Is there a way to increase airflow to the 2nd story air vents? I have tried closing the vents on the 1st floor but doesn't help. It just stays too hot upstairs and hardly any air comes out of the bedroom registers. The upstairs bathroom has good airflow out of that vent. So I don't think it is the blower.
In some cases it’s the rooms upstairs are sealed up and with the bedroom door closed there is very little airflow into the room because the door acts like a damper.
I find especially with carpet a closed door will all but stop the flow into the room because it pressurizes and stops the air.
Trimming the door is an easy fix for many homes,
when I put a system in I put dampers in the main trunk lines for each floor so a simple change can be made for summer and winter, summer I open the upstairs trunk and throttle downstairs then reverse for winter, make sure you don’t have duct dampers because if you do it can be impossible to get the flows right without adjusting them.
Also make sure if your returns have clean filters. I put filters at the intakes to keep the ductwork clean, if the downstairs intake has a clean filter and the upstairs is dirty the draw will go downstairs and not pulling as much cool air up stairs.
How old is the ductwork? Maybe it's time to get it professionally cleaned https://youtu.be/e5XQrCLLlyc
How is your duct work laid out? It could have been designed poorly.
Have you opened all of the dampers to the bedrooms? Both inline and at the register.
Do your bedrooms have a proper return path? If the doors are closed and there is no return vent in the room then the air cannot circulate properly.
There are so many factors that can contribute to low air flow. You'll have to do some detective work to eliminate some of them and find out where problems still exist. For starters:
- open all interior doors so that air can circulate freely between rooms, hallways, etc
- confirm all the supply registers and return grilles everywhere in the house are fully open and not obstructed
- confirm the air filter(s) is/are in good condition, or even briefly remove for the purpose of testing
- look for any manual or power-operated dampers and verify that they're fully open
- turn on the blower
Compare the air flow at each supply and return. This can be hard to do subjectively with just a hand so try holding something light weight in the air stream. Facial or bathroom tissue works well for this.
If you find that the flow into the return(s) is good and at least some supply registers have good flow too, but others have a barely perceptible wafting of air coming out, then unfortunately poor ductwork is a likely cause. Air currents do not like to change directions, and when a change of direction is necessary, they're awfully particular about how it is done. Too many elbows or elbows with sharp corners in the throat (rather than mitered or curved/radiused throat) can impair air flow significantly.
Closing the register in a few places where there's excess air flow might help but probably only a little. If there are places where barely any air comes out now you'd need a whole lot more air pressure in the system to force air through those branches. Closing off others isn't going to get you there -- a re-vamping of the duct design and/or additional equipment may be the only way.