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Old 06-17-2010, 01:03 AM   #1
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extending HVAC ductwork. Hard to DIY?


Hi. New to this forum here. I'm a big fan of DIYing. I don't know everything, but like to learn stuff and do it on my own. Previously, I have replaced windows, renovated my kitchen, put in new recessed lights, done tile work, etc, all by researching beforehand as well as learning as I went along. But I've never done any HVAC work before. So here goes

My main furnace/blower is up in the attic and has a hard time warming up the 1st floor adequately during the winters. Well my basement has a separate furnace/AC for the basement alone. I recently replaced this furnace with a new high efficiency one since the old one was dying. I'd like to cut out some 4"x10" floor registers on the 1st floor and extend the basement ductwork to share some of the heating between basement and 1st floor. That way, 1st floor will be heated mainly by main furnace up in attic, but also by basement furnace when it kicks on.

First of all, this is the layout of my basement. As you can see, current duct runs along right wall, almost the length of the basement



I'd like to take part of the duct down, and cut two holes on top to create feeds for the new 1st floor registers.

(1) register will sit right on top of where duct lies. So that new duct will just go straight up between the ceiling joists and thru the ceiling/floor.

(2) register will sit on the other side. So for this one, I'd like to up from the current duct into the ceiling cavity, make a 90 turn and go across between the ceiling joists so that I can have a flat ceiling when I finish off the basement.





This is my plan anyways. But since I've never done this before, I'd like to know if there's something I'm not thinking of, or not aware of. How big of a duct will I need for the cross run? Will a 7" round suffice? The cross run will be about 13-14'. What other materials will I need? What should I know when performing this. Any info helps. Thanks


Last edited by aharami; 06-17-2010 at 01:06 AM.
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Old 06-17-2010, 06:37 AM   #2
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extending HVAC ductwork. Hard to DIY?


What about return air?

If you run supplies from that furnace to your first floor. You need to also run a return. Or else you won't get much air to the first floor. And you will put your basement into a negative pressure. And could draw CO from your water heater and furnace. And harm you and your family.

Keep in mind. That the cold air from the first floor will fall through the new duct work and cool your basement off quicker after you do this.

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Old 06-17-2010, 09:50 AM   #3
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extending HVAC ductwork. Hard to DIY?


will adding two more openings really over work it though? It's a 45k BHUh Ruud 2 stage furnace that has 890 CFM at low setting, and 1140 CFM at high. Right now the basement duct has 3 registers, and the basement is 500 sq ft. after finished off, the heated area will be ~400 sq ft.



The return air will remain where it is. I will create a partition wall where the return box begins with a big opening for the return register, and the furnace will be getting it's return air from the finished part of the basement.

thermostat and return for basement furnace will remain in basement. thermostat for main furnace is on 1st floor (I wont be putting the new registers anywhere near the main thermostat). The basement furnace will kick on when it wants to, and throw some added air into the first floor as a bonus
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Old 06-17-2010, 11:35 AM   #4
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extending HVAC ductwork. Hard to DIY?


Since its hard to get a furnace any smaller then 40,000 BTUs. I understand why you have such an oversized furnace for your basement.

If you don't have a large undercut on your basement door. You'll put the basement into a negative pressure. And it will draw air from everywhere it can. Including from the water heater's chimney if you have a gas fired water heater(can't tell from the pic).

Cold air will fall from the first floor through the supplies, and cause the furnace to cycle more often. And may cause some over heating problems in the areas that you install the supplies in when it does run.
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Old 06-17-2010, 03:34 PM   #5
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extending HVAC ductwork. Hard to DIY?


Someone please correct me if I am wrong, but that looks to me like a horizontal evaporator coil laying on it's side.
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Old 06-17-2010, 06:27 PM   #6
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extending HVAC ductwork. Hard to DIY?


Might be a half case coil.

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