Ductwork

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Old 02-06-15, 12:37 PM
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Ductwork

Hello, I am going to be installing ductwork for my house, the duct is going to be ran through my finished basement. Where I am putting the air handler I can reach the bedroom, living room, bathroom hallway and possibly the second bedroom without any modification, but to get to the kitchen and my dining room, and far end of my living room I would need to build a soffit in my basement ceiling. Due to height issues the biggest sized duct I could install depth wise is 6". My Idea was to run 4 6" round ducts encased in a soffit in my basement ceiling, 2 would go to my kitchen which is roughly 18x11, one would go to my 12x12 dining room and one would go to the far end of my living room. Is there a problem with this idea or should I go with a rectangular duct which I would end up having to build myself.
 
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Old 02-06-15, 01:15 PM
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Proper duct design is far too complex to discuss in depth in an on-line forum. I suggest that you do some Internet searches as well as looking in your local library for manuals on duct design.

The cross-sectional area of a duct is dependent upon the length, the amount of airflow and the size of the room it serves as well as the temperature of the air. Generally ducts need to be larger for cooling than for heating but the temperature of the air being moved is very important. Also, the BTUs/hour requirements of the room served must be addressed before duct design can even begin. don;t forget that too small a duct will also lead to excessive noise.

I will add that far too many homes have inadequate ductwork due simply to a builder not understanding the dynamics of the system and just installing ducts that will fit in the spaces provided rather than making space for the proper ductwork.
 
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Old 02-06-15, 01:22 PM
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I understand that sizing ductwork can be a very complicated process. My friends house is very similar to mine size and layout wise, even built around the same year. The're ductwork is in the basement, all the rooms are supplied with only 6" flex duct off of a main trunk that runs about half the length of the basement, there is no cold air returns hooked to the air handler, just a whole cut in a wall in their hallway that falls to the basement. They use theirs for both heating and cooling,and it honestly does a decent job at keeping their house comfortable throughout.

So on that thought, I am not really all that worried about the system being perfect, I just want to make sure I am not going to under size the supply to the kitchen, I know the dining room and bedrooms will be getting sufficient CFM. If I under size the supply then my only option would be to tear out the soffit I built and re-do the duct work. Unlike at my friends house, I will have some cold air returns at both ceiling and floor level for my system, I plan to use this for heating and cooling. I guess I'll just do trial and error and see what happens.
 
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Old 02-06-15, 01:34 PM
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Like said a heat load is needed to know what size duct. 6 in is very small if it has to travel very far. if using flex duct it will be even worse.

6in flex duct will only give you around 60 CFM so that would be very hard to heat and cool most 12x12 rooms.
 
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Old 02-06-15, 01:38 PM
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Good luck, my friend. I can't endorse that methodology but I wish you well.
 
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Old 02-06-15, 01:41 PM
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Would a rectangular duct make any difference, say a 6x24 duct? Also I did do a heat loss calculation and came up with 480 btu/hr for my whole house.
 
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Old 02-06-15, 01:49 PM
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You have to calculate the cross-sectional area of the duct. That would be roughly 28 square inches for a 6 inch round duct and 144 square inches for a 6x24 inch rectangular duct. So the rectangular duct is about 28% larger than four of the round ducts.

It gets harder from here out but in a VERY general sense making the ducts too large is far, far better than too small.

Also, 480 BTUs per hour for the whole house is an insanely low figure unless your house is located in an area that never drops below about 65 degrees or is no larger than my bathroom.
 
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Old 02-06-15, 01:58 PM
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I figured I had calculated wrong somewhere with that heatloss calculation, math wasn't my strong point.

I would rather oversize the duct, then be to small, but I don't have a lot of experience working with sheet metal, and little to no experience making ductwork Due to sizing limitations I wanted to use sheet metal.

Its saying here a 6x24 duct can carry 925 cfm, is that correct, that seems a little high.
 
