How to determine return air duct size for a 50,000 BTU Gas Furnace

Hi, Folks, I am trying to install the 50000 BTU Weather King gas furnace on my rear house. Before, there were only space heaters in that house, so I have to "design" the ductworks and install the whole system. I can never image that it is really complicated for me to finish this project. I have a few questions hope someone can help me.
How can I determine the size of the return air duct? Where should I put the opening of the return air duct, do I have to connect the ductwork to the furnace itself? Or can I just let the return air duct connect to the furnace room (4'x4')?
Thanks
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I doubt it but here we go

Put up 4 fingers, step back, click your heels and repeat: "Theres no place like home." It should come to you by fairy messenger.

flue opening of your water heater.

You bet. Hope this helped. Bubba "Bubba Certified"
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kck3888 wrote:

duct, you have to connect it up to the furnace.

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kck3888 wrote:

experienced sheet metal duct system installer, or you will have a mess!
Here is a rough outline of what you are faced with! http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
- udarrell
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It's not Rocket Science. Any DYI can do it.
There is some guy who works at Sears and helps his boss around here. He might do it for some new shocks for the Pinto.
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<sniff,,,,,,,sniff,sniff,,,,sniff> hmmm, smells a little trollish.

Oh OK, the whole btu vs. filter size thing.......

-zero
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I have more questions than you that have to be answered before you can go much further.

First you have to determine what the requirements are for each room

What does your local building/mechanical inspector say??

You did pull a permit and are going to get the job inspected....right?? You did do a proper heat load and loss calculation.... right?? You did do the calculations to insure correct duct sizing for proper air flow......right?? You did refer to the GAMA venting tables before running the vent pipe......right??? Your fire insurance is paid up.....right??
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"Noon-Air" wrote:

oh c'mon Nooner......when the pioneers loaded up ther covered wagons and headed west, they were blazing new trails. oh sure alot died along the way, but they kept plugging along, right or wrong.
so along comes another moron, errr pioneer who wants to blaze his way into the history books, taking his family and every living critter in his house along with him. more power to him. If he lives, maybe next month he'll attempt brain surgery on himself. we should be so lucky.
the sad moron is so worried about his return air duct yet remains clueless on the combustion air requirements for his 16 sq ft furnace room......
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Yeah, your right..... Darwin will be proud :-)
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Thanks for your replies, even though most of them are discouraging.
Based on the HVAC-Calc PC application, the heat loss for that house is 38,000 BTU. The furnace is at 80% efficient rate, so it requires minum 47,500 BTU furnace. So I picked up the 50,000 BTU furnace.
For the 4 rooms that require less than 60 CFM, I ran 4" flexible ducts to each of the rooms. For the 2 bigger rooms that require around 133 CFM, I ran 6" flexible ducts to them. These are based on the HVAC-Calc calculations. All the ducts are underneath the crawl space will connect directly to the sheet metal transitional box on the discharge end of the furnance.
The vent pipe is 3", which is indicated on the furnace manuals. I will run the vent pipe straight through the ceiling of the furnace room and stays up on the roof for more than 1 foot to create the draft.
My dad, a licensed electrician will run the electrical cables to the furnace for me.
I am now stuck at the return air duct, that some of you did give me some ideas. My new question is do I need a return air duct for each of the living rooms or one big one should be OK?
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"kck3888" wrote:

far fucking out. You've got ducting for approx 500 cfm by your numbers. Whatcha gonna do with the other 700 cfm the blower is cranking out? You do realize insufficient airflow thru the hxl melt your furnace dont you?
4" flex, wot a farking riot!

post the model & serial of the furnace for an answer to your question.
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Oh, this sounds like quite a work of art. Please post some pics of this spectacle! RED LIGHT HERE!

Is that an honorable discharge? ANOTHER RED LIGHT.

Which one? Both?
Are you sure you followed the GAMA code?
ANOTHER HUGE RED LIGHT HERE!

Wrong. Distance it "stays up on the roof" has nothing to do with creating the draft. ANOTHER BIG RED LIGHT HERE.

Make him take his time and walk the cables for you. He doesn't need to run.

Here's another: GO FUCK YOURSELF.

YES.
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You need to size your system to account for more like 750cfm, not 500. The sizing has to be based on the output of the furnace, desired temp rise and pressure drop across the system, not on the amount of air each room needs.

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Thanks for your replies, even though most of them are discouraging.
Based on the HVAC-Calc PC application, the heat loss for that house is 38,000 BTU. The furnace is at 80% efficient rate, so it requires minum 47,500 BTU furnace. So I picked up the 50,000 BTU furnace.
For the 4 rooms that require less than 60 CFM, I ran 4" flexible ducts to each of the rooms. For the 2 bigger rooms that require around 133 CFM, I ran 6" flexible ducts to them. These are based on the HVAC-Calc calculations. All the ducts are underneath the crawl space will connect directly to the sheet metal transitional box on the discharge end of the furnance.
The vent pipe is 3", which is indicated on the furnace manuals. I will run the vent pipe straight through the ceiling of the furnace room and stays up on the roof for more than 1 foot to create the draft.
My dad, a licensed electrician will run the electrical cables to the furnace for me.
I am now stuck at the return air duct, that some of you did give me some ideas. My new question is do I need a return air duct for each of the living rooms or one big one should be OK?
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Then your dad is a bigger idiot than you are.

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