What is a heat exchanger in a boiler?

So What Exactly Is A Heat Exchanger
And How Does It Work In Your Boiler?

A heat exchanger does precisely what its name describes – it allows heat to be exchanged between two fluids or substances, usually water or gas, without letting the substances mix together. You may take it for granted that your boiler is able to heat water up and then deliver it around your home, either through your radiators or to your taps. You might not, however, have thought about how it achieves this. The heat exchanger is the key. Without this device, your boiler wouldn’t be able to warm up your water.