Setting the thermostat to 30 doesn't heat any faster

You get home to a living room at 14 degrees and crank the thermostat to 30 so it heats up fast. It does nothing. A domestic thermostat works like a switch, not like an accelerator: it compares the temperature in the room with the one you asked for, and decides exactly two things, on or off. Your boiler burns gas at the same rate at 21 as at 30, so the house warms up at precisely the same speed either way.

What changes is when it stops. At 21 the system cuts out when it reaches 21. At 30 it keeps burning for half an hour after you are already comfortable, and that half hour is what shows up on the bill. Spain's public energy-efficiency institute, the IDAE, puts the extra cost at roughly 7 % per additional degree, and recommends between 19 and 21 degrees in winter.

One honest caveat: heat pumps and air conditioners with inverter technology do start at higher power the bigger the gap, so there the trick works for the first few minutes. Then they overshoot and give it back in consumption.

Do this the moment you walk in: set the number you actually want to live at, 20 or 21, and leave it alone. If what you want is to come home to a warm house, don't touch the temperature: touch the clock, and have the system start half an hour before you open the door.