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Mapping Out My Ductwork

3169 Views 19 Replies 5 Participants Last post by  57TinkerMan
Hi All,

I'm looking for advice on which size of trunk & ducts to use. I've used a ductwork calculator that is associated with Menards and they drew up a plan but it doesn't take into account some things that I need. I have a 1 story house with a basement. The furnace is in the basement. I marked on the floor plan where registers need to be. The kitchen is open to the living room. I marked where I want the registers with "R" and cold air returns with "C." The house didn't have central air before so there are no holes cut in the floor which gives me some flexibility.

On the basement floor plan, I placed the furnace and plan to run the main supply trunk to the west. I'll also need something to go east since I'll have the Office Register and the Kitchen Register. (I want to go east/west with the trunk because the floor joists run north/south and that will allow me to tuck the runs into the gaps between joists which will save a ton of headroom and allow us to use the basement area south of the dotted line.)

I have a 60,000 BTU furnace and 2.5 ton AC unit (haven't installed that yet).
I will have a total of 7 upstairs registers. I drew up a plan for 4 basement registers. As you can see the fireplace and stairway block where the main trunk would extend to.

The Menards drawing said to use 8in round ducts for most of it, not sure if that's necessary but a friend of mine (that worked with heating a cooling previously) said maybe that's because there are not very many registers in the house and the furnace needs to move a certain amount of air???

What size of trunk do I need to use and when should it be reduced down?
I believe I have the cold air return's figured out because when the furnace was installed, it came with a cold air supply so I figure as long as I bring in at least that much air, it'll be good.

THANK YOU FOR ANY ADVICE!

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How did you determine you need 2.5 tons of cooling/60k BTU of heating?
Duct is designed via a room by room manual J with a manual D. And totaled to be certain the duct will move enough air for the unit.
You’ll have to make sure you can move roughly 1,000 CFM of airflow
 

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I agree with "roughneck", it sounds like your guessing. 8" round pipe for small house supply runs is pretty hefty. There is a free load calc program you can run but I can't remember the name of it, someone here will let you know.
In the attached picture (very rough), it seems you have a headroom problem in your basement (shown with X) since you are already losing a little headroom at the bottom of the steps replace the round pipe you have drawn with rectangular duct and tie a living room and basement heat run onto the duct east of the stairwell. Looks like you can get all the return (blue) you need keeping to the West of the fire place. If you can get the proper load calcs we can assist you in sizing the duct.


 

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Doesn't look like a very large house, you may only need 1.5 ton and 40k.

A room by room load calc is a must!
 

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Discussion Starter · #5 · (Edited)
Thanks for the responses!

I put a screenshot of the room sizes. That's the same thing I sent to Heating and Cooling Products. They are the company that draws up duct plans and sent you a product list that you can get at Menards. It was free.

I got the furnace and AC unit from Alpine Home Air Products & I think a local guy recommended the 2.5 ton and 60,000 BTU but maybe it was Alpine, I can't remember.

I know our electrician and friend of ours that does some new construction things were both surprised by the 8-inch recommendation. I know from reading some other threads here that 400 CFM get moved per ton, so as Roughneck said, I'll need to move about 1000 CFM...maybe that's the reason 8-inch was recommended? Maybe the furnace is bigger than needed. I'm not opposed to using 6-inch (it seems like that's what most homes use) but I did already pick up the materials for 8-inch. I'm sure I can return/exchange it though.

*The numbers in each room are the AREA. I had them labeled with different letters for another project I was working on.
 

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I’d run a manual J before buying any equipment.
Oversizing can lead to all kinds of problems. This can be compounded by undersized duct.
Then you can do your room by room manual Js and get your duct designed.
Equipment and duct is not sized by square feet alone.
 

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I may have not added sq ft up right. But looks like your just under 1000 sq ft. It looks like someone used 400 sq ft per ton to size the equipment. And unless there is no insulation in the walls and ceiling, your really over sized.

The 8" pipe sizing is to move enough air for the furnace.
 
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Basement doesn't add much to heat loss/gain unless uninsulated, in which case you should insulate if you can.

