Model numbers of all HVAC equipment proposed as well as detailed scope of work would be required to make an informed comment.

To be clear, you plan on adding ductwork. What are your intentions on the old radiator heat and boiler?

Post back.

IMO

Thanks for the response. Here are high level details, with model numbers once they respond 1) run ductwork from attic to each of the 4 2nd story bedrooms and the 2nd story bath 2) run ductwork down abandoned, 2nd chimney to 1st floor kitchen 3) run ductwork inside large closet to 2nd floor living room 4) master return in the 2nd story hallway. The home has hardwood floors with 3/4" gaps underneath the bedroom/bathroom doors.

I plan on keepin the radiators and boiler (for now). The installed system will supply heat to the attic in the winter & AC to the rest of the house in the summer. I plan on finishing the attic at some point. Tyring to upload the scope of work directly from the company.

Thanks,

Mike

So received model numbers today:
AUH1B080A942
4A7A3036A

Thanks,

Mike

Anyone?

I am not a fan of installing HVAC equipment in uninsulated attics. I consider it a last resort option.

Are you going to set a separate zone for the attic? You will need it in the winter if you are going to continue using the radiators. I also think you will need it in the summer. Even when you insulate the attic, the heat load in the summer will be higher than the first floor.

Did the contractor do a load calculation? I would be concerned there are only three 8 inch supplies providing air to the first floor.

I recommend getting a variable speed furnace. This will allow you to get a 15-16 SEER AC. If the furnace will eventually heat the whole house, then you should get a 95%+ AFUE furnace.

You need to decide whether you are going to keep the radiators. This will factor into the new HVAC design. There may be other options available. You could install a mini split in the attic after it is finished. This would give you the option of installing the air handler in the basement.

I have installed Unico high velocity systems in a number of houses (two I lived in and numerous others, including some antebellum and pre-revolutionary houses) and would recommended it.

It significantly reduces wall damage and preserves closet space (a common location for forced air duct work).

using duct-board in unconditioned spaces to make ducts has some significant advantages.
One is it s already insulated, but another is that the lower mass of the duct work reduces system losses to cooling the ducts.
More mass meas more heat holding ability.
Once the duct warms up, it takes more cooling energy to remove the heat from a heavier duct.

Flex duct has some of the same advantages, but a higher pressure loss when air is flowing.
It can be accounted for, but it meas more blower HP to move the air.

The two customers who used high velocity for heating quickly found the weakness.
Faster moving air and the stirring of the room air creates drafts.

Not very important in cooling application (remember standing front of a fan to cool off?) but a real drawback for heating.

If you put HVAC equipment in unconditioned space, one alternative is to turn it into 'semi conditioned' space by erecting a light weight insulated 'room' around the equipment.

Simple lightweight framing and thick foam insulation.