My old 19th c. house has woefully undersized ducts for the second floor, and no return. Consequentially, central AC was just never possible. You could freeze the ground floor but it never would reach upstairs. And in the winter, although heat rises, the upstairs was always a few degrees colder than downstairs. We would close off one room upstairs to maximise the heat.
Aeroseal has completely changed the situation. They spent most of the day working on my system, which it turned out was not only antiquated but insanely leaky -- probably one of the worst duct systems they'd seen. They had to make multiple repairs, in addition to the sealing, which required them to cut the drywall in the basement. One of the ducts was completely disconnected within the wall, and they reconnected it.
When they finished the difference in air circulation upstairs was obvious. They did a before/after analysis, which measured how much better the system was now working -- but really just holding a hand by the upstairs vent proved that it was working really well.
In fact, I recently got in touch with them because I needed to rebalance the heat so that LESS goes upstairs. It was about three degrees warmer upstairs! They'd advised me not to tinker with the set up without consulting, and Victor was very helpful in explaining which vents to open, which to close, to balance the heat better.
Before they did my ducts, any kind of central AC was a non-starter. Now I can get a central heat pump/cooling unit and feel confident that it will be able to actually cool my upstairs. I really didn't think this would ever be possible, so I'm very happy.
The crew was very professional and did a good job cleaning up. Highly recommend!