no doubt..that is a huge pipe..my install went really well and nice thing was they just used an expander from my piddly 5 inch to the old 6 inch pipe (not so old..put in that chunk about 1.5 years ago and in dog years (or pool years given we only run about 4 months a year) it's about 1 year old and was not cheap good luck in your search but certainly local contractor shouldn't have any problem locating for you..and yes, the double walled is class B..again, can't speak for u.s. specs, but the ngas guy brought an inspector out after install and he didn't blink at how close my shelf was to the double pipe or height relative to my roof height..he was most interested in:

accredited rating certified heater (yep)
proper ngas orifices for the altitude (yep..put em in myself..thank god i read the manual and got them)
bonded equipment..yep..6 guage..bonded bigtime.
all equipment on separate disconnects (switches, but now i know the fancy language)
pool pump and heater (electronic) on a gfci breaker? (yep...)

all is good and working nicely!