HVAC design

jrainman

jrainman

Active Member
I am a new here and have learned a lot about growing and leaned that also their is a lot of bad info here to. but overall I am learning and I am grateful, So I try to give back, when I answer a HVAC question here ,I am answering it with over 30 yrs exp in HVAC design and build, now a lot of my answers come from my many yrs of exp. but a lot of questions are answered from me entering in your design Info in to a $12,000 SMACNA LIC HVAC CAD program I am a lic. HVAC sketcher but retired now . So you can do what you want with the information I give ,use it or not that is up to you. Good luck
 
J

joe macclennan

Well-Known Member
question,

I was taught that return air duct sizing in residential systems should be about 25% larger than the main supply trunk. Does this sound right?

Can this rule of thumb be used to approximate passive intake sizes for our hobby?
 
yankiedoodledandie

yankiedoodledandie

Member
IMO In theory to keep the temps cool you want more fresh then exhaust so yeah. Think of it as cooling the air before its being exhausted. If your using a cooltube or coolhood then a 1 foot bubble around the light would need to be as cool as you can get. never going to get 25 percent more air in return, here its mainly about removing hot stale air and providing fresh o2 co2 air in. you can cut the climate of a room or closet or tent in half and come up with two completely different enviroments. At the top half of my tent its about 84 degrees right around the fixtures but at canopy height its 76 and at the floor its 74 my 6o0 watt hps is 18 inches off the canopy. Different light source requires different exhaust...Led and cfl run alot cooler. 6" in 8 inch out is what works best for me .

of course in a 4x4x6 you would need a/c in dead summer heat.
 
J

joe macclennan

Well-Known Member
yankiedoodledandie said:
IMO In theory to keep the temps cool you want more fresh then exhaust so yeah. Think of it as cooling the air before its being exhausted. If your using a cooltube or coolhood then a 1 foot bubble around the light would need to be as cool as you can get. never going to get 25 percent more air in return, here its mainly about removing hot stale air and providing fresh o2 co2 air in. you can cut the climate of a room or closet or tent in half and come up with two completely different enviroments. At the top half of my tent its about 84 degrees right around the fixtures but at canopy height its 76 and at the floor its 74 my 6o0 watt hps is 18 inches off the canopy. Different light source requires different exhaust...Led and cfl run alot cooler. 6" in 8 inch out is what works best for me .

of course in a 4x4x6 you would need a/c in dead summer heat.
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You misunderstood my question. My question for jrainman was concerning efficiency and sound levels. Basically allowing your fan to not starve for air. This is a basic principle when sizing ductwork I believe. I was trying to reafirm this from jrainman due to his experience.
 
dochickory

dochickory

Well-Known Member
I'm lucky enough then, because I can throw a stone to the Ocean, I look out at this spectacular spot on Earth and feel I have been paid in full right at that moment every morning I wake up! Thanks nice saying! :-P oh and it may depend on how your return air is made up in/out SMOKE BREAK :joint:
 
jrainman

jrainman

Active Member
Yes had to think about that for a min , 20-25% would be min , for example take a 2 ton A/c set up this setup would supply about 900 CFM fan , the min return size would be 14 inch round or lets say 14x14 return air plenum
 
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