The oldest furnace I’ve seen so far!
1936 on the front access door makes this gravity boiler 81 years old. Scary looking but remarkably simple.
In the large photo, a gas valve was retrofitted to turn the flame on or off from the thermostat. The flame heats the water in the heat exchanger and as the water warms, science takes over. Warm water rises to the radiators and cool water descends to the heat exchanger, the cycle continues until the desired temp is achieved. No motors, or circulators.