Smaller Height Ducts?

[DKAudio]

My house was built in 1954 and has pretty large ducts. They measure about 8 1/8" X 10 1/8". They are pretty obtrusive especially since the ceiling height in the basement is only about 7' to begin with. I just got a Bowflex and it is just barely passing the joists, however, because of the ducts working out is very difficult and there is not enough height to fold up the Bowflex when not in use. That is pretty important since people walk in this area and the Bowflex is large. The 2 ducts in question are a supply and return. The supply has 4 outputs and the return has 3 inputs. Is there anything I can do to get more height? Maybe installing a low profile type if they exist? My furnace is forced air by RUUD from around 1990, it says high effeciency.

Another question is would notching one of the bottom of my joists a 1/4" compromise any integrity? The joists measure 7" X 1.5" and every other one is doubled up. The reason I want to notch the bottom is so I can slide the Bowflex over a little. It is so tall (or my basement ceiling is so short) that the bolt for the upper bar sticks above the joists a little bit. I had to position the unit such that every bolt is in between a joist.

[HayZee518]

your ducts are for the most part 8x10. make up some trunk lines that are 4x20 you'll still have the same flow through them 8x10=80, 4x20=80 make up a trunk reducer 8x10 to 4x20

[DKAudio]

My question is more on installation since I have never done ductwork other than registers before. How do you go about making your own trunk lines? How are the outputs to the rooms or inputs from the returns cut and connected into the trunk? How are two trunk lengths connected together? My ducts now appear to be crimped, how do I go about removing those? Etc...

I found this website that sells oval pipe but is that for trunk or only branch? I don't understand their dimensioning either so I don't know the sq"...
Browsing OVAL

Any tips/links for installation would be awesome, thanks.

[DKAudio]

I am having trouble finding any tutorials or installation procedures online. I went to some big box stores and even online HVAC companies and do not see anything even close to 20X4. The closest was 14X3.5. My returns use sheet metal stapled to the joists which connect to the return trunk.

[HayZee518]

you'll need to find someone with a press brake to make duct work with a pittsburgh lock. my diagram shows how to make a straight trunk line 36 X 4 X 20. I'll draw up another to make a reducer for 8x10 to 4x20.