Your machine clicks on, the boiler starts warming, and a few minutes later something hisses like a tea kettle with opinions. It sounds alarming the first time, but in most cases it's exactly what the machine is supposed to do.
Espresso machines vent air, steam, and a bit of water as they come up to temperature. We'll walk through the usual sources, so you know which sounds are routine and which ones are worth a call to our team.
In This Article
Most Warm-Up Noises Are Normal
Heating water in a sealed boiler creates pressure, and machines are built with valves that let air and excess water escape safely along the way. Most of the hissing, gurgling, and spitting you hear during warm-up comes from those valves doing their job, along with condensation finding its way out of the steam wand and group.
The key is timing. Noises that happen during warm-up and then settle down are almost always normal. A sound that keeps going long after the machine is up to temperature, or that comes with steam or water somewhere it shouldn't be, deserves a closer look.
Our blog has an older video with a repair technician walking through the different pressure release valves inside an espresso machine, if you'd like to see the parts themselves.
The Vacuum Breaker: The Usual Hisser
On most boiler machines, especially heat exchangers and dual boilers, the hiss you hear early in warm-up is the vacuum breaker, sometimes called an anti-vacuum valve.
It's a small valve on top of the boiler that stays open while the machine is cold. As the water heats, air in the boiler escapes through it, and you'll hear a steady hiss and sometimes see a little steam or a few drops of water. Once there's enough steam pressure, the valve pushes itself closed, often with an audible click, and the hiss stops.
It also works in reverse. When the machine cools, the valve opens to let air back in, which keeps a vacuum from forming in the boiler. Without it, a cooling boiler could pull water or even milk back up through the steam wand.
That warm-up hiss usually lasts a few minutes. A vacuum breaker that keeps hissing well after the machine is fully heated may not be sealing, which we cover below.
Water Expanding as It Heats
Water expands as it heats. Many heat exchanger and dual boiler machines have an expansion valve that releases a small amount of water during warm-up, so the extra volume has somewhere to go. You'll typically see it as water arriving in the drip tray before you've pulled a shot, sometimes with a quiet hiss or gurgle.
This is normal on machines that have the valve. Our article on why water goes into your drip tray goes into more detail on expansion valves and the other sources of drip tray water.
Spitting from the Steam Wand
Steam wands spit and sputter when you first open them, and that's expected on every machine. While the machine sits, steam in the wand condenses back into water. Open the valve and that water comes out first, along with a burst of hissing and spitting, before you get dry steam.
That's why we always recommend purging the wand into a towel or the drip tray before steaming milk, and again right after. On single boiler machines, switching to steam mode can produce even more water at first, since the boiler is full of water rather than steam. Keep purging until the flow turns to steady, dry steam.
A wand that hisses or drips steadily while the valve is closed is a different story. That usually points to a worn steam valve seal. Our guide to poor steaming covers related wand issues.
Sputtering at the Group Head
Heat exchanger machines are famous for this one. While the machine idles, water sitting in the heat exchanger tube gets hotter than ideal brewing temperature. Run water through the group and that superheated water flashes to steam as it hits open air, so you'll hear hissing and see sputtering water and steam before the flow settles down.
That's normal, and it's exactly why heat exchanger owners run a short cooling flush before brewing. Once the sputtering stops and the water flows smoothly, the overheated water has cleared out.
A small amount of steam or a few drips from the group during warm-up is common on other machines too, especially on E61 groups as hot water starts circulating. A steady drip that never stops while the machine is idle is worth having checked.
Thermoblock Machines
Thermoblock and thermocoil machines heat water on demand as it passes through a heated metal block. When water first hits a hot block, a little of it can flash to steam, so occasional hissing, popping, or spitting from the group or wand is part of how many of these machines work.
Check your manual for what's normal for your model. Many thermoblock machines also recommend running a little water through before your first shot to stabilize temperature, which clears out any spitting at the same time.
Sounds and Signs That Aren't Normal
A few patterns suggest something more than routine warm-up behavior. Keep the diagnosis simple here, since the cause is best confirmed by someone who can see the machine.
A Hiss That Never Stops
The vacuum breaker should close once the machine builds pressure. A hiss or steady wisp of steam from the top of the machine that continues long after warm-up suggests the valve isn't sealing, often because of scale or debris on its seat. The machine may also struggle to build steam pressure or go through water faster than usual. Vacuum breakers are a common wear part, and a technician can clean, rebuild, or replace one.
A Loud, Forceful Release
Boiler machines have a safety valve that opens when boiler pressure goes higher than it should. A loud, continuous rush of steam, or steam venting from inside the machine body, isn't a normal warm-up sound.
Turn it off and unplug it. A forceful, continuous steam release or steam coming from inside the machine can mean the boiler is over-pressurizing. Switch the machine off, unplug it, let it cool completely, and contact our service team before using it again.
Water Where It Shouldn't Be
Water under the machine, on the counter, or seeping from the body panels isn't warm-up spitting. Turn the machine off and unplug it before you investigate, since water and electrical components shouldn't meet.
A Sudden Change
Machines are creatures of habit. A machine that has warmed up quietly for years and suddenly starts hissing, spitting, or sounding different is telling you something, even if the new sound seems minor.
What to Do Next
Start by noting when the sound happens, where it comes from, and how long it lasts. Sounds that stop once the machine is fully warm, or that only happen when you open the wand or flush the group, are almost always normal.
Scale is a common culprit when valves stop sealing properly, so good water matters here. Our article on hard water explains how to protect your machine, and our descaling guide covers the process for machines that can be descaled at home.
Heat exchanger and dual boiler machines need extra care. These machines have complex internal water circuits where descaling solution can get trapped, and many, including Rocket Espresso and LELIT models, should be descaled professionally rather than at home. Check your manual before you reach for the descaler.
Not sure whether what you're hearing is normal for your model? Our team is happy to listen to the details. You can reach our team through our contact page, by phone at 866-372-4734, or by email at support@seattlecoffeegear.com. Related reading: why your pump might be loud and what to check when your machine won't heat up.
Frequently Asked Questions
Is it normal for my espresso machine to hiss when it turns on?
Yes, on most boiler machines. The vacuum breaker vents air from the boiler as it heats and usually closes with a click once there's enough pressure, at which point the hiss stops.
Why does my steam wand spit water before steam comes out?
Steam left in the wand condenses into water while the machine sits. That water comes out first when you open the valve. Purge the wand until you get dry steam before steaming milk.
Why does my heat exchanger machine sputter when I flush the group?
Water in the heat exchanger overheats while the machine idles and flashes to steam when it reaches open air. That's normal, and it's the reason for a cooling flush before brewing.
Why does my espresso machine keep hissing after it's warmed up?
The vacuum breaker may not be sealing, often because of scale or debris. Have it checked, especially if the machine is also slow to build steam pressure.
When is a hissing espresso machine dangerous?
A loud, continuous steam release or steam coming from inside the machine body isn't normal. Turn the machine off, unplug it, let it cool, and contact service before using it again.
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