I've got a central heater in the basement that services the first floor, and one in a closet on the second floor that goes to the upstairs room. I want to put in AC. Do I connect it to just one system or do I need to get two units, one for each floor?

Our house has two floors with one central AC and the 2nd floor is always 5 degrees F hotter in Summer.

Our next door neighbor has a nearly identical house,
and when his single central AC died four years ago
he put in dual AC units serving a floor each and he tells me it works much better.

For us, however, we now hear two AC units starting and stopping independently, causing twice as often fan start noise.

Ugh, sorry - never thought about the noise. I've never lived anywhere that needed aircon (still don't, really) so I am not familiar with the sounds.

For better temperature control, 2 units, one for each floor, is best.

Noise wise, if you buy the cheap units, bottom of the line, like Goodman, they cane be more noisy. Units costing a little more will be much quieter. Look for a unit with a slower outdoor fan speed and an insulating wrapper around the compressor.
Ron

The new units with variable speed outdoor fans and compressors can be very quiet.

I have a Mitsubishi mini-split in my shop and it makes very little noise outside.

A few years ago I replaced my old HVAC system. the new one has a soft start on the compressor, the compressor fan and the blower inside. You barely hear it. The old one sounded like someone kicked the compressor housing every time it fired up. Not even sure they sell that style anymore, unless you are buying a super cheap one. The new one wasn't cheap but my electricity bill dropped by about 2/3's in the summer compared to having the old one.

I have thought about buying a 1/4 hp 230 volt soft starter for my outdoor unit’s condenser cooling fan.

I learned a few years ago that the 5 bladed Dodge pickup’s radiator fan’s uneven blade spacing was for noise reduction.
If my fuzzy memory is right that odd bladed fan is 28% quieter than the previous fan design.
My outdoor unit has a 4 bladed fan inside a shroud with just bent metal blades instead of “airplane wing” blades.
I wish I could buy a quieter fan design, that has the Amana hub design.
I am 99% sure a few dB quieter fan could be designed.

Your tax $ paid for this quiet AC fan design

https://stars.library.ucf.edu/cgi/v...r=1&article=1219&context=patents

Scroll down and look at Fig. 17 and Fig. 18 for “funky” fan blades and spacing.

I would like to try one of these 5 blade fans with a steeper pitch at a lower speed

https://www.ebay.com/itm/60-5611-01...6?hash=item25cae98218:g:YMoAAOSw8G1di8db

My 4 blade is 27 pitch
so the above 5 blade with 33 pitch
would probably overload the existing OEM motor

Sometimes depending on the house and the system getting it rebalanced will fix that upstairs heat problem. The hotter area gets more cooling and the temperatures end up matching better.

Our house is a big L shaped bungalow so our problem was because of exposure one side was always too hot or too cold. So each section ended up with it's own system.

Hay HVAC guys ... ever hear of WARRANTY like this ?.....

Cold air drops - hot air rises. You could store meat in our basement this time of year while the top floors are still warmer than we'd like. I'd install the a/c to the upstairs unit only, the cold air will find its way to the lower levels quickly on its own.

The thread is rather old but if anyone else is looking to install AC, you could connect it to just one system, but it's worth considering getting two units, one for each floor.

Another option would be one of the new systems that allow up to four separate thermostatically controlled air handlers to run off the same outside unit.

However you do it I don't think you would be happy with a single unit doing upstairs and downstairs.

Anyone I have known with that setup was unhappy with it.