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QUESTION:The supply trunk in my house is 20 x 6 and the cold return is 20 x 6 the
drop to the furnace is 26 x 8. Are These odd sizes?
Why would the drop to the furnace be bigger than the cold air trunk?
ANSWER: Let's take Discharge air from furnace :
The discharge air duct coming off furnace will be the size of the
furnace opening that air comes out of no matter what the story maybe.
Then it can get bigger or smaller to what the sheet metal size would be
to carry the right amount of air to each of the rooms. The Furnace size
in btu's and if it has air conditioning or not and this calculated to
get mim. size it can be. if it is this Mim. size or bigger all is fine
if it is this size or bigger.
Let's take the return Air duct :
Now the return can be the same size of the discharge if the return size
fits the mim. return duct size. If the return duct size is larger it
makes it better but just along as the min. size is obtained or larger.
now a lot of sheet metal men will pick a size that is larger than the
min. of the discharge air and the return airduct and make all the duct
work the same size for he can make the duct work faster if all the
return and discharge size of the ducts are the same size. You will see
this in a bunch of applications.
Now to the Question should the discharge duct be larger than the return
duct ? Well , Generally speaking the discharge can be smaller than the
return air duct size for it is easier to push air than it is to pull it
back by the suction of the blower fan. In your case the sheetmetal man
picked a larger size of the discharge size and the return air size and
just used that size to do the whole job.
If you want the detail run down on figuring out the return and
discharge sizes for your furnace and/or air conditioning. I will have
to give you my sheetmetal man's phone number and pick his brain. Now
don't you mess his head up for he is about 71 years old and has had 2
heart attacks already since he has been working for me. I ask him when
was he going to retire ? He said When I Die. I like that that kind of
thinking.
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