Most people need to turn the heating on when indoor temperatures drop below 18°C (around 64°F). That’s the minimum the World Health Organization recommends for protecting health during cold seasons, and it’s the point where most households start to feel genuinely uncomfortable rather than just a bit chilly. The exact moment you flip the switch depends on your home’s insulation, who lives there, and how cold it is outside, but 18°C indoors is the clearest line between “grab a jumper” and “turn the heating on.”
The 18°C Threshold and Why It Matters
The WHO’s recommendation of 18°C as a minimum indoor temperature isn’t arbitrary. Below 16°C, your body’s ability to fight off respiratory infections starts to drop. If temperatures fall further to 12°C or below, cold extremities and a slight dip in core body temperature can cause short-term spikes in blood pressure. Thickened blood and raised blood pressure in moderately cold conditions are linked to higher rates of heart attacks and strokes during winter months.
For most healthy adults, 18°C is the floor. You might prefer 20°C or 21°C for comfort, but letting your home dip below 18°C isn’t just unpleasant. It’s a genuine health risk, especially over prolonged periods.
Warmer Minimums for Children, Elderly, and Ill Households
If you live with older adults, young children, or anyone with a chronic illness or limited mobility, the safe minimum is higher: 20 to 21°C (68 to 70°F). The National Institute on Aging warns that even mildly cool homes between 60 and 65°F (15.5 to 18°C) can lead to hypothermia in older adults. Sedentary people lose heat faster because they aren’t generating warmth through movement, so a temperature that feels fine while you’re cooking or tidying up can be dangerously cold for someone sitting still for hours.
The practical takeaway: if someone in your household is elderly, very young, or unwell, don’t wait until the house feels cold to you. Set the thermostat to at least 68°F (20°C) and leave it there during occupied hours.
What Outside Temperature Triggers Heating?
Your indoor temperature obviously depends on what’s happening outside. Research tracking 249 homes found that the average outdoor temperature at which households needed to start heating was 13.3°C (about 56°F). Individual homes ranged widely, from as high as 18°C outdoors down to just 8°C, depending on insulation quality, building size, and how much heat was generated by appliances, cooking, and the people inside.
A well-insulated modern home might hold comfortable indoor temperatures even when it’s 10°C outside. A draughty older property might need the heating on when outdoor temperatures are still in the mid-teens. If you’re unsure, a cheap indoor thermometer is more useful than guessing. Place it in your main living area, away from radiators and direct sunlight, and check it in the morning before the house has warmed up from cooking or activity. If it reads below 18°C, it’s time.
Best Temperature Settings for Sleep
Bedrooms are a special case. Sleep research points to an optimal room temperature of 19 to 21°C for adults, which allows your skin to settle into the 31 to 35°C microclimate under bedding that your body needs for restful sleep. Even tiny deviations of less than half a degree in skin temperature within that range can change how quickly you fall asleep.
This means you don’t necessarily need the heating blasting in the bedroom overnight. A room at 16 or 17°C with good bedding can work well for healthy adults who prefer a cooler sleeping environment. But if you regularly wake up cold, or if the room drops below 16°C, running the heating on a lower setting overnight is worth the cost. For bedrooms occupied by elderly people or infants, keep the temperature at 20°C or above.
All Day on Low, or On and Off?
A common question is whether it’s cheaper to leave the heating on a low setting all day rather than turning it on and off. The answer depends on your system. If you have a gas or oil boiler, it’s more efficient to heat the home only when you need it. Leaving the boiler running all day uses more energy and costs more. A programmable thermostat set to warm the house before you wake up, drop while you’re out, and rise again before you get home is the most cost-effective approach. The U.S. Department of Energy estimates you can save around 10% on annual heating costs by lowering the thermostat 7 to 10°F for eight hours a day.
Heat pumps are the exception. Because they work differently, producing a steady, lower level of warmth rather than burning fuel in bursts, they’re more efficient when left running at a consistent temperature throughout the day. If you have a heat pump, keeping the thermostat steady rather than cycling it up and down will actually save you money.
Practical Signs Your Home Is Too Cold
Temperature readings are the most reliable guide, but your body gives signals too. Persistently cold hands and feet indoors, visible breath, condensation forming on the insides of windows, and difficulty concentrating are all signs the room is too cold for comfort and possibly for health. If you find yourself layering up heavily just to sit on the sofa, the heating should already be on.
Condensation deserves a specific mention because it leads to damp and mould, which create their own health problems. A home that regularly drops below 16°C, particularly one with poor ventilation, is at much higher risk of mould growth on walls and around windows. Heating to at least 18°C and allowing some airflow helps keep surfaces above the dew point where moisture collects.