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Old 02-06-15, 02:04 PM
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It has been far too long since I actually did any duct design (some 35 years) that I simply do not remember the fine details. On top of that I no longer have any of my reference materials of those days long ago. There is a wealth of information on the Internet however, all you need to do is find it. I know, not all that helpful. Here is one page to get you started. Residential Air Duct Calculator - EfficientComfort.net
 
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Old 02-06-15, 02:06 PM
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Thank you Furd, I had actually looked at that calculator for, and for the life of me couldn't find it again, so I thank you for linking it to me!

I want to oversize the duct as well because In the future I may finish that side of the basement and would like to have enough cfm left over to add to it.
 
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Old 02-06-15, 02:14 PM
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How about the returns to get the used air back to the furnace/air conditioning source? In my home I fortunately have plenty of supply vents, but not enough returns at the right areas. - Especially important in 2 story homes or homes with open stairway access that can be come a chimney.

Dick
 
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Old 02-06-15, 02:16 PM
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I am going to have a return in the living room, one in the master bedroom, and the rest of the return will come from the basement.
 
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Old 02-06-15, 02:28 PM
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I've been reading along and I saw you mentioned a high and low return. Since my focus is on saving energy I emphasize to home owners the need to balance the supply and return air flow. Up to a point a system will self adjust for opening and closing registers for summer and winter, but there is a penalty for doing so and a big penalty for having insufficient return capacity IN EACH ROOM.

What it boils down to is the equivalent of installing 25% of your supply registers on the outside of the house and trying to heat the neighborhood. Excess supply pressure in a room with a limited return path drives the warm humid air out through your walls, not only wasting energy but risking moisture problems within your walls. And with all of that warm air being forced out, a lot of cold air must leak back in to replace it. PA is not the warmest climate to live in and even at our current lower energy prices, heating season can be painful.

Best,
Bud

A couple of posts beat me to the enter key. You say returns in the basement, but that risks return demand competing with the need for combustion air, unless you have a sealed sysyem on both heat and hot water.
 

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Old 02-06-15, 02:34 PM
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Bud, what your saying is every room should Have a cold air return? Wont air naturally flow from a high to low pressure assuming doors are open?

I guess I should fill in on my whole situation and my impression of this.

I've seen plenty of houses that have been retrofitted with forced air system. Every single forced air system I have seen, unless it was a new construction only has 1 return or no returns at all. Like I said my friends home doesn't even have a return from the first floor to the return of the system. Others I have seen have one large return in a central location. This is going to be a retrofit system, and I have very limited amount of space for ductwork.

My water heater is electric, I have an oil fired boiler that I don't use, my primary heat is a coal stove located in the basement, I was just adding in a heat pump for backup if needed, my coal stove is going to be tied into the ductwork as well later on.
 
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Old 02-06-15, 03:22 PM
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Obviously between the coal stove and the heat pump your are working to reduce your energy costs. All I'm pointing out is that a poorly designed duct system will create high and low pressures throughout the house which results in expensive air being forced out and replacement air sneaking back in. The fact that many other homes have also done it wrong is not really a good reason for not doing what you can to avoid some of their mistakes. Remember, the cost of energy when they skipped over these details was one tenth what it is today.

I deal with this all of the time in the retrofit business where the best way is somewhere between expensive and impossible but people need to move forward as best they can. Air seal the house and air seal those ducts. Be aware that drawing your return air from the basement can cause backdrafting. That oil furnace (not being used right now) is allowed 5 pascals of depressurization in the combustion zone. Your furnace blower is capable of more than 50 pascals of depressurization, far more than enough to where the wrong combination of doors being closed or whatever could result in harmful exhaust fumes being pulled back in to the living space. I can't comment on your coal stove, no information.

I have rattled your cage, you have more information than you asked for, now you get to choose. I do wish you the best.

bud
 
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Old 02-06-15, 03:40 PM
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I really want to do this the right way, its just with my finished basement, its making things almost impossible. I could simply encase the the ductwork in a soffit, that part is simple, the difficult part is that I have doors that I can't make any lower(at least I don't think I can) The only places to run the duct and encase in a soffit there is a door.

