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Old 10-24-2008, 05:27 PM   #1
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I need help with duktwork


I don`t have any experience with heating, but I feel I can do it. Let start: I have small mountain cabin with very bad insulation and wood fireplace as heat source:1. Main room 520 sq ft with high ceiling=9200 cubic ft ; Bedroom 120 sq ft=960 cubic fc ; 3.Bathroom 60 sq ft= 480 cubic fc
I just purchase 50k BTU Goodman Electric Furnace and want to make ductwork. It will be in located basement.
Furnace is 17.5 x 21.0 and i want to ad plenum 24 inch high ( will it be ok ? ) then run to room # 1 2 pcs 8" round flexible vent ; to room#2 1 pc 7" vent and to bathroom 1 pc 6" . Then from room #1 returning 8" vent to furnace. Form all vents i want use 12"x4" registers
Did I calculate everythink correct? Plase advise, thank to all for great previous posts I learn from.
Andy

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Old 10-24-2008, 07:40 PM   #2
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I need help with duktwork


Add up all of your supply air ducts 6"=30sq/in, and 200cfm/1000fpm
7"=40sq/in, and 260cfm/1000fpm and 8"=50sq/in, and 350cfm/1000.
Return as much as you supply. Having more return is better than to little.
Make sure you use all the supply. You will be O.K.

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Old 10-25-2008, 11:51 AM   #3
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If you use those duct sizes to move those CFMs, you will have one very noisey system.

At 50,000 BTUs output.
You need to move rouhgly 900CFM for a temp rise of 51F.

If you want a quiet run. The return duct should be a 16" round for flex.
14" is ok, but will give you a little noise.
A 10 x 16 floor register, or equal sq in is what you should have for the return.
Or you'll be tripping the thermo limits, and burning up strip elements.

You may have selected a furnace a bit large for your place.
Can't tell from here.

But, if you want the heat to be some what even between the rooms, without seeing the layout.

I'd say your going to need to supply.
Room 1 with 780CFM, 3-14 X 4 8" Flex, Velocity 750FPM
By rights, this room should be 4-12 X 4 8" Flex, Velocity 570FPM

Room 2 with 80CFM, 1-12 X 12 6" Flex, Velocity 420FPM
Room 3 with 40CFM, 1-12 X 4 5" Flex, Velocity 300FPM

The above sizes are for noise considerations. And enough air flow to prevent limit trips.
But without any length considerations.
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Old 10-25-2008, 01:46 PM   #4
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Thank a lot, that infos clears my concerns.
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Old 10-25-2008, 05:50 PM   #5
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Quote:
Originally Posted by andy62 View Post
Thank a lot, that infos clears my concerns.
I thought it was a cabin and not the white house
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Old 10-25-2008, 11:24 PM   #6
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Old 10-26-2008, 06:49 AM   #7
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Quote:
Originally Posted by 1610 CUB View Post
I thought it was a cabin and not the white house
Those CFM's are based on the CFM required for a temp rise of 51F, at the amount of heat his furnace produces. To keep it from cycling on the thermo limits, and to prolong the life of the strip heaters.

He could lower his CFM to 675 total, and a temp rise of 68F.
But it would shorten the life of the heaters some.

Which would change the rooms to.


Room 1 with 584CFM, 3-14 X 4 8" Flex, Velocity 570FPM
Room 2 with 61CFM, 1-12 X 4 6" Flex, Velocity 310FPM
Room 3 with 31CFM, 1-12 X 4 5" Flex, Velocity 230FPM

His furnace may be twice the size his cabin needs, or more.

The CFM has to be based on what the furnace needs, in order for the furnace to be able to work.

If the furnace was sized properly, then the CFM would be based on what the rooms need, within the operational bounds of the furnace.
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Old 10-26-2008, 11:54 PM   #8
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I think your furnace will be close to what you need. When you say mountain cabin I assume cold weather but then again there are mountain cabins where it doesn't get very cold.

