We do a lot of North Atlantic coastal installs and it's not uncommon for us to find installs done by others using side-wall where the strong winds affect inducer issues, even with sealed combustion. We've solved a number of problems that way by venting up an old chimney, all the way to the top with the exhaust. But due to a lack of space for a 2nd vertical pipe, we've opened the chimney base sufficiently to get the combustion air intake into the bottom of the chimney so it pull fresh air down the chimney, around the exhaust PVC. We've never had an issue doing this, solved several wind problems for customers and made a handsome profit in doing so. I personally have installed old HeatMakers and others with concentric pipes and over a long run, it can be a painful ordeal.
This sounds promising. What is involved in "opening up the chimney base"? i.e.: Do you mean masonry work to increase the width of the chimney (sounds expensive).
Also, have you had any issues with frost forming on the opening of the exhaust? It sounds like your climate would be similar, although a bit warmer, to mine (Toronto, Canada).
I'm leaning that way now, but I'm still puzzled on how to vent it. I want to maintain sealed combustion, so I'd prefer not to draw air from inside my home.
I'm specifically looking at the Viessmann Vitodens 100. In the venting manual (link below) it states that I can use coaxial venting. It looks like this provides both intake and exhaust. Can I do this through my chimney? Alternatively, is it okay to use both an intake and exhaust PVC pipe through the same chimney flue?
There is no problem taking the combustion air from the side and running the 3" 636 PVC up the chimney (last time I checked 2" was not allowed in Ontario for this boiler, but they were trying to get it passed). I have done it many times. That said, lots of people will vent out 7 ft above grade in Toronto and because venting can be a bit tough in the city, sometimes it is accepted. I am not advocating it but it has happened and been accepted (not by TSSA but by some local young fitter that is sent out by Enbridge).
Your best bet is the Viessmann 100 with the DHW package vented up the chimney then seal up the chimney, top and bottom.
Stay away from those other boilers you mentioned. They pale in comparison.
I don't know about Canada, but in the US we have been using 2" pp to vent the Vitodens 100. The 2" ridged slides right down the chimney no problem. Placing the intake near the driveway shouldn't be a problem (as long as it's above the snow line).
We are licensed for steam and hot water in Minneapolis and factory trained (BC school) for Viessmann Solar and Vitodens. We also have installed many Viessmann boilers vented to or across the driveway. It is not a public path and rarely used as a sidewalk. The Viessmann makes one of the best concentric (co-axial) vent kits on the market today. The kit may use an existing "B"vent, masonry or new chase with zero clearance to combustibles. The coax also assure sealed combustion and direct -vent with the added advantage of pre-heating the intake combustion air. This has come in handy in high altitude or extreme weather installations. The coax vent is also perfect for side venting through a rim-joist with little chance of frosting or cross-contamination.
Extending a vent beyond the exterior must be done with caution since the water vapor can freeze up the vent if the vent is cooled too long before hitting the open air. I know of know condensing boiler manufacturer that will allow a 7 foot flue extension beyond the envelope...