Warm Floor Function Proposal for Better Thermostat

[rradkov]

Hi everyone,
I’d like to propose a “Warm Floor” function for users with underfloor heating (UFH). I believe this feature would be a valuable addition to Better Thermostat, and parts of the logic could also benefit radiator-based systems.
Starting with version 1.8.0 beta 15+, Better Thermostat already exposes several advanced variables that seem suitable for implementing such functionality, for example:

Heat Loss
Heating Power
MPC Gain
MPC Insulation (Ka)
MPC Loss
Sun Intensity Heatup

Based on these parameters, it should be possible to calculate and control the system in a way that keeps the floor comfortably warm, while ensuring that the room air temperature does not rise excessively. For UFH users, comfort is often more about floor temperature and thermal stability.

Additional key benefit: UFH thermal inertia
Another important reason for a Warm Floor mode is the high thermal inertia of underfloor heating systems.
If UFH is turned off for a long period of time, the floor slab cools down significantly. Once heating is needed again, it can take a very long time to rebuild the stored heat, leading to: delayed comfort, higher peak energy demand, less efficient system behavior overall.

Maintaining a low but continuous heating level—just enough to keep the floor warm—can: drastically reduce recovery time, improve overall comfort, and avoid large temperature swings.

A dedicated Warm Floor function could help balance energy efficiency with thermal stability by using the existing MPC and heat-loss data already available in Better Thermostat.

Additional idea – UFH and fan‑coil cooperation
In my setup, I use underfloor heating together with fan‑coils in each room, with a target temperature of 23 °C.
Proposed behavior:

If a window is opened and the room temperature drops by more than 5 °C,
once the window is closed, the fan‑coil automatically switches on to quickly bring the room back to 23 °C,
after the target temperature is reached, the fan‑coil switches off,
while the underfloor heating continues running to maintain steady comfort.

This approach would significantly improve recovery time after ventilation, while still leveraging UFH for stable, long-term heating.

I believe both ideas align well with the existing MPC-based design of Better Thermostat and could bring meaningful comfort and efficiency improvements, especially for UFH users.

[cl445]

Both ideas are rules about when to heat and with which device, and we keep those in Home Assistant while BT provides the settings to act on. Keeping the floor slab warm is a low preset temperature that an automation or schedule switches to instead of turning the room off (https://better-thermostat.org/setup/presets, https://better-thermostat.org/deep-explanations/schedule-and-night-mode). The fan-coil boost after airing is an automation as well: after the window closes, turn the fan-coil on while the room is more than a few degrees below the target and off once it reaches it, and leave the underfloor heating under BT. Closing this, since it does not fit the direction of BT.