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

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How to determine return air duct size for a 50,000 BTU Gas Furnace kck3888 09-19-2006
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Posted by Steve Scott on September 22, 2006, 8:24 am
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.


>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?


--
A cat would be man's best friend, but
never stoops to it.





Posted by kck3888 on September 21, 2006, 4:50 pm
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?


Posted by aka-SBM on September 22, 2006, 12:39 am

> 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.

Then your dad is a bigger idiot than you are.


>
> 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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