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Many say you can't down fire a boiler and I agree that with a gas atmospheric there is not much to control. It comes down to removing and plugging orifices or putting in smaller ones or playing with the gas pressure.

I would never attempt it without combustion testing. With oil we used to down fire all the time with the old boilers especially the old coal fired ones. A firebrick in the flue pipe and adjust the barometric damper , watch your stack temp and combustion #s. You can overdo it like a match heating a tea kettle.

@HVACNUT said: "No other criteria other than the house is still standing, and no dead people for a safe 'successful' operating system?"

What other criteria does a DIY usually require?

I think that falls into the "I don't Know what I don't know" stage of unawareness.

Lets add to that criteria: "It's heating the house!"

I wonder if a mechanical exhaust system might be in order. You could install something like a Field Controls PVG-300. That can be dialed in to the exact draft required to maintain proper boiler venting without the constant changes caused by weather. Seal off the chimney and vent thru the side wall. For more control over the drafty house, add the Field Controls Fan-in-a-Can. or maybe just a combustion air inlet from outside to the boiler room.

You can then seal off the boiler room and the boiler can operate independent of the chimney and the rest of the house for combustion air and exhaust.

That oiler have a draught hood on it? Atmospheric — it should. If so, you really have two separate issues: draught through the boiler, which should be controlled by the boiler's dampers, and draught up the chimney

It's the draught up the chimney which needs to be controlled. The stack damper is really the simplest (and cheapest) way to do that, but as you have found it isn't set and forget — or, actually, it is: you set it for adequate draught on warmer days, and on nice cold days off it goes. Bad news: any fixed restriction on the chimney will do exactly the same thing — stack damper, reduced size liner, you name it. That's the way chimneys behave…

How did it go from 480K to 225K? Was gas pressure adjusted, or burners removed?