Oversized overtall chimney?

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…

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.

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

Would a simple dryer vent be just as effective by placing the vent flap hood inside the room where the boiler is?

All draft is is the difference in weight of a collum of air at a lower temp versus a column of air at the stack temp. Of course, if the flue id restricted, or a lack of combustion air, the wrong chimney cap or some trees can affect things.

I think it is legal on a gas atmospheric to install a flue restrictor (that's not the correct name which I can't remember) and with the correct MU air and correct burner adjustment on a warm day it will help control the draft. But the draft may increase on a cold day not a 100% fix.

The fix for that on an atmospheric gas burner would be a double swing barometric damper on the flue pipe, and an unrestricted supply of combustion and dilution air.

I believe that is what a combustion air inlet looks like but the flap is missing and replaced with a mesh screen to repel critters. That said, a common dryer vent is only 4" and that would not be sufficient for the amount of combustion air required for that boiler. I might guess that @dandub1960 might require an 8" or 10" intake hood. Since field controls only lists the IAH-6 as the largest part available for their combustion air systems, @dandub1960 may need to have one custom built in the size required using the IAH-6 as a template for the larger one needed.

There are bigger hoods for kitchen and bath exhaust fans. Also made by sheet metal fitting manufacturers although it looks like all the brands i knew of for them have been bought and renamed.

Yes because only DIYers are incompetent....

@Steamhead yes a 'neutral pressure point" TY

I was trying to remember where I saw this I Think it was in one of Timmie's books that I have i will have to look

I think Jim Davis would disagree in this scenario.

That is just mean Chris. I know of many professionals that are incompetent! I trained several of them to competency before they went into business for themselves. At which time they became incompetent businessmen.

I just would like to mention that a professional might baulk at converting a boiler with a 480,000 input to operate at 225,000 input. That is less than 1/2 the rated firing rate. Even an inexperienced professional would caution against that.  When asked how this might happen, I could only think of one answer.  Someone experimented with their own equipment.  Since the results did not burn down the house or cause any of the residents to pass away from CO poisoning, then the experiment was a success and the system was left to operate that way ever since.   

Nothing wrong with that assumption IMHO.  I have downfired many a coal conversion from over 1.50 GPH (the rated BTU of the original boiler) to less than 0.90 GPH  by placing a flame retention burner in the firedoor of the boiler to put the fire black in the place it was designed to be.  Does that make me incompetent?  The customers with the lower oil usage don't think so. And those boilers were still operating 15 years later just before I converted them to new gas boilers once the Gas Co. buried mains down the street.    

Then they saved $$$ again with ModCon boilers.  Repeat business. The glory and reward of doing the right thing for the customer. A professional job for a fair price.