duct size

[danno1]

if i have my forced air wood furnace 20 ft from my houes should the duct from the blower and the return be the same size?

 

[Duetech]

My un-professional reply is yes. That is based on the interpretation that if one is smaller than the other it will cause a restriction of the air flow unless both are oversized to begin with.

P.S. Welcome to the forum!

 

[laynes69]

If you size your supply side to the blower correctly, I would go maybe 20% larger on the return. This way you have adequate return air for the system. You don't want your return duct too small. What type of furnace are you installing?

 

[danno1]

daka . the person i bought it from had 8 inch duct on both. it also has a big blower on it. but it was only a few feet from his house. but my insurance co. says i have to be 20 feet from my mobile home.

 

[ikessky]

How are you setting this up? Give me a little more info on your plan.

The reason I ask is because I know a guy who tried to run a Vogelzang forced air unit outside of his house. He built a little shed for it, ran insulated duct work from the shed to the trailer and built chases around the insulated duct work. He abandoned the project because the fan didn't have enough power to push warm air all the way into his trailer's duct work and to pull return air back to the furnace shed. He would have had to buy duct boosters or come up with something else to make it work, but he didn't want the added expense (first to buy the fans and second to run then and use more electricity).

Not trying to discourage you, but just letting you know what I've seen in the past.

 

[danno1]

that sounds like the same thing i was going to do but i was going to put the duct in 12inch pipe and put it in ground 2 feet.so what if i use smaller duct from the blower? any thoughts?

 

[Gooserider]

Presumably the numbers for ducting would work like that for hydronics, only w/ different scales... Given that you will be having air under pressure from the fan going to the house, but be sucking air back throug the return, it makes sense that the return should be bigger than the supply duct. I would try to figure what the airflow you need to achieve in order to deliver the heat is, and size the ducts accordingly - I believe the Engineering Toolbox site has some references on duct sizing, there may be better sources.

Also remember that you are going to need to insulate the heck out of the ducts, or you will be ending up using more wood to heat up the great outdoors than you get in your house.

Gooserider

 

[laynes69]

An 8" duct is rated to carry around 200 cfm. So with that that would heat my living room. Since its going outside, you really do need a larger blower on the unit. From there you could enlarge the ductwork and it would work alot better. I wouldn't like the idea to burying ductwork underground, mold, moisture if something ever happened with it. Running it above ground and insulating it is what I've seen with some outside forced air furnaces. If you would buy a larger blower and modify the jacket, you would need a Honeywell Limit/control so you could decide the on and off for your temps.