/Cool", here's how Smart Vents will operate for a home set point of 72F:
This hysteresis value is called Vent Hysteresis. While Vent Hysteresis is not used for mini split set point handling per se, it is used in the calculation for switching mini splits between heating and cooling.
Mode Switching
Flair uses the sum of the Heat Cool Mode Hysteresis (default 2F) and the Vent Hysteresis (default 1F) as the mode switching value. By default, this value is 3F.
So if the average home temperature rises or drops 3F above or below the home set point, Flair will switch modes. 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.
Switching also depends on the current mode. If the heat is on, when the room temperature goes 1F above the cool-to set point, Flair will switch to cooling. If the AC is on, 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:
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 2F range around the home set point for creating the heat-to and cool-to set points
- Vent Hysteresis: the 1F range around a set point that determines when Smart Vents open/close
These values are adjustable by contacting Support.