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Old ductwork... new furnace

We have recently purchased a 1920's bungalow style home, story and a half. The house was originally setup with a gravity furnace as evident by the large baseboard vents. I haven't lived in the house long enough to know if it stays warm in the winter but during the summer it seems like the a\c is on long periods of time throughout the day and doesn't really keep the house cool. We have had the refridgerant checked so I know its not that. Here are my questions concerning the system.

1. Our cold air return grate is only 12x12 for the whole house and is located in the living room. The plenum underneath is is quite a bit larger and goes directly down to the furnance. there is a small return vent on the side of the plenum in the basement also. Is the C.A.R. too small to get enough air flow?

2. Our upstair does not get very cool at all. There are three rooms and the only room that gets cool has a direct line from the furnace. The other two room's lines pass by the large old gravity base vents in the living room and dining room. The only way we get air upstairs is if we close both of these vents and even then we barely get any and of course our downstairs doesn't cool properly then. these two vents are supplied by 6 inch round duct but at the base of each vent is a large round plenum looking thing that is about 18 inches in diameter. Are these large plenum type things destroying the air flow? (photos attached)

FIrst is the base vent from old gravity system. You can see the damper inside. When the vent is open the upstair is closed and vice versa. When its in the middle there barely any air on either side. Mainly all on the first floor side.

Second is the plemum type thing. I'm afraid its cause too much air flow disturbance and decreasing the air output of my system.

If anyone has dealt with something like this in the past any comments would help. I'm contemplating removing these plenums and giving each room (both up and down) their own supply by Y -ing off the supply that currently goes to it but am afraid my furnace doesn't have enough power to push air to all these rooms (8 all togther) especially with the C.A.R. the way that it is.


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