# Duct work, what size for main trunk?



## lugoismad (Jul 23, 2008)

I have a wood furnace I'll be running the duct work for soon. 

What size would you recommend for the main trunk? I have a 1400sqft 2 story house. I plan on running a 12" duct from the furnace, and T'ing it, half will go to the attic, and half to the crawl space. From there I'll split it off to the 3 main downstairs rooms and 2 main upstairs rooms with 6" lines. 

The line to the attic will have an inline blower installed in it as well, because I'll be using it for an AC I'm installing too.


----------



## maurice (Jul 23, 2008)

If you want the house balanced, you will need to calculate the heat loss/ heat gain on each room/space, (ashrae Man J?_ then size the furnace to the total load, then calculate the duct sizing (Manual D?).  Some computer programs can do this for you.  

Is the duct round?  If you talk with an HVAC guy or an HVAC supply house, they might be able to do a "seat of the pants" estimation.


----------



## lugoismad (Jul 23, 2008)

maurice said:
			
		

> If you want the house balanced, you will need to calculate the heat loss/ heat gain on each room/space, (ashrae Man J?_ then size the furnace to the total load, then calculate the duct sizing (Manual D?).  Some computer programs can do this for you.
> 
> Is the duct round?  If you talk with an HVAC guy or an HVAC supply house, they might be able to do a "seat of the pants" estimation.



Duct will be round.

I'm not going to size the furnace...because I already have it, and got an extremely good deal on it. I'll look for a computer program. Otherwise I'll just wing it and modify it as I go to make it work.


----------



## maurice (Jul 24, 2008)

Thats ok if you already have the furnace.  Most furnaces can have a plenum box sitting on the output side.  (Thats where the AC coil is.)  I've got about 1800 sq ft in an old farm house.  Originally it had 1 (8x16") duct going to 3 rooms, one 8" round going to a kitchen, 6" round to another room.  Then when we added on to make the 1800 total, I changed it (without calculating) to have 2 (8x16") mains going off the plenum.  We now have good air flow (heat and cool) in each room.

So I might guess that one 12" round might be a bit small?  Really, I'm not critisizing (sp), as you see, I was willing to experiment on my own house.  (come to think of it, maybe I should calculate cross-sectional area of 8x16 compared to 12" round....)


----------



## Redox (Jul 24, 2008)

How many CFM is it?  How many branches will there be?  I can give you rough seat of the pants numbers if you want.  I have a Manual D program around here somewhere, but it runs on DOS and really isn't any more accurate on small jobs.

edit:  Oh, you want to put AC on it?  Different matter!  Will the blower support the AC?  You need 400 CFM/ton to make it work right.  How big will the AC be?

Chris


----------



## heaterman (Jul 24, 2008)

In the absence of any info usually required to make that kind of a decision, what is the rated CFM of the blower on the furnace? You should at the very least size it to that because the manufacturer has determined what is required to keep it at normal and acceptable operating temperatures.

Basically speaking, if you undersize your duct system, either main or branch, you will be inviting a host of problems to visit you on a permanently recurring basis.


----------



## lugoismad (Jul 24, 2008)

heaterman said:
			
		

> In the absence of any info usually required to make that kind of a decision, what is the rated CFM of the blower on the furnace? You should at the very least size it to that because the manufacturer has determined what is required to keep it at normal and acceptable operating temperatures.
> 
> Basically speaking, if you undersize your duct system, either main or branch, you will be inviting a host of problems to visit you on a permanently recurring basis.



I called my moms boyfriend, and he said he still has the plenum that goes from the furnace to his AC and ties into the duct work, and I can have it. Its 12"....so I'm guessing as long as I stick with that, it should be fine, because it heated his house for years.


----------



## lugoismad (Jul 24, 2008)

Redox said:
			
		

> How many CFM is it?  How many branches will there be?  I can give you rough seat of the pants numbers if you want.  I have a Manual D program around here somewhere, but it runs on DOS and really isn't any more accurate on small jobs.
> 
> edit:  Oh, you want to put AC on it?  Different matter!  Will the blower support the AC?  You need 400 CFM/ton to make it work right.  How big will the AC be?
> 
> Chris



I'm going to assume so, because I bought the furance and blower from my moms boyfriend, and he heated a roughly 3000 sqft house with it. My house is 1400. I know he said he put an oversized blower on the furnace. It should be fine I'd guess, but I'll look into it.


----------



## Gator eye (Jul 24, 2008)

I am heating 3200 foot and using a 18" round duct off the wood furnace plentum into the gas furnace plentum. I did put a flap, or one way check valve in the duct work so if the gas furnace turns on it doesn't back feed through the wood furnace.

Oh yeah one more thing if you got cold air return, it's worth taping your wood furnace into it. It heats up the house a lot faster and the air seems to flow better.


----------



## Redox (Jul 24, 2008)

Gator eye said:
			
		

> I am heating 3200 foot and using a 18" round duct off the wood furnace plentum into the gas furnace plentum. I did put a flap, or one way check valve in the duct work so if the gas furnace turns on it doesn't back feed through the wood furnace.
> 
> Oh yeah one more thing if you got cold air return, it's worth taping your wood furnace into it. It heats up the house a lot faster and the air seems to flow better.



Yes, definitely have to connect the return.  Otherwise, if someone closes the basement door, the smoke will be sucked town the stack and bad things will happen.  I hear it's a code violation to have a return within 10 feet of a stove.  MUST CONNECT RETURN!!  You can also create a new second return duct if it is easier.

Chris


----------