Now this door does only go to a closet, and from codes I have read I haven't seen anything against codes about making a door shorter( I would have to shorten the door and frame about 3") If I did it this way I could easily fit 2 8"x16" ducts in there, one for supply and one for return, This would easily allow me to get an adequate amount of return air from that end of the house.

I do have a question that you guys may be able to answer for me, so for lets say my living room, if I added 2 8" supply lines to the living room (roughly 350 cfm) Would 2 wall ducts that are 4x15 for the return be sufficient?(roughly 320cfm total)

I really appreciate the help, I do want to do this system right, I was being arrogant and cheap when I started this thread, but in all honesty its better to just do it right to begin with.
 
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Old 02-06-15, 04:17 PM
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We have become somewhat accustomed to "arrogant and cheap" it happens when things get complex, it is good you recognize it. I have built homes from the ground up and remodeled many existing ones. Ground up is sooo easy. Just an FYI, but our friendly government is working hard to tag every home with a MPG sticker (a HERS score) so new buyers will know how energy efficient their new purchase will be. IMO, it will be a total nightmare for existing home owners to try to compete with each other to achieve the best score. There are just some things out government should stay out of.

The pros will have to answer your air delivery questions.

Bud
 
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Old 02-07-15, 08:38 AM
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Okay so I am just throwing this idea out there, but don't know how well this would work. If I put the unit and all the ductwork in a central location of the attic I could easily cut cost of ductwork by over 3/4, and not to mention it would overall be a million times easier to do.

The reason for wanting it in the basement was to be able to hook into the duct from my coal stove to distribute heat throughout the rest of my house via duct work. Would it be possible and even realitvly efficient if I put the duct work in the attic and then snuck a duct from the basement to the attic through the first floor of the house some how. I feel like there would be a ton of heat loss this way and it would not work.
 
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Old 02-07-15, 09:59 AM
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Locating a heating systems and or ducts in an attic in cold country should be avoided. Some do it, but the energy penalty is difficult to avoid. Being a new install, you could go to extremes with sealing all of the system and insulating it from the cold up there, plus avoiding adding heat with related problems. There may be a trade off that you could work around. You are there and we are not.

Bud
 
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Old 02-08-15, 01:50 PM
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Okay here is an idea I came up with including both cold air returns and supply lines.

Picture is attached.
Supply side. All 14 and 10" duct is solid round pipe.

Off of the air handler plenum box is 2 8" lines that feed the the living room at two different locations. Another 8" lines runs to the back bedroom of the house. 1 14" round duct comes off and runs towards the front of the house, this splits off to two 10" lines that run to the far end of the house. One line gets ruduced to 8" and supplies the dining room, the 10" lines tie together and one 10" lines runs to the kitchen. 1 7" lines runs from the plenum to the spare bedroom, 1 4" line runs from the plenum to the bathroom.

Return side.
All returns will be cut into interior walls and cut through the floor dropping into the floor joist below.
Return air from master bed will follow enclose floor joist, then flow into 8" line into return plenum, one cold air will come directly out of wall cavity in living room and attach into cold air plenum. One 10" round solid line will run the length of the house and receive cold air from spare bedroom and from hallway cold air return. One addtional 10" line will run in the same soffit as the 2 10" supplies and receive cold air from the dining room and the other end of the living room. This will return the same amount of cold air as warm air is supplied.

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Old 02-09-15, 11:55 AM
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What size unit is this? Calculating return size you are at 740 CFM (best practice) Not even 2 ton.
 
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Old 02-09-15, 02:31 PM
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Its going to be a 2 ton or 2.5 ton system. With the vents in place in my plan I calculated about 900 cfm of return (150 at each return)
 
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Old 02-10-15, 08:48 AM
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10 inch duct is about 300 cfm
8 inch is about 155 cfm
 
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Old 02-10-15, 09:29 AM
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I understand that. I changed the plan anyways, I am going to build a rectangular duct to return the air back to the air handler. This system is going to be more for cooling than heating, heat pump is just going to be a back up so I am going to go with a 2 ton system, that is more than sufficient for cooling.
 
 

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