Explainer · Espresso machines
Single boiler, heat exchanger or dual boiler?
How a machine heats its water decides how long it takes to warm up, how consistent the coffee is from one shot to the next, and whether you can steam milk while the coffee pours. This guide explains the four common designs and who each one is for.
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Espresso needs water at about 90 to 96°C. Steaming milk needs steam, which means heating water well above boiling point. A home machine has to manage both from one box that plugs into a kitchen socket, and there are four common ways of doing it: the thermoblock, the single boiler, the heat exchanger and the dual boiler. Which one a machine uses tells you more about what it will be like to own than almost any other spec.
This guide is researched from manufacturer specifications and long-term owner reports. We haven’t used these machines ourselves. The warm-up times in the tables come from each product’s data, which you can check on its page.
Thermoblocks heat up in seconds
A thermoblock heats water as it flows through a hot metal block, so there’s no tank of water to bring up to temperature first. The fastest designs are ready to brew a few seconds after you switch them on, and the slower ones take about a minute. They use little electricity when idle, because there’s nothing to keep warm.
The downside is that one heater does both jobs. After making the coffee, the machine has to switch over to steam, and you wait a few seconds before you can froth the milk. You’ll barely notice that with one flat white, but it adds up over a round of four. The temperature can also drift if you make several shots in a row. The better thermoblock machines have a PID, an electronic controller that holds the water at a set temperature, to keep that in check.
Most compact machines use a thermoblock, and so do nearly all machines with a built-in grinder. If you want a machine that’s quick and easy to use, start by looking at these.
Single boilers are simple but slower
A single-boiler machine keeps a small boiler of water hot. Most have the same 58mm parts as café machines and a heavy brass group head, the part where you lock in the portafilter (the handle that holds the coffee). All that metal holds its heat, which helps the shots taste consistent once the machine is fully warm. There’s also very little to go wrong, so a well-made single boiler lasts for years and can be repaired.
The boiler can only be at one temperature at a time. To steam, you flick a switch and wait while it heats up further. After steaming, the boiler is too hot to brew with, so you run water through until it cools back down. Without a PID, a simple thermostat lets the temperature swing up and down by several degrees, and owners learn to time their shots around it. This is called temperature surfing. Some people enjoy it and others find it a chore.
- The Classic E24 is ready in about 5 minutes; the Silvia takes about 10 minutes.
- The Classic E24 comes with pressurised and non-pressurised baskets; the Silvia with non-pressurised ones.
- The Classic E24 stands 38cm tall; the Silvia 34cm.
| Classic E24 | Silvia | |
|---|---|---|
| Price from | £493.99 | £582.99 |
| Heat-up time | about 5 minutes (better) | about 10 minutes |
| Filter baskets | Pressurised, non-pressurised | Non-pressurised |
| Width | 23cm (better) | 23.5cm |
| Height | 38cm | 34cm (better) |
| Depth | 24cm (better) | 29cm |
| Water tank | 2.1L (better) | 2L |
Many people looking at single boilers start with the Gaggia Classic and the Rancilio Silvia, and we’ve written up how the two compare. Neither has a PID, so you’ll need some patience with both. A single boiler with a PID, such as the Lelit Anna PID, takes away most of the guesswork and keeps the simple design.
Heat exchangers brew and steam at once
A heat-exchanger machine has one large boiler that’s kept hot enough to make steam all the time. The water for the coffee doesn’t sit in that boiler. It runs through a tube that passes through the boiler, and picks up heat on its way to the group head. So you can steam milk while the coffee is pouring, and there’s plenty of steam for several drinks in a row.
In the traditional design, water left sitting in that tube gets too hot while the machine is idle. Owners run a little water through before each shot to cool it down, which is called a cooling flush. Some newer designs avoid this. The Lelit Mara X, for example, controls its temperature from the group head, so you don’t need to flush before every shot.
Heat exchangers also take a long time to warm up. A big boiler and a heavy group head can need twenty minutes or more to settle at the right temperature, so a lot of owners put the machine on a smart plug and have it switch on before they get up.
- The Mara X has PID temperature control; the Appartamento doesn't.
- The Mara X is 22.5cm wide; the Appartamento is 27.4cm.
- The Mara X is 52cm deep; the Appartamento 42.5cm.
| Mara X | Appartamento | |
|---|---|---|
| Price from | £1,138.99 | £1,336.99 |
| Heat-up time | about 24 minutes (better) | about 25 minutes |
| PID temperature control | Yes (better) | No |
| Width | 22.5cm (better) | 27.4cm |
| Height | 35.5cm (better) | 36cm |
| Depth | 52cm | 42.5cm (better) |
| Weight | 18.5kg | 20kg |
The Mara X and the Rocket Appartamento are the two heat exchangers people most often weigh against each other. Our Mara X and Appartamento comparison explains who each one is for.
Dual boilers have a boiler for each job
A dual-boiler machine has two separate boilers, a small one for brewing and a larger one for steam. Each has its own temperature control, so you can set the brew water to the exact degree and leave the steam boiler alone. You can brew and steam at the same time, and you never need to flush to cool anything down.
It’s the most capable design and also the most expensive. Two boilers use more electricity than one, and they mean more parts and more descaling. Warm-up times vary, but they’re usually shorter than a traditional heat exchanger’s because the brew boiler is small.
- The Dual Boiler is ready in about 6 minutes; the Elizabeth takes about 8 minutes.
- The Dual Boiler comes with pressurised and non-pressurised baskets; the Elizabeth with non-pressurised ones.
- The Dual Boiler is 37.9cm wide; the Elizabeth is 32cm.
| Dual Boiler | Elizabeth | |
|---|---|---|
| Price from | £1,099.95 | £1,139.99 |
| Heat-up time | about 6 minutes (better) | about 8 minutes |
| Filter baskets | Pressurised, non-pressurised | Non-pressurised |
| Width | 37.9cm | 32cm (better) |
| Height | 38.6cm | 38cm (better) |
| Depth | 37.7cm (better) | 38cm |
| Weight | 15.2kg | 15.3kg |
A dual boiler is the most capable design. Whether you need one depends on how many milk drinks you’ll want to make at once.
Which design to choose
If you want a coffee quickly before work, choose a thermoblock. Nothing else is ready as fast, and a good one with a PID makes consistent shots.
If you’ll mostly drink straight espresso, a single boiler is a good choice, ideally one with a PID. You’ll rarely need steam, so the wait between brewing and steaming hardly affects you, and the heavy group head helps each shot come out the same.
If you’ll often make several milk drinks in a row, look at a heat exchanger or a dual boiler. Steaming while the coffee runs saves time when you’re making a round, and both designs produce far more steam than a thermoblock or single boiler.
If you want the most control and don’t mind paying for it, buy a dual boiler. You can set the brew temperature exactly and there’s plenty of steam, without the flushing or waiting the other designs need.
If you’re still unsure, your budget will probably decide for you. Our guide to how to split your budget between machine and grinder explains what each price bracket buys once you’ve set money aside for a grinder.