Guide · 3 November 2026
What happens inside the machine: single boiler, E61, PID, flow control
How boilers, heat exchangers and thermoblocks heat, why the E61 brew group stays warm without electronics, what a PID controller does better and what flow control is for. — by Dieter Eitz
The data sheet of an espresso machine lists words such as single boiler, heat exchanger, E61, PID or flow control. They describe how the machine heats water, how it holds the temperature and how the water flows through the coffee. If you know what is behind them, you understand your machine better and see where you can step in yourself.
1. Single boiler: one boiler for everything
A single-boiler machine has exactly one boiler. For espresso it keeps the water at brewing temperature, usually around 93 °C. For frothing milk, however, the steam wand needs steam under pressure, and for that the boiler has to get much hotter, around 120 to 130 °C. So the machine switches between two modes: first espresso, then heating up, then steam. If you want the next espresso straight afterwards, the boiler is too hot and has to cool down first, usually by letting some water run through the brew group or the steam wand. If you only drink espresso, you will not notice any of this. If you often make several milk drinks in a row, you will wait more often.
2. Heat exchanger: one boiler, two circuits
A heat-exchanger machine keeps its boiler permanently at steam temperature. A pipe runs through this boiler: the heat exchanger. The fresh water for the espresso flows through this pipe and is warmed by the hot boiler water on the way, without mixing with it. This way you can pull a shot and froth milk at the same time.
The catch: if the machine stands for a while without brewing, the water in the heat exchanger gets hotter than you want. With many heat-exchanger machines you therefore let a little water run through the brew group before brewing, until it no longer hisses and spits. This is called a cooling flush. Some newer heat-exchanger machines measure the temperature in the heat exchanger or in the circuit to the brew group and control the heating accordingly, so the flush is shorter or not needed at all.
3. Thermoblock: hot on the go
A thermoblock does not store hot water. The water only flows through a heated metal block when you brew and is brought up to temperature on the way, much like an instant water heater. That is why such a machine is ready after a few minutes, while a machine with a large boiler often needs 20 to 30 minutes until the brew group is warmed through as well. Machines with two thermoblocks, one for coffee and one for steam, can do both at the same time.
What a thermoblock does not have is much heat in reserve. A large boiler with plenty of metal around it acts like a heavy cast-iron pot: it takes a long time to heat up, but cold water hardly throws it off balance. With a thermoblock, the control has to reheat quickly at every shot. For a few cups in the morning that works well. Many shots one after another and long steam phases are the strength of large boilers.
4. The E61 brew group: warm without electronics
The E61 is the distinctive, usually chrome-plated brew group at the front of many portafilter machines. It came onto the market in 1961, hence the 61 in the name, and is still one of the most widespread designs today. It consists of a heavy block of chrome-plated brass that stores heat itself.
This block is kept warm by a circuit that needs no pump: a thermosiphon. Hot water is lighter than cooler water. It rises from the heat exchanger (in machines with two boilers: from the brew boiler) through a pipe up into the brew group, gives off heat to the brass there, cools down a little in the process and sinks back through a second pipe. This circulation runs constantly as long as the machine is on. The group stays evenly warm this way, but it also needs its time to heat through after switching on.
The E61 also includes a small pre-infusion chamber: before full pressure arrives, this chamber fills, and the water first meets the ground coffee more gently.
5. PID: fine control of the temperature
Older machines control the boiler temperature with a pressure switch or thermostat: heating on until an upper limit is reached, then off until it drops below a lower limit. The temperature swings by a few degrees as a result.
A PID controller works more finely. The three letters stand for proportional, integral and derivative. Put simply, the controller looks at how far the temperature is from the target, how long it has been off, and how fast it is changing at the moment, and heats in measured doses accordingly. Like cruise control in a car instead of alternating full throttle and braking. The result is a temperature that fluctuates only a little. On many machines you can also set it to the exact degree on the display, for example a little higher for light roasts and a little lower for dark ones.
6. Vibration pump and rotary pump
A pump builds up the pressure for the espresso. Most home machines have a vibration pump: a magnetic coil moves a piston back and forth in time with the mains frequency. It is small and reliable, but audible; the pressure rises rather slowly and pulses slightly.
A rotary pump is driven by an electric motor and comes from the catering trade. It runs much more quietly, reaches full pressure quickly and holds it evenly, without pulsing. Many machines with a rotary pump can also be connected permanently to the water supply, so you no longer have to refill a tank.
7. Flow control: steering the flow yourself
On most machines you start the shot and the pump pushes at full pressure straight away, as a rule about 9 bar at the brew group. With flow control, a valve sits between the pump and the coffee that you operate yourself during the shot, often with a lever or a dial on the brew group. With it you decide how much water flows and how quickly the pressure builds.
A typical sequence: at first you open the valve only a little, so the water slowly moistens the ground coffee. Then you open further up to full pressure. Towards the end you ease it back a little. The gentle pre-wetting can help the water flow through the puck more evenly so that no channels form. This is especially interesting for light roasts, which are harder to extract. Flow control is a tool for experimenting, not a must: a well-dialled shot at constant pressure tastes just as good.
Your next step
Look at the data sheet of your machine and pick out the terms: How does it heat, how does it control the temperature, which pump does it have? From that you can see where to start. If it has a PID controller, try two temperatures two degrees apart with the same bean and taste the difference. With a heat-exchanger machine it is worth trying the cooling flush once with a cup and a thermometer.
Single boiler → one boiler, espresso and steam one after the other
Heat exchanger → steam boiler with heat exchanger, both at the same time, possibly a short flush before brewing
Thermoblock → heats on the go, ready quickly, little heat in reserve
E61 → heavy brass group, kept warm by thermosiphon, with pre-infusion chamber
PID → fine temperature control, often adjustable to the degree
Rotary pump → quiet, even, often mains water connection possible
Flow control → steer the flow, and with it the pressure curve, by hand during the shot
