We have an older 2 level home where the lower level is a walkout basement. Stairs connect the two levels, each is about 900 sq/ft. We have a Carrier furnace plus a heat pump for HVAC. This system heats/cools our entire house... sort of. It blows “cold heat” in the winter, never quite warming if it’s actually cold outside, and coolish air in the summer, never quite cooling if it’s actually hot outside. Maybe this is normal but it seems ridiculous. That aside, we’ve got a single return on each floor, in the middle of the house. AND those returns are stacked... the main level return is in the floor directly on top of the lower level return in the ceiling. Whether this is good design or bad, it also has the strange effect of zero sound insulation in our home. There’s basically a hole going straight thru between floors. My questions is: how can the return issue be fixed/changed to eliminate the sound issue and improve hvac performance? Thank you!
Your heat pump is not operating efficiently - how old is the unit - has it always done this ...
What is sounds like is that your reversing valves might be defective or most likely the unit is low on Freon - have you had the unit serviced to make sure it is up to snuff? Also in winter the colder the temp outside the worse the heat pump will work. When temps are below 32 F - Aux Heat is best..
As for the hollow noise - the cheap route was done to build the house - you would need to do a dual duct system to get rid of that - perhaps you can split the duct into two halves and feed to the Air Handler ending up as one at the air handler .