bbeaton wrote:I'm sure it's common knowledge to other people on the forum, but what's the difference between an anti-vac valve and a pressure relief valve? I've done a quick search but can't find a clear answer. I have an early 90s Salvatore with just a pressure relief valve, is that why I have to bleed the boiler as the machine is warming up?

Yes. However, it doesn't matter if the machine is HX or whatever, the need for a pressure relief valve is dictated by the size of the boiler.

The usual procedure in a machine like yours is to start the machine with the steam valve open, and to close the steam valve when a bit of steam starts to come out of the steam wand. You can do the reverse at the end of a session - open the steam valve a bit to bleed off steam, but this isn't necessary. However, you may find that your boiler pressure gauge will eventually read 0.2 bar to 0.3 bar when it is actually zero bar if you don't do the reverse part. This is just cosmetic, just remember to add 0.2, 0.3 bar to get the actual boiler pressure reading.

The pressure relief valve is for safety - it only opens when the boiler pressure is too high. You'll usually see a value like 2.2 bar or 2.5 bar, etc., on the valve, and the valve will have been fixed/peened so that the pressure can no longer be adjusted.

The anti-vacuum valve is open when the machine is cold. As the boiler heats up, steam from the boiling water will force the valve to close, leading to normal operation. When the machine is turned off, the valve eventually opens again. Thus, you no longer have to bleed pressure manually if the boiler has an anti-vacuum valve.

Note anti-vacuum valves are usually fairly easy to retro-fit as long as there is room in the case for the additional fittings, typically a T and an elbow (because most styles of anti-vac valves have to stand upright and be at the top of the boiler). Plenty of older machines do not have anti-vac valves. I run my machines on smart plugs so I've put anti-vacuum valves on most of them.