Installing branch Duct

[jasonwiest]

I am finishing my basement, and I am trying to run a branch duct off of the main duct. However, the exact place that I want to run the branch duct takeoff, there are cold-air returns from the first and second stories, running along the floor joists. Will there be a major heat loss if I run my duct straight through a cold air return for about 3 feet along the floor joists?

[dash]

Give us the duct sizes involed.doesn't sound like a good idea.

[jasonwiest]

The duct for the warm air is 6 inches, and is metal. The return air is just pulled through the floor joist cavities, and sealed with cardboard(I don't know what it's actually called)

[Gunnery Sergeant Hartman]

Bad idea.
That duct will not make a difference in your basement anyway...

[jasonwiest]

I know you had mentioned that it's a bad idea to run the duct through the cold air return. Why would that be bad? Is it because it will make the forced air too cold? You had also said that "That duct will not make a difference in your basement anyway" What do you mean by that? In my basement, I am pulling 5 branch ducts to supply heating to the basement, but running them from the main branch, across the ceiling, and down the walls to the floor. (So that the warm air will come off of the floor, and not the ceiling) It's just that the one joist has that return that I would like to pass through.

[dash]

Originally posted by jasonwiest
I know you had mentioned that it's a bad idea to run the duct through the cold air return. Why would that be bad? Is it because it will make the forced air too cold? You had also said that "That duct will not make a difference in your basement anyway" What do you mean by that? In my basement, I am pulling 5 branch ducts to supply heating to the basement, but running them from the main branch, across the ceiling, and down the walls to the floor. (So that the warm air will come off of the floor, and not the ceiling) It's just that the one joist has that return that I would like to pass through.

Running the supply thru the return,will decrease the air flow in that return.

Adding five supplies to the basement ,will "consume" considerable air flow of the furnace.Where will the extra air come from.You really need a Pro,to design this.

One warning,most important,if the furnace needs combustion air ,and it most likely does,where will it come from,THIS can KILL you and yours,be careful.

[jasonwiest]

The furnace does need combustion air. The air is being pulled into the furnace through returns in every room of the house(about 10 returns in total), and I was planning on putting 1 return into the basement to pull in the cold air as well. In your statement below, where you say "One warning,most important,if the furnace needs combustion air ,and it most likely does,where will it come from,THIS can KILL you and yours,be careful." What is the part that is bad, and would kill us about adding in the 5 additional supplies? Is it because it needs fresh air from outside, and the air being returned through the house is not sufficient?

[cem-bsee]

any device having an open flame needs oxygen to burn the fuel -- normally this comes from ambient air -- ambient air in a house comes from infiltration thru the cracks & opening of doors to outside -- "tight" houses need a pipe to the outside with its inner end near the burner --

the forced air systems are circulators of air -- warmer | colder air thru the supply registers, cooler | warmer air returned to the blower via return grill -- in ducts -- ducts must be sized to deliver the correct amount of air to condition each room -- cooler | warmer determined by summer | winter --

BTW,anyone having open flame burners are damm fools if they do not have CO detector!

[Edited by cem-bsee on 02-27-2005 at 09:06 AM]

[Chill]

It sounds like your over your head. Getting advise here from someone that as not seen the house, is not enough. I have read some of your previous posts, and sounds like you do not have enough information to do a quality job. Get a contractor to look it over for you.

[Shophound]

What Dash is getting at is that if by the addition of the new ductwork to the basement you upset the air balance to the point where the furnace begins to backdraft products of combustion from its vent stack into the house, that's a bad thing, and can be deadly.

This scenario could be caused by installing a return air opening for the basement along with your supplies, but the return pulls in more air than is replenished (or has a leak somewhere), causing the room to go into a negative pressure. This would lead to backdrafting of combustion appliances such as water heaters as well as your furnace.

I don't live in an area where houses have basements, but I would think a basic requirement of a basement finish-out would be to build a closet to enclose the furnace and provide the closet with a dedicated source of combustion air from outdoors. Here in Texas many homes have gas furnaces in closets within the house, and are provided with a source of combustion air from the outdoors.

I will weigh in with the others and recommend a qualified HVAC pro tackle your ductwork issues.

[jasonwiest]

Thanks for the advice. I'll take your advice and have someone look over the design to ensure things are done properly. I thought it was simpler than it actually is.