To save you some money on heating get a ceiling fan. Vaulted ceilings are expensive to heat especially since no one really lives up their.

I notice a 10 degree difference in my vaulted ceiling versus chest level in the same room. If I turn on the fan it drops the difference to 3 degrees. This is an average measured over several weeks.

Also if the duct work is not going to carry any air conditioning but the return up high if you can. This will pull down the heated air and the delta T of the air will be less compared to floor heat.

I do this in the winter for my home. It is set up to where the vents are used to vent air conditioning in the summer cause the upstairs gets pretty warm. In the winter I have the duct system set up to where I turn a couple of baffles on each run to the upstairs and it becomes a return. I did this two years ago and so far it has really made a huge difference in keeping my house even temperature wise.

I agree with Beenthere. Trying to push too much air through a pipe will cause noise. I prefer quiet vents. There is a balance between moving air too fast and moving it too slow.

For my installs I shoot for 300 cfm for 8 inch, 210 cfm for 7 inch and 120 cfm for 6 inch. This is straight pipe which has better laminar flow compared to flex.

You might consider 2 vents in the main room to get more even spread for the heat. Depending on the layout it could be cold on one side and warm on the other. Also if you have vents in the walls versus the floors you will get better coverage. In my opinon floor vents suck. They just heat furniture most of the time.

Also consider varmits. They love the warm air and insulation. Make sure the vents are will protected.
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Old 10-27-2008, 12:55 AM   #9
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I calculate my cabin for 30000BTU and 900 cfm , but i find old quote for propane furnace rated 54k BTU and 1200 cfm- so i trust proffesionals and change my order for furnace with bigger heat element.And in my case is very easy and not expensive ($70)to change heating element from 15 kW to 10 kW and i think is much easier go "down" then "up".
Thank again for helping me
Andy
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Old 10-27-2008, 06:11 AM   #10
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With 10KW of strip. If you use 630CFM total, you'll will have a temp rise of roughly 50F.
At 900CFM your rise would only be 35.

Few contractors will use the right size furnace.
Most tend to over size, using the excuse of quick recovery.

Last edited by beenthere; 10-27-2008 at 06:38 AM.
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Old 10-27-2008, 06:49 AM   #11
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Quote:
Originally Posted by Marvin Gardens View Post


For my installs I shoot for 300 cfm for 8 inch, 210 cfm for 7 inch and 120 cfm for 6 inch. This is straight pipe which has better laminar flow compared to flex.

In my opinon floor vents suck. They just heat furniture most of the time.

.
I'm the opposite.
Side wall vents, cause some people to complain of cool drafts across their feet or legs when the blower is near the end of its run time, and the temp rise is low.

Also shoot for lower static pressue which requires a lower CFM in the pipes sizes.
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Old 10-27-2008, 08:41 AM   #12
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I'm the opposite.
Side wall vents, cause some people to complain of cool drafts across their feet or legs when the blower is near the end of its run time, and the temp rise is low.
I'm talking high on the wall or on the ceiling.

Floor vents don't heat anything except the furniture sitting on top of them. Plus you don't get any air flow.

Most of the time the furnace has to stay on longer to heat up the house. Then there are parts of the room that are hot and parts that are cold.

I did a year long test in my house that had floor vents. In just one room (120 sq ft) there was a variation of 6 degrees from the outside wall to the wall near the vent. There was a desk in the way of the vent. The key was to move the desk of course but I wanted the desk where it was. I shouldn't have to arrange furniture to fit the vents.

In the summer it was even worse. That room would just cook except where the vent was and that was really cold. So cold in fact that it was miserable to sit there for very long.

The vents were removed in the remodel and put high on the wall. Now there isn't one degree difference in the room even without the fan on. And the desk is in the same place.

Floor vents are nice if you want to heat furniture.
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Old 10-27-2008, 08:58 AM   #13
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Generally.
If I have floor supplies, I have returns in every room(except kitchen and bathrooms of course).
Don't have the problem of stratifing the room from one end to another then.



Floor supplies aren't for every application.

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