ductwork questions

[rice]

hi everyone.
i have read alot around here but never asked any questions. i am almost finished my new shop and its getting cold here. i am installing a forced air oil furnace in my shop. 30x40 14' ceilings. r20 walls, r40 attick. 83000 btu furnace that can be adjusted down to 60000. i have my light fixtures recessed in the ceiling. i am going for a clean looking ceiling. on the front 30 foot wall i have a 12x12 door. the furnace is going against the opposite 30 foot wall in the corner. i have a few questions for someone who knows about ductwork.

i was planning on running a main duct up the back wall and into the attic and then branching off to a bunch of round ducts. my attic will be blown in insulation and i will cover the ducts good.
-what size of main duct?
-what size of branch duct?
-how many branches?
-is one return against the back wall going to do it?
-if the heat regesters are 14 feet up is that going to properly heat the place?
-if i come off the main duct with some sort of manifold to 6 or 8 round ducts and ran r6 flex to all of the registers, instead of running a main trunk the whole way would that work?
-is it a good idea to have ductwork in the attic? northern canada -40c

thanks in advance to anyone willing to take the time to help me.

 

[Carsonb]

May I just suggest a few things about the furnace first? Get it on a frame or pedistal (non combustable) 30 inches off the floor. There are some nice multi-position furnaces out there too that you could suspend closer to the ceiling and leave yourself more usable floor space too. Your capacity heat wise sounds OK but I'm thinking you have a fairly small fan in the furnace? 1/3 hp G9 fan? Thing is your cfm isn't a lot to push the air down from over head ducts. It'll work but if you want the shop to heat up fast you want some push behind the heat! I'll assume you have a low boy furnace? Hot in front over the burner return over the back? Anyhow built a decent sized plenum over the hot side, you'll likely need to leave an inch over the top clearance from combustables. 6 feet tall is not too much the furnace needs room to build pressure. I'd run an 8X18 trunk line down the center of the building dropping ot an 8x14. Take off 2 per side 6 inch round to the outside walls. Drop them with 90's and get round duct registers. Drop the 8x18 to 8x14 with a tapered transision then run the 8x14 close to the door end. Then I'd take 2 more 6 inch runs off to the side walls 1 each side then 2 more to the door. If you can tolerate dropping pipe or 8x8 square duct down beside each door track to wash the wall with heat you might like it better! Add dampers near the outlets so you can balance the air flow if needed. Normally they go near the trunk line, but I'm thinking you'll not have easy access.

-is one return against the back wall going to do it?

Yes, it''d pull more evenly across the shop if there was more than one but at least centralize it and make it bigger, say 8x24. Drop it down to 30 inches from the floor too, you want to pull the warm air down. Use a good mid range disposable air filter not those Hepa things but not the super cheap disposables.

-if the heat regesters are 14 feet up is that going to properly heat the place?

Yes just not fast, if you open and close the door you're going to be cold some of the time.

-if i come off the main duct with some sort of manifold to 6 or 8 round ducts and ran r6 flex to all of the registers, instead of running a main trunk the whole way would that work?

Not nearly as well as a trunk line to round metal ducting. Technically it should work, but slowly. Flex duct, if you must but its not the greatest.

-is it a good idea to have ductwork in the attic? northern canada -40c

No, you're outside the vapour barrier so every joint will need to be sealed every register opening and you're going to have to make a metal timble (sealed) to extend away from the hot air supply where it enters the attic space. Guessing 12 inches to a wood frame, then the vapour barrier can seal to the metal. The supply will be too hot for super 6, Tuc tape, or accoustic seal stuck directly onto the duct! It can be done, anything can be done it just takes money!

HTH