I would guess that modifying the duct work in a 1920s masonry construction home will be prohibitively destructive and involving removing or damaging irreplaceable trim that contributes to the building's character.
Mini split heat pumps:
An alternative, which will not be inexpensive but may be significantly less destructive, would be investigating installing a multi head mini split ac unit with one head in each bedroom. Such a unit costs perhaps $2500 to $5000 for the unit, installation extra and depends on the difficulty of install. If you live in a heating climate, especially one with cheap electricity in the winter or might install solar and have sunny winters, you should likely invest in the heat pump versions. Models are available that operate down to -13 F now (Mitsubishi M-Series) in single head. Supposedly multi-heads that operate that low are coming out this spring.
Advs:
Individual temperature control for each bedroom
- Really high efficiency available for both heating and cooling
- Avoid destruction of interior trim
Disads:
- Potentially a lot of $$$
- Still a newer technology in the US (used all over Europe and Japan). If you plan to sell soon, recovering your investment at resale could be a challenge
- Some people hate the look.
Other than minisplits:
Air leakage:
I'd suggest you have a blower door test performed. All the insulation in the world doesn't matter if you are changing over the air in the house too quickly. If you didn't air seal the attic before filling it with cellulose, you could have this problem. An infrared gun on a cold night can help show these leaks too. Insulation installers will say things like seals in air but cellulose, fiberglass insulation, and mineral wool insulation are actually very air permeable. If you find that is the problem, you could move insulation out of the way in the attic to use canned foam (get a commercial gun, much nicer job to do than with home depot cans). If you can't move the insulation, you could install a vapor permeable sheet membrane like Tyvek or a smart vapor retarder like IntelloPlus on top of the insulation to block air movement. Do not use a vapor impermeable barrier like black plastic or visqueen (clear plastic); you could trap moisture and cause a mold problem. Attic access hatches are also causes of large volume air leaks. There are special hatch covers you can buy or build to address this problem.
Duct work in unconditioned space:
Do you have any ducts in unconditioned (uninsulated) space like an open crawl space or an unconditioned attic? If so, you could be losing huge amounts of heat to the crawl space or attic. If your ducts are older, they likely aren't air tight either. You could use specialized foil tape or duct mastic to seal the joints and you could insulate this duct work, though it won't be fun work. If your ducts are really leaky, you could get big gains in system performance from this work.
Humidity control:
Have you measured your humidity when your system is running? If your humidity is high in the winter or summer, you will feel colder or hotter. Your humidity could be high in the house for many reasons (improper shower venting) but dirt crawl spaces are a big source in the summer (not in the winter so much). Seal off the dirt crawl space to eliminate humidity, mold spores, radon, etc. from getting in the house. A radon membrane company, in areas with radon, might be a cost effective contractor to use.
Variation of the temperature from room to room that is different from the thermostat is a basic fact of operational behavior. you will have to get used to some variation!!
It is possible to even out the differences by changing how much of the heated air gets to each room. This is usually done by using vent controls on each room to permit more or less air into each space. Sometimes there are also vent pipe dampers or diverters that can be adjusted as well. A balancing exercise is usually an iterative process that takes time and patience.
A well balanced system will only stay that way if the same conditions apply as when the balancing was done. Some of these conditions would include outside air temperature, state of window coverings, whether doors are open, closed or partly ajar and if windows remain in the same state (closed or open).
Note that a system balanced for heating can take an entirely different set of adjustments to optimize for cooling during warmer parts of the year.
So as you can see you could try making small gradual changes in your setup. Try closing off more of the the vents that feed into the area where the thermostat is located and open those in the colder room a bit if not already fully open. Remember - iterative process.
Best Answer
No, not really. The work you're describing doesn't sound remotely cost-effective, or effective at all for that matter. Running a huge duct from the basement to the attic will choke off the airflow. Not to mention that if you're going to to that, you're already doing half the work necessary to redo the ductwork properly to connect the two zones.
It sounds like the real solution would be to abandon the attic unit (possibly selling it for some quick cash if it still works), and then integrate the second floor ductwork with the first floor ductwork such that the basement-mounted unit can service the whole house adequately.
If your second floor gets really really hot, the first and probably most consequential step is to air-seal and super-insulate the attic floor, and possibly adding a radiant barrier under the roof rafters if you can perform the work yourself (don't hire that part out; the payback time will be on the order of a hundred years). Low-E storm windows can help too if your house's windows are old and crappy.