How are vacuum breaker valves supposed to work?

[tsquared]

I googled "vacuum breaker valves operating principles" and got some good hits. However, I still don't know how they are supposed to work in espresso machines. The valve is supposed to open as the boiler cools down and boiler pressure drops below room pressure? As boiler pressure equalizes with room pressure, the valve closes? I'm confused because:

1. I open the steam valve all the way and then close it
2. I then turn on my machine
3. Boiler pressure reaches between 1.2 and 1.4 bars after about 10 minutes and then the boiler shuts off. If I leave it alone, the boiler will cycle, but time between cycles is a few minutes, much longer than when it's in steady state.
4. I open the steam valve, pressure drops to zero, and then builds back up to 1.2 in a few minutes
5. I open the steam valve again, pressure drops to under .8 bars, and then quickly builds up again

Now I leave it alone and the machine warms up in about another 25 minutes. Why, when I open the steam valve all the way in step 1, didn't that "break" the vacuum? I just want to understand things a little better before I replace the vacuum breaker valve.

-Tuyen

[barry]

it sounds like your vac breaker is stuck closed.


vac breakers are open when the machine is cold (they generally open by gravity), and close when the boiler comes up to pressure (they close by steam pressure).

when the boiler cools off, the vac breaker opens and vents the boiler to the atmosphere. when the boiler is heated, the vac breaker allows the air in the boiler to vent until it is closed by steam pressure, and then the boiler comes up to pressure and operates as normal.

if your vac breaker is stuck closed, then the boiler builds what is known as 'false pressure'... air pressure, not steam pressure. when you vent off that false pressure after the initial heatup, the boiler can then heat up properly.

try leaving your steam wand open during heatup, and shut it after the wand starts to vent steam.

[cannonfodder]

If you listen closely while your machine heats, you should be able to hear the breaker hiss as the boiler pressurizes.

Think of it as the exact opposite of a pressure cooker. On a pressure cooker, the valve opens once the pressure reaches a point; on the espresso machine it is open until the boiler reaches a certain pressure then closes.

[tsquared]

Thanks -- I left the steam valve open this morning and saw some water being forced out as the boiler heated up. Kept it open until steam started come out -- boiler pressure never dropped below 1.0 again even on subsequent opening of the steam valve. So I gather the vacuum breaker valve has to be open a lot longer than just a minute or two. So doesn't this sort of mean that vacuum breaker valves have to be sized to the boiler and heating element? Once the vacuum breaker valve is "fixed" does it stay fixed for awhile or is it meant to be changed yearly?

Also I took a peek and the valve is already the larger sized one; looks like the one that CC is selling.

[barry]

So I gather the vacuum breaker valve has to be open a lot longer than just a minute or two. So doesn't this sort of mean that vacuum breaker valves have to be sized to the boiler and heating element?

the valve has to be open "long enough". how long depends upon boiler size and element capacity. and no, the vac breaker doesn't have to be sized to the boiler. they vent whatever air is in there and then close under steam pressure; they don't have to be very big at all.

Once the vacuum breaker valve is "fixed" does it stay fixed for awhile or is it meant to be changed yearly?

vac breaker life depends upon frequency of use and water hardness. if the machine is left on all the time, then the vac breaker works that first startup and then doesn't have to do a thing until the machine gets shut off again, months or years later. they often get stuck closed in this scenario. if the machine is cycled on and off frequently, and there is any hardness in the water, then the valve will get minute scale deposits on the valve seat, which will cause it to leak steam when closed (my marzocco at home has this problem); just a wee leak, but a leak nonetheless. the valve can also suffer from degradation of the valve seat or o-ring, with age & heat exposure.

personally, i only worry about them if they leak; but i leave my machines on for long periods of time.

--barry