Installing a new furnace, main duct size?

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Old 04-08-19, 06:31 AM
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Installing a new furnace, main duct size?

Putting in a new downflow furnace on the main floor of my 1000 sq ft addition. Running my main duct down through the floor and across a 30 ft span over a 4 ft concrete crawspace, I'll then connect my heat runs to each room. Where the furnace has to go there is a joist running very close to where I'll have to cut my floor, joists are 16" apart. I need to know what size main duct is usually used coming off the furnace and down through the floor so I'll know if I will have room for it, thanks..
 
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Old 04-08-19, 07:15 AM
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The duct is sized to the airflow needs of the furnace, and how many BTU it is.
There is no common size.
Let’s start with the basics. How did you size the furnace for the space?
 
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Old 04-08-19, 08:27 AM
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You need a room by room load calculation to see how much heat each room needs.

The ducts are sized based on the load calculation and the airflow needs of the furnace.

If you're doing a decent job with insulation and won't have ducts in an unconditioned space or slab, doubt you'll need more than 40 000 btu at 90%+.
 
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Old 04-08-19, 01:55 PM
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I was looking into a 40,000 or 45,000 at 80%+ and 90%+. The duct size I was looking for was the square or rectangle one that would come off the back or side of the furnace and down through my floor. That duct will run across the middle of my floor in the crawspace which I'll connect my round heat ducts to going to each room. Never installed a downflow, not sure what kind of duct work connects to the back bottom of the furnace..
 
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Old 04-08-19, 03:01 PM
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Nothing gets connected to the back unless allowed by manufacturer.

Without calculations, you're guessing. There's a whole science to sizing and duct design.

HVAC Sizing Calculation Software The Heat Loss Calculator HVAC, HVAC/R, HVA
 
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Old 04-09-19, 01:53 PM
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I understand what you're saying, like I said forced air furnaces are all new to me, boilers are more up my alley. All I know is I'm going to have to cut a hole in the floor under the furnace to connect my main duct to, the furnace I'm looking at doesn't show any specs and I just needed to know what size hole, not sure if I'm going to miss that floor joist where the furnace is going to sit. I'll see if I can find more info online showing ductwork off the bottom of the furnace...
 
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Old 04-09-19, 02:00 PM
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What material do you plan on using for ductwork?
 
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Old 04-10-19, 06:31 AM
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Plan on using metal ductwork to run across my crawspace floor joist then wrap with insulation, there's a concrete floor in the crawspace, then connect my heat runs going to my registers with insulated piping? The furnace will sit in the main floor closet next to an outside wall, I believe I'll have to run a rectangle duct from under the furnace all the way to the other side of the house which is 30ft. I know a friend who does heating that is going to help me but he's on vacation right now. I just didn't know how they connect the duct going to the bottom of the furnace, thought I'd cut the hole in the floor and move any floor joist if I had to and have it ready. Found a couple of pics, I believe the hole and main duct should look something like this?
 
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Old 04-10-19, 07:29 AM
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The penetration for the main plenum passing through the floor will be at least the dimensions of the furnace.
You’ve posted a photo of a floor register cutout.
 
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Old 04-10-19, 09:53 AM
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The supply duct should be the same size as the opening on the AHU or two inches bigger if lined duct is being used. The return should be sized at .05 friction loss
 
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Old 04-10-19, 01:16 PM
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ok thanks, that's what I needed to know, I'll get the dimensions of the furnace. Now at least I can outline where the furnace is going to sit so I can see if it's going to miss my floor joist, thanks again..
 
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Old 04-12-19, 09:21 PM
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The ducts need to be sized for the cfm at required friction rate. (there are calculations based on total equivelent length to determine required friction rate)

Just using the outlet size can get you in trouble.

HVAC systems are engineered from the ground up, you don't just get a bunch of components and cobble them together.

As previously suggested, do a room by room load calculation since it's needed to size the furnace and ducts to each room.

You'll have to learn a few things to design your own system.

Tools:
Residential Air Duct Calculator - EfficientComfort.net
HVAC Sizing Calculation Software The Heat Loss Calculator HVAC, HVAC/R, HVA

Google "manual D"
 
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Old 04-22-19, 04:52 AM
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Never had a forced air furnace before, I installed a boiler in my main home years ago and loved it. I'm still up in the air on a furnace or hot water heat, I'd really rather have a boiler instead. I never did the sizing part of it only the install, if my addition calls for a 45,000 BTU furnace would I also need a boiler with a 45,000 BTU output?
 
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Old 04-22-19, 05:22 PM
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Yes - at least. Same goes for furnaces.

Hot water system also needs to be engineered - the rads have to be sized to deliver correct btus at the water temperature the system is designed around.

Pump sizing, pipe sizing need to be address.

putting in a heating system from scratch isn't diy friendly.
 
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Old 04-28-19, 05:07 AM
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Yes I understand about the pump and pipe sizing, I've installed a few boilers before. The guy I worked for is deceased now and he was the one who did all the heat loss calculations for the baseboard sizes, I just did the installs. Trying to find something online now to help me figure the heat loss of each room, going to run 3 zone loops. Seeing I'm leaning more on going to a hot water boiler rather than forced air, should I post any questions I may have in the boiler section?
 
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