I understand what a vacuum breaker valve is and how one works, but why is it needed? I've heard they prevent vapor lock, but I'm not sure I understand how a vacuum could prevent a steam boiler from heating up and operating normally. As the water gets to 250 degrees the steam will fill the vacuum right?

Fair enough (all you smarty-pants types.........)

Here's something that might baffle a Google search but is still a question many owners of some of the new machines WITHOUT p-stats on the steam boiler (like the Alex Duetto II and Vivaldi) might be asking themselves.

Quiz: Do they also need a vacuum breaker? And if so, why?

No machine needs a vacuum breaker, but they make life more convenient by eliminating the false pressure.

Endo wrote:Quiz: Do they also need a vacuum breaker? And if so, why?

Also, there's no reason to assume the method of temperature control of a boiler has an impact on how vacuum is released once the boiler cools down. AFAIK, the P-Stats are a closed system, not allowing any transfer of pressure to atmosphere, thus they wouldn't allow for vacuum relief upon cool down.

another_jim wrote:The miracle of Google site search reveals these amazing facts...

I had no trouble finding how they work, but not a good "physics level" explanation of why it's necessary.

True confession... My days of college physics are long behind me. However, I believe the vacuum forms because the water contracts as it cools. Of course some of the suspended water in the saturated steam returns to liquid, but not enough to offset the reduced volume of the cooled water, hence a vacuum.

At the moment, why this situation does not neatly reverse itself upon reheating eludes me. Anyone care to explain why it doesn't?

Jim, thanks for the links. I tried searching but so many forums here have the words "vacuum" or "vacuum breaker valve." Next time I'll be more diligent.

HB wrote:At the moment, why this situation does not neatly reverse itself upon reheating eludes me. Anyone care to explain why it doesn't?

I find it easier to explain this way:

What is the noise you hear before the vacuum breaker closes? Air escaping (of course). If this air wasn't to escape (either through the valve or by purging the wand), this extra volume of air would result in a higher boiler pressure than if the system was sealed, and therefore trip the p-stat earlier. This extra pressure created by this purged volume of air (pre-boiling air) is called "false pressure".

I think endo raises a good question about PIDed boilers. Since the PID would not be affected by a false pressure reading it would heat the water up to 250 degrees and raise the pressure in the boiler beyond its 1.3 bar setting. It's been a while since I used PV=nRT so I'm not going to try to calculate the pressure it would get to. If it's too high, it could damage the machine or risk injuring an unsuspecting operator. But I guess SOP is to purge the water out to the drip tray.

So would a PIDed steam boilers be better served by an OPV or a VBV?

HB wrote:At the moment, why this situation does not neatly reverse itself upon reheating eludes me. Anyone care to explain why it doesn't?

The only way I can explain it on a machine like the Maximatic is if the vacuum causes air to get sucked in over time.