Supply weak

There is a whole lot easier faster cheaper way. Look at what size the pipe is going to that room. Do and estimation of how many feet of pipe and elbows are hidden in the floor and walls. Do a friction loss and figure up how many equivalent feet of pipe you have on that run. There's your answer. An improperly sized run. C'mon, whad you really expect? Some giant size dust ball just happened to be clogging the run?........maybe a nerfball fell down the pipe or some dead gerbil? Bubba

Cheap installs lead to problems everytime...

Although Bubba and KJPro are right, there still remains the possibility of broken duct. Use your flashlight, look around at any exposed ductwork you can find. Replace the air filter [a clogged air filter will reduce total airflow]. If all else fails, it might be a poor design. Most designs fail in the return air sizing [too small.] If you are willing to pay for the time, a qualified HVAC contractor who's licensed in your area, and knowledgeable will find and solve your trouble, but not for free.

Once on a new build resi job a long time ago I watched a clean up guy do a bang up job sweeping the floors. Right into the supply openings in the floor.

It wouldn't take much to block a piece of oval duct shoved in the wall cavity feeding the upstairs.

Bowling ball storage?? ;-p "Duct busters" does scope out ducts though.

That's a reasonable hypothesis, but every other supply on that floor is pretty strong.

The pipe to the weak one is longer than the one to the bathroom, but not nearly enough to explain the difference.

Right. The guy who came out looked around and couldn't find anything; he spent a long time looking.

The house is really old (1948). I'm sure it's had a number of upgrades in that time. The guy who came out pointed out a number of stupid things. E.g. there are two small bedrooms on the second floor with two supplies eacha no return.

There's been lots of years for people to have done poor design and work.

Yeah, not expecting free. Quite happy to pay a reasonable fee, though of course I would give up if the $$ required got close to what it would take to just put in room-only heating and cooling.

That's why I'm wondering if any qualified contractors _do_ use scopes, because I can't see any other way to diagnose the problem. (One poster here already commented that one outfit does.)

The guy who checked out the ductwork for me says he's seen all sorts of crazy stuff. Like a plumber who stood on ductwork in a floor and crushed it, then covered the floor up. And a homeowner who had this expensive refinish job done on their basement, then discovered they forgot to look into putting in ductwork.

A friend of mine who use to do concrete work once told me that some guy he was working with took a dump on a freshly poured basement floor then floated the hunk of s*it into the fresh concrete. How nice..

Then you should stop taking dumps on freshly poured concrete floors, Mikey. Bubba