Heating bills drop more from sealing leaks than from turning the thermostat down. Here is the order to tackle things in.
A large share of heating goes into warm air escaping through window gaps and the front door. Before lowering the thermostat by a degree, check weather strips, insulating film and curtains. Old window frames leak the most, and a single strip makes a noticeable difference. Closing curtains at night alone cuts heat loss through glass considerably.
Turning the boiler fully off while you are out lets the home cool, and reheating it costs more than holding a lower temperature. For a few hours away, 'away' mode or a setting 2–3 degrees below normal is cheaper. Switching off only makes sense for a day or more.
The same 20°C feels colder when the air is dry. A humidifier or a damp towel bringing humidity to 40–50% lets you drop the setting by a degree or two with the same comfort — far cheaper than heating more.
Closing valves and doors in unused rooms shrinks the area you actually heat. Fully closed rooms grow condensation and mould, so air them occasionally. Under-floor heating is trapped by thick rugs, so keep coverings thin where you sit most.
Many boilers can run either by room temperature or by the temperature of the water flowing through the floor or radiators. Room-temperature mode reacts only to the room where the thermostat sits, so a thermostat in a sunny window or near the front door can be out of step with how the home actually feels. Where heating needs to run for long periods, a moderate water temperature over a longer time is often steadier than short bursts at high temperature. Timer settings that heat mainly before bedtime and before waking are also useful. Exact settings vary by model, so check the manual.
If the boiler runs longer than it used to for the same warmth, its efficiency may have dropped. Sludge in underfloor or radiator pipes leaves some rooms cold and keeps the boiler working longer. Before the heating season, do a trial run and check for noise, leaks and error codes. Leave pipe flushing and internal servicing to the manufacturer or a qualified technician. Never stack things around the boiler or block its flue; keep the flue area clear and check that its joints are secure and not rusted.
Heating the space close to your body takes far less energy than heating all the air in the home. Layered clothing, thermal underwear, indoor socks and slippers make a lower setting feel much less cold, and a lap blanket helps wherever you sit for long periods. Heaters that touch the body, such as electric blankets, raise comfort with less power than space heaters, but long direct contact with skin can cause low-temperature burns, so keep the setting low and use a timer. Do not use them folded or bunched up, as that can cause overheating.
Saving on heating can lead to its own mistakes. Cutting ventilation completely raises humidity, causing condensation and mold, and leaves the air stale. A few short bursts of cross-ventilation a day refresh the air with little heat loss. Gas or kerosene heaters used indoors can produce carbon monoxide, so always ventilate and switch them off before sleeping. If you feel a headache, dizziness or nausea, turn off the heater, open the windows, get outside and seek medical help if needed. A carbon monoxide alarm adds a layer of safety.