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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 Noon-Air on September 19, 2006, 11:57 pm

> 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.

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

> How can I determine the size of the return air duct?

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

> Where should I put the opening of the return air duct, do I have to
> connect the ductwork to the furnace itself?

What does your local building/mechanical inspector say??

>Or can I just let the return air duct connect to the furnace room (4'x4')?
>
> Thanks

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??




Posted by on September 20, 2006, 12:47 am
"Noon-Air" wrote:

>
>> 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.
>
>I have more questions than you that have to be answered before you can go
>much further.
>
>> How can I determine the size of the return air duct?
>
>First you have to determine what the requirements are for each room
>
>> Where should I put the opening of the return air duct, do I have to
>> connect the ductwork to the furnace itself?
>
>What does your local building/mechanical inspector say??
>
>>Or can I just let the return air duct connect to the furnace room (4'x4')?
>>
>> Thanks
>
>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??
>
>


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......

Posted by Noon-Air on September 20, 2006, 8:31 am

> "Noon-Air" wrote:
>
>>
>>> 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.
>>
>>I have more questions than you that have to be answered before you can go
>>much further.
>>
>>> How can I determine the size of the return air duct?
>>
>>First you have to determine what the requirements are for each room
>>
>>> Where should I put the opening of the return air duct, do I have to
>>> connect the ductwork to the furnace itself?
>>
>>What does your local building/mechanical inspector say??
>>
>>>Or can I just let the return air duct connect to the furnace room
>>>(4'x4')?
>>>
>>> Thanks
>>
>>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??
>>
>>
>
>
> 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......

Yeah, your right..... Darwin will be proud :-)



Posted by kck3888 on September 21, 2006, 4:45 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 on September 21, 2006, 8:57 pm
"kck3888" wrote:

>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.


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!


>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?

post the model & serial of the furnace for an answer to your question.

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