Open Air Return in a former chimney chase- Help

[jruf]

I am having a 3-ton geothermal HVAC system installed in an older 2-story home (1918) and the contractor is using an open air return in a former chimney chase due to space limitations. The entire HVAC ducting is part of the install. We formerly had baseboard hot water heat with an oil burner. The oil burner and chimney were removed as part of the construction. A supply duct was installed within the former chimney chase, but there was no room for a return duct. Is an open air return consistent with best practices and how will this affect air quality in the *

The contractor spray foamed the majority of the chase (Demilec HeatLok 200), but there is still dust in the return. In addition, there is a sheet metal box that is attached to the foundation wall at the unit which is exposed to the foundation. Shouldn't this be enclosed and not exposed to the *

Any help would be greatly appreciated.

Thanks
Jason

[Freon]

I am a little confused. Let me see if I get it... The supply duct (air into the rooms) is in the chimney. The open air return is at the furnace with no ducting feeding it return air. I am not sure about the metal box's function other than a form of plenum. If that's true there may be filters there and not in the usual furnace location. If this is correct, an open air return will work if there is enough clearance under every room's door that has conditioned air entering. Sometimes you may need to cut 1 to 2" off the doors to provide enough clearance above flooring and carpet for air to 'return' from each room to the furnace air intake. I assume there are no closed hallways stopping the air from returning from each room to the furnace.

Ideally you'd like returns in each conditioned room but older houses don't make that easy. So you do the best you can. I hope I understand your situation correctly.

[jruf]

Thank you for your response. I may not have explained it quite correct. We will have two central returns one in a hallway upstairs ducted to a central hub in the attic and a central return in the downstairs ducted to the basement hub/unit. The open air goes from the central hub located in the attic to the unit located in the basement. It is run where the chimney used to be. In the former chimney chase is a supply line (metal duct) and an open return mostly enclosed by spray foam. The metal box in the basement provides a seal from the open air return to the unit. Does this help explain the *