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How vacuum can cause steam problems

How vacuum can cause steam problems
Back in the old days when most folks burned coal, vacuum was just about everyone’s friend. But when the oil burners arrived, the system problems arrived with them. Today vacuum still causes problems in steam systems, but in different ways.
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Comments
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
HeatingHelp.com
Reminds me of a job where the water kept flying out of the boiler. It wasn’t really a vacuum problem so I won’t put it here. But someday I’ll tell you my lettuce story.
When you say the following: "But then when the oil burners arrived, the system problems arrived with them", I'm going to ask if I am safe in assuming you mean some systems may encounter problems, but surely you didn't mean that all will. As presented it is possible to read this that you are suggesting vacuum's usefulness in residential heating ended with the arrival of intermittent fire.
I do understand that vacuum does cause many issues, and most of the equipment you refer to here I am not familiar with. I do run a process steam system with some of these devices, and vacuum is a problem in some areas just as you say and I employ vacuum breakers in several locations.
That said, in many simple residential applications that have no need of any of these devices like my Mouat system, natural vacuum between firings is very simple to achieve and can provide very significant performance improvement; my results being very much just as Hoffman described quite clearly in their literature on vacuum vents for use in oil and gas fired systems.
I bring this up as I think it would be unfortunate if some readers who might be inclined to try vacuum operation were inadvertently discouraged from doing so by this article.
I removed them both. For years now the only vent has been one single 1/2 NPT solenoid valve at a remote location in my garage. That is how little venting is actually needed to run these things. On an average day that small opening is open to the atmosphere about 10% of the total time.
https://heatinghelp.com/assets/documents/541.pdf
At least one two pipe system not designed for vacuum that I can report does very nicely with it.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
Seems to me that the way to achieve the above is to expel air and limit its re-entry? Didn't Mouat utilize a wimpy ejector to help move steam? It's a waste of steam to use it to compress air. Wonder if Mouat contractors sealed joints somehow?
Another thing to remember is that these systems all used crossover traps (well, almost all, anyway); the use of those (which have at least the capacity of the modern Big Mouth) meant that the progress of steam in the mains was limited by how fast the mains can heat up, not how fast the air could get out.
It's really fascinating to observe one of these things in action -- the boiler fires up at full song, and you expect the vent(s) to begin letting air out in great quantity -- but if you go and put your finger (or tissue or whatever!) up there... maybe a gentle wisp, and that's it.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
There is one important difference however. When the dry return is open to the atmosphere, header pressure at least slightly over atmospheric pressure is required to push steam into the radiators. Not much, but something. The pressure condition at the exit of every radiator is the same...atmospheric. When the burner goes off the dry return then almost instantly becomes the highest pressure place in the system so flow from the mains stops right then. The void created from steam condensing in the radiators is replaced by an unlimited supply of air from the dry return. When the dry return is closed off from the atmosphere and the system pressure drops rapidly, more of the residual heat in the boiler is made into steam, much of it and what steam is still in the mains continues to flow to the radiators as the dry return now retains its status as the lowest pressure place in the system. It isn't a huge amount of steam but the flow of what there is now also favors the radiators in colder areas more than it does when the dry return is at atmospheric pressure. Small differences in condensation rates in different radiators controls the flow more now than when every radiator is facing the same atmospheric pressure at its discharge. In these conditions the steadily deepening vacuum level in the dry return is being effected separately by all radiators individually. This small percentage change in flow every burn cycle adds up to a surprising amount of balancing that does not occur open vented. And it is completely automatic changing with the conditions.
The(to me) very annoying condition of warmer upstairs/colder downstairs in the bitter cold and colder upstairs/warmer downstairs in very mild weather in my house disappeared with the switch to vacuum operation.
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch