Ductwork for finished attic...


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Old 03-27-07, 09:09 AM
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Ductwork for finished attic...

Were planning to finish the attic in an 800 square foot bungalow in southern Michigan. The house is early 50s vintage and pretty poorly insulated. The attic and eaves will be somewhat better insulated than the rest of the walls by the time were done.

The furnace is a Lennox Pulse, 60000 btu, that was installed in the mid-80s. It takes combustion air from outside and vents exhaust through the back wall by way of a pair of PVC pipe runs. All the large (12-inch?) round ducts for the original gravity furnace were replaced at that time with 6-inch. Right now the central air conditioner is inoperative, but will be replaced pretty soon.

Does it sound like the furnace is in the ballpark for that size of frame home with finished attic and basement? My impression is that it might be a bit of overkill.

The furnace is in the middle of the basement so that the longest duct to any of the 5 ground floor registers is on the order of 12 feet from plenum to head. There are cold air returns on outside walls in each ground floor room except the kitchen/dinette and bathroom. In the finished basement there are two ceiling level registers plus a cold air return right at the base of the furnace.

I want to divide the 380 square foot attic into two roughly equal sized rooms. I believe I need separate ducts to bring heat to each? Plus similar sized cold air returns, as well?

Each of these duct runs will be closer to 20 feet long, at least, with a couple of 90-degree bends to get them where theyre going. Do they need to be larger cross section than the short runs to the ground floor?

Using 8 inch round and 3.25 x 14 stacks would give me 50 sq. in. per room compared to 28 for the 6 inch. This sounds better for both the distance and the large-ish size of the rooms.

Ive worked out how to get everything through the walls to the attic and back and it seems pretty do-able. My preference is to do as much of the labor ourselves as we can, but the design is going to have to pass muster with the local building department.

How involved do you think planning this should be?

Just use the larger duct size for these longer runs and hope for the best?

Have a HVAC designer work up a full Manual J and Manual D analysis (or even apply an experienced eyeball to the job) and do the work ourselves to his specs? Will I be able to find anybody willing to do that?

Or will I have to call in a contractor to do everything?

Im doing what I can to educate myself on this subject. Are there any decent materials in print or online that would give this capable do-it-yourselfer homeowner a reasonable introduction to what he needs to know?

Your advice, experience, and suggestions will be appreciated.

Mike D.
 
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Old 03-27-07, 05:45 PM
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First you may want to read this
http://www.inspect-ny.com/heat/lennox1.htm

Next your furnace is getting up there in age; I'm a stickler on replacing old furnaces.
Seen too many cracked heat exchangers causing carbon monoxide to be released in one's home and your furnace concerns me.

If you plan on getting a new furnace the A/C contractor will size the furnace for your total space including you new space and should guide you on your duct insulation.
 
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Old 03-28-07, 03:08 PM
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Dear Dave,

This was my father's home before he passed away. Now that you remind me, his first Lennox furnace was replaced or repaired under warranty somewhere around 1990. Whether the replacement was any better than the original will remain to be seen.

In context with everything else we're trying to get done with this house right now, I do not intend to replace the furnace before we move in unless it demonstrates a detectable problem. Now that you've brought this up I will contact Lennox and install some CO detectors right away, and be sure to at least have it inspected before next heating season begins.

If it turns out I can avoid engaging a contractor immediately because of the furnace, the rest of my questions remiain in play...

How detailed do I need to get in planning the runs for the attic?

Will I want both a supply and return in each of the two attic rooms?

Am I likely to be able to find anyone to hire to look at this professionally and help us with the design, if we're going to be doing the installation ourselves? Or will I have to hire a contractor no matter what?

I'm making at least a tentative effort to try to calculate the duct requirements for the attic per materials I've dug up in the county library system. If anyone would like to mentor me a little bit in this, I would appreciate it very much.

Meanwhile, is my intuition at least approximatly correct that these ducts that are twice as long, plus bent a couple of times, are likely to need to be bigger than the shorter ground floor register runs?

Thanks,

Mike D.
 
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Old 03-28-07, 08:55 PM
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Ok get it checked, cleaned too, CO2 detectors are a must! Even knew furnaces need these.
I feel better now, but I will say this is my last Furnace post do to I am a professional contractor there is too much liability for me. So in note, use at your own risk and take nothing I say as exact, it is only an opinion if equipment is new. Plus add I have no Idea how your bungalow really is.

Now to answer your question,
If you are going to add a room approx. 380sq ft.
This is added to 800sq ft? Total living space would be 1180sq ft?
Does that include your basement sq ft?
is your new attic space going to be where you sleep?

A 60,000 BTU furnace, normally produces 1200CFM of air depending on the model of the unit so adding your attic space will probably meet your needs.
I would change all collars from the furnace to have damper adjustments if possible.


Taking your (2) 8' Flex duct that would give you 420-480cfm
Which divided by 2 would give you 210-240cfm per room using 8" duct per vent.

or (1) 10" duct that would give you 380-420 total cfm.
Tie the 10" duct into a insulated triangle box in your attic section tap out each side with your
8" flex to your rooms.

That leaves you about 700 to 800cfm of air for the rest of the house.


For strapping flex duct you want to make the 90 turn with 2 straps 1 perpendicular to the horizontal bend in the duct and another strap about a few inches away and at around 45 to 20 degrees off of that. Where the 90 Flows and does not pinch.
Keep the duct from wobbling dipping and peaking, firm and tight is good and air flows more freely.

For return, This is hard, I have NO CLUE. If the rooms were well insulated and air tight propper return calls for a 12x12 grill in each room with 10" flex for each. I would tie both into a insulated box and run 14" flex down to the return area where the furnace is. You may be able to use 12" flex but it could cause air noise in your return.

That's about it, don't forget the damper collars!
 
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Old 03-29-07, 07:09 AM
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Originally Posted by ClassicDave View Post
Ok get it checked, cleaned too, CO2 detectors are a must! Even new furnaces need these.
Add cleaning to the list.

I feel better now, but I will say this is my last Furnace post do to I am a professional contractor there is too much liability for me.
Believe me, Dave, I understand that I AM THE ONLY ONE RESPONSIBLE FOR WHAT I DO WITH INFORMATION I GET FROM STRANGERS ON THE INTERNET! I'm only looking for whatever guidance I can get for my own research and study.

Now to answer your question,
If you are going to add a room approx. 380sq ft.
This is added to 800sq ft? Total living space would be 1180sq ft?
Does that include your basement sq ft?
is your new attic space going to be where you sleep?
Including basement, total living space would be about 1900 sq ft. The attic will be divided into a bedroom and a sitting area of approximately equal sizes.

Does that change your opinion below any?

A 60,000 BTU furnace, normally produces 1200CFM of air depending on the model of the unit so adding your attic space will probably meet your needs. I would change all collars from the furnace to have damper adjustments if possible.
All the ducts have dampers in them, as will the ones I add.

Taking your (2) 8' Flex duct that would give you 420-480cfm
Which divided by 2 would give you 210-240cfm per room using 8" duct per vent.
I'll be using galvanized round duct in the basement and 14 x 3.25 rectangular for the stacks. These are all stud walls on 16" centers. How much effect do the transitions and duct shape have on flow calculations?

I should be able to run a separate supply duct to each half of the attic. At the top of the runs I will be in the eaves space very close to where they terminate. I shouldn't need any flex at all.

Does flex have less effective diameter due to surface roughness?

That leaves you about 700 to 800cfm of air for the rest of the house.
It seems I might be spreading things a little thin. However, if so it will be temporary. A couple of years down the road I'd like to add on a sunroom, which will be a good opportunity to replace the furnace with a larger one and zone controls.

For return, This is hard, I have NO CLUE. If the rooms were well insulated and air tight propper return calls for a 12x12 grill in each room with 10" flex for each. I would tie both into a insulated box and run 14" flex down to the return area where the furnace is. You may be able to use 12" flex but it could cause air noise in your return.
How about I drop another three (or even four?) 14x3.25 ducts and connect them at the top in a manifold that I can split between the two rooms? It happens that I have an interior wall below the eaves space near the attic divider wall. I should be able to convert at least three and possibly 4 bays of that into cold air return. At the bottom, they'll all dump into the same joist space where I can re-combine and duct them directly back into the main return that runs along the center of the basement ceiling.

The only thing left to check is if I need to enlarge the return duct for the short distance between the added returns and the furnace. I have not dismantled the basement ceiling there to see what I have yet.

Today's assignment is to try to do a constant friction method calculation for the system and see if I can make any sense of things that way. <sigh>

So, what do I have glaringly wrong? What can I improve? What have I not thought to ask???

Thank you very much for your help.

Mike
 
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Old 04-01-07, 12:58 PM
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U-values for heat load calculation

Is there any place I can find tables of U-values for various kinds of construction on line?

Does the ASHRAE Fundamentals manual have this data? It is available from our county library reference section. The ACCA Manual J is not.

Is there any other way I can derive those values?

I am trying to do a heat load calculation for my house to check if the furnace will be big enough to heat the new finished attic space. I'm using procedures described in Audel's HVAC Fundamentals Volume 1.

Mike D.
 
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Old 04-01-07, 08:59 PM
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I think you will be better off to call some one in to give you a bid on the job.
From what you have said you are going to need a lot of sheet metal duct work made up to get up to the new rooms. plus walls that you have to open on the first floor to put the duct in
Might go to http://warmair.net and compare fuel cost. See if you can put a ductless heatpump up there. Or put a small furnace and AC for the rooms there. You are always better off with a 2 story home to go with to units
 
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Old 04-24-07, 02:35 PM
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Return duct construction methods...

Well, I've been able to do a heat loss calculation that shows my 60000btu furnace is in the ballpark, including the two new rooms in the attic. See this thread for details of the house and project:

http://forum.doityourself.com/showthread.php?p=1149338#post1149338

My supply runs to the attic rooms are figured out. Two galvanized 8" round ducts will come almost straight out of the furnace, elbows feed into the bottom of two 3.25 x 14 stacks, more elbows at the top, and almost straight out to the backside of the registers in the wall. Total length of the runs is about 22 and 24 feet. It's about as good as you can get and still go where they need to.

My question now is the right construction methods for the return air ducts. Code no longer allows the use of framed spaces as air returns, right? We have to use enclosed ductwork throughout?

I'll put grilles on the opposite sides of the rooms from their supplies, and I can duct them to as many as four 3.25 x 14 stacks through other interior walls to return the air to the basement.

All the original cold air returns in the basement ceiling are 14 x 7 inch joist spaces that are enclosed by sheet metal on the bottom. They feed into the main return which runs below the joists in the center of the basement ceiling.

For a lot of little reasons it would be much easier to do the same thing for these new air returns for the attic, but is it allowed? Otherwise I need to do what, line these spaces with sheet metal, duct board, or fully enclosed ductwork?

Of course, I realize that anything you tell me I'll have to clear with my local inspector.

Please let me know some workmanlike ways to do this part of the job right?

Thanks,

Mike D.
 
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Old 04-27-07, 09:52 AM
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Question One more time...

It's been a couple of days and it looks like I'm not getting an answer.

Would anyone please point me toward a reasonably accessible source of information on what methods are permissible to construct my air returns?

The books I've been able to get through our local library system rarely get down to the hands-on detail, which I assume you professionals got in trade school and from your apprenticeships. Despite all the wonderful theory I've been able to absorb, I have not found a basic methods and materials tutorial to fill in these gaps in my practical knowledge, although I've tried.

I'm not your typical do-it-yourselfer. Before we move in, I will have relocated our electrical service, re-wired most of the house, given the kitchen a major face-lift, and re-built the bathroom from the framing out. All the work will be permitted and inspected and the quality will be as high as I can make it.

If I could persuade my wife to let me, I'd tear out our supply plumbing and replace it as well, it's so damn old... And I HATE sweated copper. <sigh>

So, is giving me the help I'm asking for within the mission of the doityourself.com forums or not? You can send me a PM if you'd rather not be seen talking to me...

Thanks,

Mike D.
 
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Old 05-02-07, 10:40 PM
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I find its best to check code for where you at. As to what can be used for the cold air duct returns and how. they are different all over
 
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Old 05-03-07, 11:34 AM
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Red face getting a manual j

There is a forum on this site where you can get help from others on doing load calculations, or get someone to complete one for you:

Manual J

There is also some info on software.
 
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Old 05-03-07, 02:35 PM
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Thanks...

Thanks everyone.

Manualj, some good resources and links there. The forums look like they have potential. Thanks for the link.

Ed, I'll be checking with our inspectors on local requirements. At least now I've gotten enough responses (on my thread in the other sub-forum) to know that my question will not be an unreasonable one.

The last big detail for me to confirm is if I will want to re-size the section of return duct between the furnace and the main return air trunk. It may be borderline. The good news is it's extremely accessible, unlike just about everything else about the job.

Once I get things together I'll do some pressure, flow, and temperature measurements to make sure the furnace is not being abused. If there's a problem, I'll be back, I'm sure! <LOL>

Be well,

Mike D.
 
 

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