Whether finished or not, it gets included in the load calculation.

The trunk size should be the least of your concerns right now. 1000 cfm in a small house blasting out a few 8" pipe registers will be very uncomfortable.

Normally for 1000 cfm (for 60k/2.5 ton) you would have 10-15 registers depending on size.

A 2.5 ton a/c won't run enough in a house of that size to properly dehumidify unless the house is a complete basket case in a brutal climate.

You need to start from scratch and do things right!
 

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Shouldn’t have to figure out your trunk size. That should be sized to the total air volume your equipment delivers. The equipment that’s sized to the manual J report.
Basements often only need a little bit of heat and dehumidification. It’s often more comfortable with basements on their own system, such as a minisplit heat pump.
 

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They can need some supplemental heat in the shoulder seasons (gas fireplace or if electric is cheap enough baseboards) when on the same system as upper floors, mini-split is overkill.
 

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I'm just trying to figure out my trunk size and when to reduce it based on the two drawings I posted with register locations.

Here's what your supply duct system might look like, treat this post as a visual aid only. Green runs denote basement To keep it simple use 4x10x6" round floor boots, they come in angle, straight or end (torpedo). Use 6x10x6" round angle ceiling boots for the basement. All runs 6" round, don't forget to install balancing dampers (especially the bathroom). You can use 14x8 duct if you wind up downsizing your equipment, 16x8 if you keep what you have. You don't have to reduce the duct anywhere won't make much of a difference for this small a space. Err on the side of too much return air. Loads calcs are your friend, the guys here asking for them are on your side.

 

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All runs 6" round, don't forget to install balancing dampers (especially the bathroom).
Or design it properly and use the right size pipe and register boot for each space of the house based on the heat loss/gain calculation, cfm and required friction rate.

Bathroom may only need 4" and 3x10 register.

Balancing dampers are only for minor adjustments, if a damper needs to be closed 80 to 90% the system was done wrong.
 

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I agree with you, however (and this doesn't make it right) this is a DIY project, the owner has the equipment and the motivation to do the work. Load calcs are not infallible, if you size each run to the gnats ass there is a lot less wiggle room and let's face it, that's probably not a great idea if he is using the equipment he has. Granted a 4" run in the bathroom would probably be adequate, as for the rest of the house, 6" runs with dampers is more than most houses get today. He's not building a shrine to the HVAC gods, he just wants the best he can get with what he has. 6" round runs w/dampers will give him that without dicking around with different size runs, boots and registers. I've seen too many screwed up heating systems based on someones Manual J inputs and oversized equipment. I think overall the OP should allow some leeway (damper adjustment) in his system. What I have suggested will work for the equipment he has. If he wants to downsize the equipment based on Manual J calculations, great, he can then size all runs per your suggestion.
 

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The majority of the cost is in the ductwork and labour (everyone's time is worth something), not the equipment.

If the equipment is incorrect for the house, it should be returned if at all possible - if not, resold and the correct stuff ordered.

It's best to do things right or the house will be uncomfortable and drafty. If it's over the op's head, time to call a pro in.

Putting 6" pipes everywhere is a complete cop out. There's no need to do that.

There will always be tolerances, it's not an exact science, but come on, 6" flows almost twice as much air as a 5".

Also, for cooling, it's bad to use 4x10 on everything especially with floor vents. If room only needs 40 cfm, the velocity will be very poor and the supply air will stay near the floor instead of mixing properly.

----------------------------

Wise to check local permit requirements before doing the job, a load calc, inspections, etc may be mandatory for a new installation.
 

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What I have suggested will work for the equipment he has. If he wants to downsize the equipment based on Manual J calculations, great, he can then size all runs per your suggestion.

Sounds like we are arguing, but the tone should be taken as normal conversation. My 6" runs everywhere comment is only based on his existing equipment (not all jobs in general or Manual J), other than the bathroom all the runs would provide an approx. base 90 cfm (cooling) that can be balanced up/down a bit with dampers. What does the owner want to do? Until we know that, it's all conjecture.
 
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