How does a (HX) Heat Exchanger machine work?

"Here is a diagram of the workings of a heat exchanger machine.

The pump pulls water from the tank and directs it through the heat exchanger, where it is heated to make coffee with. Note that it passes the expansion valve; the expansion valve is what regulates the pump's maximum output pressure before it reaches the heat exchanger. After the water passes through the heat exchanger, it passes through the three-way valve. This valve's purpose is to release built up pressure in the portafilter once you are through pulling your shot. If this valve were not there, hot coffee grounds could squirt all over you when you remove the portafilter from the group after pulling a shot. It releases the pressure into the drip tray and is a safety feature. When you perform back-flushing, this three-way valve is what you are cleaning besides the obvious shower screen.

Note: If you look closely at the diagram you will see that when the water exits the pump and heads toward the heat exchanger, there is a boiler fill solenoid valve in the path. When the level of water in the boiler is low, the boiler fill solenoid opens and the pump is activated to refill the boiler. A common question we get asked is why does my pressure some times drop while pulling a shot for a few seconds and then go back up? The simple answer is that your boiler called for water while you were pulling the shot. When this happens, the water from the pump has two paths to follow: the one directly to the boiler or the one through the heater exchanger and your coffee in the portafilter. The water naturally follows the path of least resistance: the one directly to the boiler. How do you prevent this from happening? The easiest way is to pull a little water from your hot water spigot before you make you coffee. Just pull enough too active the pump and cap off the water level in your boiler. This will greatly reduce the chance of it calling for water while you are making coffee.