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My house has two HVAC systems. One is in the attic and has supply and returns in the ceiling of the second floor. The other is in the basement with supply and returns at the floor of the first floor. Each has a thermostat on the respective floor.

This makes one terribly poor at heating and one poor at air conditioning and overall difficulty to keep the house comfortable. The staircase is open so I can't isolate the floors.

I'm thinking of connecting ductwork between the two so each will circulate some air to and from both floors. Probably 6 or 8 inch duct between them. Will this cause any issues?

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    Hello, and welcome to Home Improvement. First, it looks like you've created two accounts; you should use the contact form and ask for your accounts to be merged. Second, do you know why one system is poor at heating and the other poor at cooling? Until you do, any solution we offer will be speculative. – Daniel Griscom Dec 7 '18 at 12:34
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Connecting the 2 may make things worse. Trying to control 2 different systems I have done but interconnecting the ducts would need to be done on the supply and return with dampers to be able to adjust them. An 8" duct is not going to provide a lot of change over that distance. I use 6-8" ducts for single bedrooms with just a short feed from a trunk line. My concern would be the system efficiency could be reduced for example sending cool air upstairs to the 2nd unit could cause that evaporator to freeze up. But if dampers in the duct were added you could possibly improve the mix I would just be concerned about freeze ups.

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Connecting the two systems outright probably doesn't make sense. It seems to me like it would make controlling them difficult, you'd have return air problems unless (and maybe even if) you also connected the return supplies, you'd need relatively big vents to make a difference, and because the systems are sized to handle half the house you're potentially overworking them by having them each now handle the entire house.

One big system

Replacing both with one larger system may make more sense -- especially when eventually you have to replace one or both anyway.

Independent recirculating vent

The intermediate alternative that comes to mind for me is to build something to recirculate air, like a single large vent that connects the two floors (but is not connected to other HVAC vents) with a fan (though it's not immediately obvious to me if the fan should blow up, down or be reversible based on cooling season). The open space from your stairs would essentially be the 'return' for this system, as the goal is to simply try to circulate air between the two floors to more evenly balance the temperature.

At increasing levels of complexity, this could be controlled manually by a simple switch, on a timer, or based on temperature differential (probe upstairs and down, and fan runs if difference passes some threshold). You'd probably have to custom build the last one.

Whether this makes enough difference to justify the cost (both install and ongoing cost to run the fan) I could not say.

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