The thermostat reads 21 degrees and the room is cold. This is a common domestic argument and it usually gets resolved by someone deciding the thermostat is broken or the other person is imagining it. Neither is likely. The thermostat is measuring air temperature, and air temperature is only one of the things that determines whether you feel warm.
You are a heater, and comfort is about the rate of loss
A resting adult produces heat continuously, roughly comparable to a small light bulb, and has to shed it at about the same rate to stay at a constant temperature.
What you experience as warm or cold is not a reading of the air. It is whether you are losing heat faster or slower than you are producing it. Anything that changes the rate of loss changes how the room feels, whether or not it changes the air temperature at all.
There are four routes out, and a thermostat only knows about one of them.
Radiation: the one nobody thinks about
Every surface emits infrared radiation according to its temperature, and every surface absorbs what falls on it. You are exchanging radiation constantly with the walls, floor, ceiling and windows around you.
If those surfaces are warm, the exchange is roughly balanced. If a large surface is cold, the traffic is one way: you radiate to it, it gives you back very little, and you lose heat steadily to something that is not touching you and is not changing the air temperature.
Building engineers deal with this by talking about mean radiant temperature, the average temperature of everything the room is surrounded by, and it matters roughly as much as air temperature for comfort. A room with warm air and cold walls is not a comfortable room.
Where this bites hardest is glass. A single-glazed window on a cold night can sit far below room temperature, and sitting next to one is like sitting facing an open fridge: nothing is blowing on you, but you are steadily radiating heat away. It is also why a room with solid uninsulated external walls feels chilly at any setting, and why underfloor heating feels warm at a lower air temperature than radiators do, because the warm surface is large and radiating back at you.
Convection: why draughts feel so much colder than they are
Skin warms the air immediately next to it, and in still conditions that warmed air lingers as a thin insulating boundary layer.
Move the air and you strip that layer away and replace it with cooler air, again and again. Heat loss rises sharply, even though the air temperature has not changed at all. That is the whole of wind chill, and it works indoors too.
An unnoticed draught from a letterbox, a gap under a door, or a chimney is a continuous supply of moving air across the floor, and it makes a properly heated room feel cold at ankle height.
Conduction and evaporation
Conduction is direct contact, and it matters mainly through your feet. A tiled floor at the same temperature as a carpeted one feels much colder because tile conducts heat away from your foot far more readily than carpet does. The floor is not colder; it is better at taking heat from you.
Evaporation matters less indoors in winter but is not nothing. Very dry air increases moisture loss from skin and breath, and each gram of water evaporated takes heat with it. This is a small effect compared with the other three, but it is part of why heavily heated, very dry rooms can still feel unwelcoming.
And the thermostat is in one spot
Worth adding, because it is the simplest explanation and often the correct one. A thermostat measures air at one point, usually on an internal wall at chest height in a hallway.
Warm air rises, so the air at head height in a hall can be a couple of degrees above the air where you are sitting. If the thermostat sits above a radiator, in sunlight, or near a lamp, it will read high and shut the heating off early. If it is in a draught, it will read low and overheat the rest of the house.
What to fix, in order
- Find the draughts. Doors, letterboxes, floorboards, unused chimneys, badly fitting window frames. Cheapest fix, largest immediate difference.
- Deal with cold glass. Heavy curtains closed at dusk, or secondary glazing, cut radiant loss substantially for very little money.
- Do not sit facing a cold surface. Moving a chair away from a large window changes comfort more than a degree on the dial.
- Cover cold floors where you stand and sit.
- Check what the thermostat can see. Not above a radiator, not in the sun, not in a draught.
- Then, if still cold, raise the temperature. Last, not first.
Heat, moved around
For a machine built entirely around moving heat from one place to another, how a refrigerator actually works. For evaporation doing the same job on purpose, the swimming pool example in that article is the clearest version of it.
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