Lesson 9
Combi Boiler Behaviour
Understand what your combi does when a tap opens, what the radiator dial really sets, why it cycles on mild days, and when to stop resetting it.
One burner, two jobs
Priya’s combi has two jobs: heating the radiators, and heating tap water the moment a hot tap opens. It has one burner to do both.
Inside, the burner heats a loop of water called the primary circuit. For the heating, that water goes round the house through the radiators and comes back. For a hot tap, the hot primary water passes through a small plate heat exchanger instead: a stack of thin metal plates where it warms the mains water on its way to the tap, without the two waters ever mixing.
All of this happens behind the case. Everything in this lesson is something you watch and use through the controls on the front; as The Gas Boundary explains, the case stays on.