Shop HVAC Ductwork Questions

[fidowanttobe]

Folks, I don't know who else to ask!
40X40X12H shop, 2x8 walls, 6/12 pitch, flat ceiling. All metal interior and exterior. Propane 70K BTU furnace with hopes of future AC.
I would like the propane furnace close to the ceiling in the corner furthest from doors, but am clueless to the duct work. I asked around about 12" R-8 insulated flex hose in the attic to 4 diffusers and got mixed answers. One that made me think the most was that flex gathers dust inside more than others. Really was intrigued by the idea of hidden duct, but that would be an issue. But I would also assume that the heat would be dispersed more evenly than 1 duct running along 1 wall and I don't have the room for more than 1 wall. This will be a well insulated building with a twice daily opened door.
I've been scrambling with the wiring to get the ceiling in so I can get SOME heat in there while I work with a space heater, cuz right now all the heat flies out the vented ridge cap, but now I'm stumped and would appreciate any experienced advice.
Thanks in advance.

 

[KennyG]

In an open shop, you don't really need a lot of ductwork. I would not run anything above the ceiling. Just blow a lot of insulation above the ceiling. If you have the doors in one end, I would put the furnace in the center of the opposite end, run a metal duct down the center 20 feet or so and put registers in the end and on each side about 2/3 of the way.

 

[KennyG]

Also, I wish I had put my electrical in surface mounted conduit instead of in the walls and ceiling. It would have been a lot easier and more flexible.

 

[sandman2234]

First thing I would do is put a tarp over that ridge cap! Maybe a couple of layers of tarp to keep that precious heat from leaving while you are working on it!
I am not a fan of flex duct, never have been. Ductboard is getting to be a lost art which is what is in my house. Another method is internally lined steel duct, but that stuff is crazy expensive these days! They make another method that is basically a cloth tube with mesh openings that carry the air toward the other end of the building. I have seen it used in open gyms and basketball courts.
Good luck, let us know what you decide!
David from jax

 

[mikester]

Radiant tube heaters are a low profile option.

Schwank | Radiant Tube Heaters

Schwank radiant tube heaters provide comfort and high efficiency suitable for warehouse heating, distribution centers, and manufacturing applications.

www.schwankgroup.com

 

[KennyG]

If you keep the ductwork under the ceiling, you don't need insulated ducts.

 

[EddieWalker]

I'm not an HVAC guy, but I am a Remodel and Repair Contractor. I like your plan for a flat ceiling with insulation. That will allow the roof to vent and make it more affordable to max out your R value with insulation. Are you going to blow in loose insulation?

I also like the idea of putting your duct work in the attic. Unlike the others, I'm a big fan of flex line. It gives you a smooth run that doesn't come apart or leak like metal is famous for. I've been in a lot of attics. I've never seen flex line leak, but I've enjoyed the cool air coming out of metal ducts on just about every attic that I've been in!!!

I can't comment on 12 inches being the correct size for your main distribution line. I would base the size of that line on the size of the opening coming out of the unit.

Something an AC guy told me when deciding where to place the ducts was in dealing with the outside influence on that room. Windows let more heat or cold into a room then a solid wall. Place the duct to force air in that area.

With your big doors, that's where the ducts need to be. That's where you will have the most heat loss.

 

[MAX-24-Dean]

No ductwork required.

https://www.supplyhouse.com/Modine-HDS75AS0121FCAN-HDS75-Hot-Dawg-LP-Separated-Combustion-w-Aluminized-Steel-Heat-Exchanger-75000-BTU

 

[bigtiller]

No ductwork required.

https://www.supplyhouse.com/Modine-HDS75AS0121FCAN-HDS75-Hot-Dawg-LP-Separated-Combustion-w-Aluminized-Steel-Heat-Exchanger-75000-BTU

Agreed, I see no need for duct work in a garage.
You can hang a 75000 btu from the ceiling like the one Max-24 links to and be done.

 

[workinonit]

If you want to disperse the air just run a round sheetmetal trunk the length of the shop and put 8 or 10 inch stubs in the trunk.