dantyvogel wrote:What is it that happens that could cause damage?

leaking boiler fill valve

dantyvogel wrote:I have an 8-10 year old GS3 and I'm told the first thing I should update is the vacuum breaker.... I'm no technician at all and don't know much about espresso machine parts or repairs, but people talk like it can possibly cause some serious damage. What is it that happens that could cause damage? Seems like a major design flaw if this is the case.

It seems that there may be a misunderstanding here - this thread is referring to vacuum breaker valves, which have the function of avoiding negative pressure in a cooling boiler. They can output water vapour for a short time, when the boiler next starts to warm up, before closing. In case of failure, the valve is likely to fail open rather than closed, which is fail safe. If it failed closed that could cause some low pressure induced damage.

There is a separate or combination OPV (over pressure valve) on most machines, which avoids boiler or system damage from a thermostat or temperature control (PID etc) failure. You'll be able to check a GS3 handbook or water circuit diagrams for the presence of this device, or just ask LM.

kolu wrote:leaking boiler fill valve

+1

On a plumb-in, a leak there allows the boiler to overfill and it will flow out of the anti-vac. If the anti-vac has a hose to the drip tray that flow is safely directed away from electronics and electricals inside the machine.

dantyvogel wrote:I have an 8-10 year old GS3 and I'm told the first thing I should update is the vacuum breaker.... I'm no technician at all and don't know much about espresso machine parts or repairs, but people talk like it can possibly cause some serious damage. What is it that happens that could cause damage? Seems like a major design flaw if this is the case.

On the GS3, the vacuum breaker is placed right above the brain box.
On the newer machines, there's a tube that goes from the breaker to the drip tray.
However, I think that on the older models, this tube wasn't there - so in case the breaker leaks, it may drip right into your brain box...
That's why they recommend replacing it every year.

Graham J wrote: ... In case of failure, the valve is likely to fail open rather than closed, which is fail safe. If it failed closed that could cause some low pressure induced damage.

That is highly dependent on the primary cause of failure. When it's Anti-vacuum valve failure, you're right. But when other components fail (boiler-fill valve in most cases) then there could be slow leak of water to the boiler, even with machine off - that ultimately causes to leak water from Anti-vacuum valve. Or you can have scaled up level probe that cause machine to overfill up to the point where there is incredible amount of water dispelled by anti-vacuum valve during heat up.
Other modes of failure (piece of debris stuck in fill valve, forced overfill in case of level probe failure) usually end up blowing water through safety valve, but nevertheless its better to have the Anti-vacuum valve routed so that in case of flooding, the water escapes to the drain.

Phaedrus wrote: On say, a Profitec Pro 800, the vac breaker has no such line and will spew water onto the boiler until it seals. Is there a technical reason why this design was chosen?

Odd because the Profitec Pro 700 drains to the drip tray.

Yes. Maybe I'm overthinking it, but it seems like these decisions are deliberate design decisions. I just can find a reason as to why.