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It looks like the vent may be reduced and unless that is something insulated for 0 clearance, not just type b pipe, it is too close to combustibles on the outside.

Your vent location may be the cause of your problem. Here is an excerpt from a typical B-Vent manufacturer’s installation instructions.

6. VENT LOCATION AND ENCLOSURES

Selkirk Type B Gas Vents are recommended to be installed within the heated portion of the structure wherever possible to reduce heat loss which may in turn lead to poor draft and/or condensation and/or ice problems.The building can cause reverseventing action when the appliance is off, or operating on its pilot.In multi-family residential, highrise and many other types of buildings, codes specify that vents must be located in fire rated shafts or chases. Building Code requirements in such cases must be carefully followed with respect to wall construction, access, clearance, support, initial penetration of breaching, and method of termination. If an exterior location is necessary, an exterior located Selkirk Type B GasVent (as per Figure 4) must be:- Enclosed by a chase spaced out 1” from the vent at least to the roof line.- The enclosure should be caulked to prevent entry of moisture.- It is also recommended that his chase be insulated to a minimum of R-12(RSI 2.1).- Rigid type insulation is recommended. If a non rigid type is use, it must be restrained in such a manner so that the 1” airspace clearance to combustibles respected.- The bottom of the enclosure must be closed off.- It is recommended to provide an access panel for future inspection and cleaning of the vent Appliances served by an exterior gas vent must have an air supply to the appliance room adequate to balance indoor and outdoor pressures. Otherwise,“stack action” of the heated building can cause reverseventing action when the appliance is off, or operating on its pilot. In multi-family residential, highrise and many other types of buildings, codes specify that vents must be located in fire rated shafts or chases. Building Code requirements in such cases must be carefully followed with respect to wall construction, access,clearance, support, initial penetration of breaching, and method of termination.

In order to reduce the “reverseventing action” of that B-Vent, commonly called a Down-Draft, you may need to enclose that venting system as indicated in this photo.

Be sure that you use the proper insulation on the interior of the enclosure so as to prevent the insulation from coming in contact with the galvanized vent.  If you have questions about this, you can always contact the build permit folks and ask the fire code official what is the best practice to enclose the vent.  You may need a variance if the enclosure encroaches on the property set back.   

OR just do it on the weekend and hope no one calls the building inspector on you. 

And why all of a sudden, do I need to do this now, since it has not been a problem for 15 years? you might ask

Your old boiler may have has the same problem and someone fixed it by eliminating the roll out switch. which would have been dangerous because that would make it possible for carbon monoxide to enter the building on a regular basis. And it was cheeper that following the B Vent manufacturer's instructions. I do not recommend bypassing the Spill switch limit device. I recommend fixing the problem the correct way. @Bob Harper is the Flue and Exhaust expert on HeatingHelp.com. Lets see if he agrees.

Here is the link to the Selkirk B-Vent manual. You can also find the one for your particular brand online. it. will say something similar I'm sure.

https://www.selkirkcorp.com/literature/Model_RV_Cdn_Installation_Instructions.pdf

Just curious, how was it determined that 199,000 was the right size?