Someone who is keeping him/herself warm is well-off financially. Of course this is meant figuratively, but when this proverb is taken literally, this was certainly the case in the past. Someone who was rich, had it in the winter months warmer than someone with much less money. If the canals and the Amstel froze, the water supply was in danger. Winter inventories should be enough until the spring. And there was a need fuel to cook and stay warm. A lot of fuel.
The poor of the city received weekly support, in the form of bread, butter, and a basket of peat. With that peat a one room house could be stoked hot, but most of these houses were not. A basket of peat was hardly sufficient. Often, the peat was still a bit damp, which caused a lot of smoke and odor. Those who could afford wood were therefore better off. Dry wood burns stable, gives little smoke and almost no smell.
A good hearth was essential. In an open-hearth a fire was built. A good chimney caused the smoke away upwards, an iron fireplace plate behind the hearth radiated extra heat into the room. Good ventilation was important and sloping backs in the chimney could yield some increase, but still a drawback was that one would lose in all pretty much warmth. Even in a well-heated room you had dress with extra layers and often a foot stove was used as an additional source of heat close to the body.
In order to optimally use the combustion of fuel in the Golden Age, one had to have dig deep into their pockets. In the ideal situation, people bought a cast iron stove, which was coated on the inside with refractory brick. In this closed stove they stoked a fire, so that the iron plates gave the heat evenly off into the space. An additional benefit was the fire prevention. The smoke was derived via a pipe through the chimney, which was also better for one’s health.
Cast iron stoves from the seventeenth century are very rare. We know that these heaters have stood in City Hall* on Dam Square and were also found in prominent houses. Who wants to get an idea how a heater from that time looks like, can go take a look at the one in museum Ons’ Lieve Heer op Solder (Museum Our Lord in the Attic. A 17th century canal house with a hidden Catholic church in the attic) or in the Rembrandt House Museum.
*City Hall on Dam Square is better known nowadays as the Royal Palace Amsterdam.