The principle of the donkey is that hot water rises (like hot air) and as the fire warms the water, the warmer water will be what comes out of the top pipe. ... While firing up the drum full of water & detergent and later draining it, the water temp from the bottom pipe (where the flames were) was almost cold.

What is donkey boiler?

: an auxiliary boiler (as one carried aboard ship for use in port)

Why is it called a donkey boiler?

Name. Steam donkeys acquired their name from their origin in sailing ships, where the "donkey" engine was typically a small secondary engine used to load and unload cargo and raise the larger sails with small crews, or to power pumps.