Every winter, the same question pops up in kitchen chats and group texts: should you leave underfloor heating on all the time? Bills climb, thermostats blink, and someone swears switching it off at night is the magic fix. The scientific answer points in another direction: inerzia termica.
m. The hallway tiles felt lukewarm, the bedroom still cool, and the thermostat had that guilty “working on it” look. My neighbor had switched his radiant floor off overnight to “save,” then spent the morning waiting for warmth to climb back through concrete and wood. Coffee went cold before the room warmed up. He wasn’t alone. We’ve all had that moment when the house seems stubbornly a beat behind your day. The trick is inertia.
Inerzia termica, felt under your feet
Underfloor heating doesn’t warm the air first. It warms the mass. Pipes or cables heat a screed and slab that then radiate back to you, slowly and steadily. That mass is a giant buffer, and it doesn’t like sudden changes. The slab is a battery. Once charged, it releases heat for hours. Turn it off and the battery doesn’t go “cold” on command; it coasts down, then you pay again to ramp it up. *Yes, the floor keeps “thinking warm” even when the thermostat tells it to stop.*
Last winter I followed a family in Modena who tried two routines for a week each. Routine A: steady 20°C setpoint, gentle water temperature around 30–32°C. Routine B: nightly off from 11 p.m. to 6 a.m., then a morning push at 38–40°C to catch up. Their smart meter told the story. Energy use dipped a bit overnight in B, then jumped more in the morning than they saved in the night. Comfort also lagged: the kids ate breakfast in hoodies. Nobody cheered the “savings.” The slab was playing its long game.
The physics are straightforward. Concrete and screeds have high heat capacity, so they store energy per degree like a slow, heavy flywheel. Heat moves through that mass at a measured pace, and the surface heat flux is capped for comfort. Try to sprint and you hit a wall. With a heat pump or condensing boiler, forcing a catch-up demands higher supply temperatures. That crushes efficiency. Lower, steadier temperatures keep the system in its sweet spot and let the mass do its quiet job.
Run it right: small moves, big comfort
Think “set-and-hold,” not “stop-and-go.” Pick a day setpoint you actually like — say 20°C — and let the system hover around it. If you enjoy cooler nights, use a mild setback of 0.5–1.5°C, not a blackout. Pair that with weather compensation so water temperature follows the outdoor chill. A slow curve is your friend. Cap floor surface temperature at 26–28°C for comfort and wood health. Set down, don’t shut down.
Don’t chase the thermostat. Big tweaks confuse a slow actor. Two changes per day is plenty: morning and evening. If a room drifts, nudge by 0.5°C and wait half a day. Rugs are fine, just not wall-to-wall — leave breathing strips so heat can rise. Air the house in short bursts instead of leaving windows cracked. Let’s be honest: nobody really does that every day. But a five-minute blast of fresh air beats hours of trickle-loss through a leaky window.
When people say “I turned it off and it was still warm,” they’re revealing inerzia termica in plain language. That lag is not a bug. It’s the feature that stabilizes your comfort and your bill.
“Radiant floors love boring schedules,” says a building physicist I called. “Keep the supply temperature low, let the slab idle, and resist the urge to mash the thermostat.”
- Night setback: 0.5–1.5°C in most homes; larger only for long absences.
- Heat pump users: target 28–35°C supply water on typical winter days.
- Boilers: run condensing, low-return temps, steady circulation.
- Rugs: cover less than 30–40% of a room, especially on heat pump systems.
- Away for days: drop to 16–17°C and restart early via app.
What this means for your winter, your bill, your mood
Leaving underfloor heating “on” doesn’t mean blasting. It means letting a massive, gentle system do what it’s built to do: store a bit of heat and give it back calmly. You trade spikes for a smooth line. Small, steady heat wins against big, spiky bursts. In practice that looks like fewer cold mornings, fewer panic boosts, and a heat source — pump or boiler — that stays in its efficient lane. Some homes are lighter or leakier and can live with a slightly deeper night dip. Others, heavy and tight, barely notice set-backs at all. If you change one thing, change the rhythm, not the goal: comfortable rooms, minimal drama. Share a meter screenshot with your neighbor in February. The curve that looks boring will probably be the cheap one.
| Point clé | Détail | Intérêt pour le lecteur |
|---|---|---|
| Thermal inertia | The slab stores heat and releases it slowly, acting like a battery | Explains why on/off cycles feel slow and costly |
| Setback strategy | Use small night setbacks (0.5–1.5°C) instead of shutting off | Reduces morning catch-up, improves comfort |
| Low supply temperature | Weather-compensated 28–35°C water on typical days | Boosts heat pump COP and boiler condensation |
FAQ :
- Is it cheaper to leave underfloor heating on all day?Often yes, if “on” means steady low output with small setbacks. The mass avoids expensive catch-ups and keeps the heat source efficient.
- How much should I lower it at night?Start with 1°C and test. Go to 1.5–2°C only if the system still recovers without high supply temperatures or morning chill.
- What if I’m away for a weekend?Use a deeper setback, around 16–17°C, and preheat a few hours before you return. Longer trips can go lower, factoring frost protection.
- Does the advice change for heat pumps vs boilers?Yes. Heat pumps gain most from steady, low temperatures. Condensing boilers also prefer low returns. Old non-condensing boilers are less sensitive but still benefit from avoiding big swings.
- Do rugs or wood floors block the heat?Thick rugs slow it a bit. Leave open paths and avoid total coverage. For wood, keep floor temps under ~27–28°C to protect the boards.
