Is the coal filling port of the coal-fired water heater always open or closed?

Normally open.

The coal filling port of the coal-fired water heating furnace is always open to allow air circulation and support combustion. Opening it can increase ventilation, the stove will be strong, and the room will be warm. It should be noted that the larger the opening, the more briquettes will be consumed.

The heater gradually evolved from the brazier. It was first invented in the Tang Dynasty and became particularly popular in the Ming and Qing Dynasties. In the middle and late Ming Dynasty, the furnace craftsmanship reached a state of perfection, and furnace-making talents emerged in large numbers, and now electric heaters appeared.