New heater - ducting question

[yardwork]

Happy New Year.

Currently I have an old Dayton 125K BTU heater that has a 6" exhaust vent through the roof that I would like to reuse with a new separate combustion garage heater. With the new heater, can I run the combustion air duct horizontal and use the existing vertical ducting (with two increasers 4"-5" and 5"-6"?)

The garage is 694 sq ft with 12' ceilings for ~8300 cu. ft. No windows or service doors, two R13 garage doors (18'x7' and 9'x7'.) Walls and ceiling have poly with R13 in the walls and ~R30 in the ceiling. I'm leaning towards the 75,000 Beacon-Morris from Menards with the separate combustion kit. Online calculators suggest this setup will easily achieve a 40 deg+ rise in temp. Anyone see any issues?
Thank you.

 

[yardwork]

Ok, more research make leads me to believe I'm ok, good actually with the 6" going through the roof. I was operating under the assumption that I had to have two wall or roof penetrations (which is how I've seen them installed) and wanted to avoid that. I now see with the combustion air inlet box I can rework my existing roof vent and have both fresh air and exhaust through that opening.

Am I missing something? Thanks.

 

[bobbyjean]

i think you will be ok with that setup- but just so i'm clear....it's one 6" pipe that will handle both intake and exhaust? seen it before,should be ok.
75k is oversized but will work well...get a thermostat that you can set duty cycle or cycle rate on