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?
Best Answer
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.