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Well, vapourstats could be regarded as more fine tunable pressuretrols, although it might be more accurate to say they are more sensitive. They come in two ranges -- 0 to 16 ounces and 0 to 4 psi. One would use one of the other, depending on one's application.

Now that you have a nice 0 to 3 psi gauge, though, the first thing (before you change anything!) to do is to watch it as you go through a heating cycle. What you should see (I hope) is that the gauge will register very little pressure -- say less than half a pound (lower is better! for quite some time after the boiler fires up. Indeed, if you have a collaborator, you could have them keep an eye on the slowest radiator, and let you know when that radiator starts to heat up. Ideally, when that radiator starts to heat up you should still be at the low pressure reading -- it shouldn't have changed much. What you should see then is the pressure should start to rise. That is the pressure -- for your system -- at which you want the pressuretrol or vapourstat to shut the burner off, to let the system catch up to the boiler.

If it's less than about 1.6 or 1.7 psi -- which is about as low as a pressuretrol can reliably operate -- you should have a vapourstat. But before you rush out and get one (yeah, I know, it's Christmas and a shiny new vapourstat under the tree would be nice!) there is at least one thing to think about.

Specifically, what was the more or less stable pressure which the system ran at, before it started to rise? If it was more than half a pound or so, how is your main venting? That more or less stable pressure is a direct reflection of your main venting and piping, since that is the pressure it takes to get the air out of the mains. It really shouldn't be over that half a pound or so; in fact, ideally it should be around 0.2 psi for a more or less typical residential system -- even a fairly big one.

Would the reduced fuel burn from not raising the pressure beyond what is needed to get that last radiator going save enough to pay for the vapourstat? Hard to say. It certainly isn't even close to proportional to the pressure setting, but it will save some fuel.

You mention a concern about interrupting the heating cycle. It will interrupt the burner running, if the heating cycle is long enough (on the main system I care for, it sometimes interrupts on very long recoveries). But this doesn't hurt the heating cycle -- those radiators will keep on delivering heat quite happily while the burner is off, working on the excess steam which was produced before the burner shut down. Most important, though, this causes at most a minor reduction of efficiency. Consider. While the burner is off, waiting for the system to catch up, it isn't burning any fuel. When the system catches up, it will fire up again. True, there is a small loss in there -- if there is a post purge and a pre purge, the gasses left in the fire chamber will have cooled off -- but the boiler itself will not have cooled significantly (which you can verify by noting that steam is produced almost immediately when the boiler fires up again)(I have, parenthetically, a disagreement with my delayed restart using a timer friends on this one -- you want as short an off period as is reasonable, so the boiler doesn't cool off).

Some would say that the additional wear on the burner from stopping and starting is a consideration. However, with any kind of luck at all (unless you got the system at midnight plumbing supply, off the back of an unmarked box truck) all the components should have a service life of many tens to hundreds of thousands of starts. Even if you ran 24 heating calls every day all winter, and each call involved the burner cycling on pressure four times (in which case your boiler is wildly ovesized, and you have other problems) that's only about ten thousand starts per winter. If the boiler is reasonably matched to the system, so that you have only perhaps three or four pressure cycles per start, and only on recovering from a setback on a cold morning, the difference is even less. So yes, it is a consideration, but in my view a minor one.

There is a consideration in the opposite direction, too, however: neither vents nor traps are really happy at higher pressures. Yes, they are rated for it (typically up to 3 psi working pressure), but they will last longer at lower pressures. Again, to cite the system in the main building I care for, all the traps are original to the system -- 80 years old -- as is one of the two main vents (I added one a few years back; don't really need it) -- since the system has never subjected them to a pressure differential greater than 12 ounces per square inch; I have heard others quoting a ten year life for traps.

I'm rambling -- but I hope my thoughts help!