The first thing to consider is why we need heating systems at all. The reason is not to warm us up but to stop us losing too much heat. This may seem pedantic, but appreciating this point keeps our thinking in the right direction; we are looking for comfort, not a means of cooking ourselves.
Our body temperature is around 37°C while the highest acceptable room temperature is about 23°C. So we are always emitting heat to our surroundings just like a radiator or an electric fire.

Measuring heat
If you leave a cup of hot coffee it goes cold; if you put it in the snow it cools faster, so the rate at which heat is transferred is proportional to difference in temperature. The rate of transfer is measured in watts and, because the watt is a small unit, we often use a unit of 1000 watts called a kilowatt (kW).
Wherever the coffee is left, it will be colder after four minutes than after two. The total heat lost depends on the rate of loss and how long the rate is maintained. If an electric fire emits heat at a rate of 1 kilowatt for a period of 1 hour, it will transfer a total of 1 kilowatt-hour of energy (1 kWh). However, unlike the forgotten coffee, it does not get colder because the electricity generators provide replacement energy at the same rate as the fire is losing it.

Keeping warm
The average human rate of loss is 100 watts and in a party of 30 people the heat given out will be equivalent to a 3kW electric fire. Provided it is regularly stoked with food, the body can keep up with the loss just like electricity generators. It is only when the rate increases or decreases too much that we become uncomfortable and then we need some control over it.
The first controls were clothing followed by shelter even if only a cave dwelling. Up to the 19th century the dwelling was still the main protection. Fire was a bonus; the only fuel was wood, it was hard work getting it and, unless you owned the woods, quite expensive.
Homes were built for maximum protection at minimum cost, generally in sheltered spots with thick walls to store heat and low ceilings so that only a small volume of air had to be warmed. The windows were small and often omitted on the windward side of the house.
As fuel became cheaper houses were designed more for appearance and by the 1960s showed hardly any regard for conservation. Why bother? You could always fill up with cheap heat later. Things are different now. Fuel is again a luxury and we must return to the idea of the structure itself contributing more to our comfort.

Building heat losses
As mentioned earlier, when the body loses heat too rapidly we become uncomfortable. The obvious remedy is to keep our surroundings at a level acceptable temperature. However, when the house is heated, the temperature difference between it and the outside increases and it begins to lose heat itself. You can’t just heat the rooms up to comfort level and then switch off; replacement heat must be provided continually at the same rate as the house is losing it. The heat is lost in two ways; by heat passing through the structure, and by warmed air being replaced with cold.