flue pipe for switzer boiler

[loggie]

I am finally getting time to put my boiler together an I have all the class A chimney components which will go from the draft inducer strait up through the garage and attic and out the roof.The question is about the flue pipe from the boiler in the basement to the inducer in the garage.I core drilled a 9" hole at a 45deg angle thru the 8" thick wall between the basement and the garage and I was going to use double wall flue pipe or DSP for a portion of the run where I cannot maintain 18" clearance to combustibles but after speaking to there tech support he told me I cannot switch from single to double and back to single again.I would use DSP all the way but I am unsure how to connect to the inducer as the only fitting I see is a stove adaptor or a class A adaptor.The run of flue pipe is close to 20 feet about 6 horizontal.The switzers I have seen all used single wall pipe (galvanized) but the codes here say black pipe must be used because of toxic fumes from galvanized pipe.Should I just use single wall and make heat shields.Any installer's out there ever run into this?