New Ductwork For Beach House

HouseMedic

Hi,
I am a small contractor in Delaware and I am looking for some advice on ductwork for my personal beach house. It is a two part modular that sits on a block foundation with an unheated crawl space. I need to re-do all of the duct work. I just had the house moved to my lot and most of the duct work was taken out when the house movers messed things up. This was a project house to begin with but now I have a little more to do. Anyway as you can see by the attached drawing the old ducts (light blue) were two 3 ¼” x 14” main ducts connected by a 10” crossover duct. (Not sure where crossover duct was connected) The original just used the floor joist as ducts to the registers. I would like to put in new main ducts with 6” pipe to each register. The original main ducts where the 3 ¼ x 14” and I was wondering if I should change them to maybe 8 x 10” and take off the 6” from that. Also where would be the best place to hook up the crossover duct? Total living area is about 900 sq. ft. and I have a down flow furnace with I think is 65000 BTU. If anyone could give me any info I would be greatly appreciative.
Thanks,
Ron

northarktech

ductwork should be resized to handle whatever cfms your unit produces if you keep 2 trunks then each trunk should be sized to handle half the total cfms. if you use 2 trunks then the main supply should be tied into the trunks in the middle of the total lenght hope i didnt confuse you.

snapperhead

the heater looks like it supplies most of the duct under the kitchen side of the house with the crossover sending it to the front of the house..were there dampers to get the air to the front but closing the back area duct work originally?suggestions from what see from the drawing is....don't use the back duct work as show off the heating air handler,(disconnect it from the heating unit)move it more towards the back of the house and closer to the registers.measure the opening off the bottom of the unit to take say a 24" deep plenum this will be your discharge duct that the cross-over will connect to foward and to the back..with a balancing damper in each just down from the new plenum box,with this the cross over is your supply duct to the back and the front.if not a box plenum a "pair of pants" coming of the bottom of the unit then to each section old cross over(the crossover becomes your front supply and rear supply to the registers directly from the unit fan)sizewise i would keep the reister duct work the same and let the unit opening dictate the connections to the new or exsisting supplies....what they call a "pair of pants" is square...say 24" x 24" to the unit then splits off on a 90 degree elbow 1/2 the original opening to feed 12" x 6"...8"...10" your needs (depending on the depth of the drop)both front and back if your familiar with this sorry...how the heating unit itself is it door louvred to allow return air from the spaces heated to cycle back and the stat location should be in the hall way return to the unit.to make it simple after my confusing redo taking the heating unit and place it in the middle of the crossover n the drawing so it becomes the supply to front and back

HouseMedic

Thanks guys, snapperhead Is this somewhat of what you mean? (see picture) I don't know if it is possible to move the front duct that close to the front because of plumbing. I will have to check it out. I am not down at the house now. If at all possible I would like to keep the main ducts up against the floor joist running across and the take offs up in between the joist. The cross over has to be below the joist. This would make it easier to insulate. I am not sure but it seems like the original just had a short connection down from the heater to the 3 1/2" x 14" duct. I guess this would be the plenum? Some others had told me to use flex duct to the registers but I don't like that stuff. If I did leave it the way it was originally, Do you think the cross over duct was right under the heater? This also has one of those self contained AC units that connect to the duct some place in the run.

If there is anyone out there that does duct design is there a way that I could get a calculation made up with a diagram. I could possibly pay some $ but like with all of us money is a little tight right now.

Thanks,

Ron

snapperhead

the only reason for the rear duct moved back towards the wall was to get it away from the heating unit so you could use the crossover as the main supply duct to the front and back.how about dropping the unit plenum down to the sand or concrete crawl space floor then go(duct run out) front to back and back up to make your connections to the duct that supplies the registers.thats what i meant exactly...the main supply duct should go to the duct work with the registers on it ,not have them on it(back duct work)as the 1st drawing was with the unit connected there...down discharge.go over to WWW.HVAC-TALK.COM and look at the "wall of pride" section for some install pix "unit to ducts"it depend on the opening from the unit into the crawl?