Vaporstat

When startung up from a cool boiler, about how long does it take for the vstat to cut out? Then how long does it stay off, and how long does it un before cutting off again?

If the boiler is oversized it's normal for it to cycles, maybe a minute or two on and then off for a like time.

Is the pigtail clear?

Bob

A low pressure gauge would be very helpful to the diagnosis... I don't see the video, so I would ask: is the movement of the mercury bulb smooth, or is it rather jittery, jouncing back and forth? If it is the latter -- which is not at all uncommon -- it is responding to instantaneous, short term variations in the steam pressure. These occur for any number of reasons, and are unavoidable.

There is, however, a solution: a pressure snubber. Grainger has them, as do a number of other folks. It would be placed between the top of the pigtail and the vapourstat inlet. A word of caution on them: the filter type, which is entirely satisfactory for the purpose, does have a tendency to clog and should be replaced annually.

As to the timing of the problem -- both in terms of run time before it starts, and cycle timing after it has started. The run time is good. That suggests you have at least adequate main venting on your system. The within cycle timing, however, tells me that the boiler is significantly oversize for your system -- at least half again as big as it needs to be. You might consider having a qualified tech. down fire it, if that option is available.

Maybe a better shot.