Trouble Setting Up Vaporstat

Bought a Honeywell 408A . . .

. . . yesterday and installed it on the boiler in my 2-pipe system. The pigtail is clean and correctly oriented, and I leveled the unit according to the arrow inside. I set the main to 10 oz. and the subtractive diff to 6 oz. But it is not working correctly.

When I first fired up the boiler, the pressure built up to 2 psi, according to the gauge on the boiler, before I shut it off at the power switch. It is not a very precise gauge -- it doesn't have a tick mark for 1psi; so it's very hard to guess where 4 oz. and 10 oz. would be on it.

By re-leveling, I was able to get the vaporstat to cut out at just about 1-1.5 psi (again, hard to tell exactly). But it does not cut in at all. In the end, I had to re-install the old pressuretrol; it pushes steam into the returns, and causes some other problems, but at least it keeps the house warm.

I really need some help here. I figure one of the following might be the problem:

1. I damaged the vaporstat by leaving it in the cold in my car for several hours.

2. I damaged the vaporstat by letting the pressure get too high when I first fired up the boiler.

3. I haven't gotten the knack of leveling the thing (though I'm not typically stupid about such things).

4. There's some internal problem with the boiler that is preventing the vaporstat (and old pressuretrol, for that matter) from acurately sensing the pressure inside.

5. I have gremlins in my basement.

As to #4, the boiler is badly in need of cleaning. I moved in here in May; the boiler moved in in 1997, and no skim pipe was ever fitted to it. The near-boiler piping is not according to the installation instructions, so I wouldn't be surprised if it was never cleaned after installation.

So could very dirty, oily, silty water in the boiler prevent the vaporstat from functioning? The PA440 pressuretrol works poorly bad too; it's set to cut-in at 0.5 oz. and cut out at 1.5 oz., but regularly runs up to at least 2 psi, sometimes 3.

I guess my main question is whether the vaporstat is likely to be malfunctioning, in which case I simply need to exchange it. Beyond that, any additionaly wisdom would be most gratefully received.