A zoned, 2-stage, Trane XV95 was installed recently in my house. One zone is for the majority of the house, the other is for a much smaller in-law suite. I have some general questions. I'm an engineer and understand enough basic HVAC principles to wonder about some of the things I see.
Do return ducts normally have dampers? Mine do not. I ask because when the house is heating and the suite is not there is considerable "suction" on the suite side. The door to the suite is pulled open if not latched.
The furnace supply and return plenum have a 9" round duct directly connecting the two with a damper inside that is attached to a weighted arm on the outside. The damper opens more when the furnace goes to high heat. What is the damper's purpose?
Is there a limit to the number of dampers that can be wired to a zone? I ask this because two 6 inch supplies are on the wrong trunks. Example: A basement supply heats when the suite does. It should be on the house zone instead. It would be simpler to install a zone damper in the 6" duct than re-route it due to space constraints.
What causes the furnace to switch from low to high heat? Is it a timer on the furnace, or is the rate of rise at the thermostat factored in?
I have attched two cartoons/schematics. One shows the addition heating and the rest of the house not. The other shows the opposite.
Thank you in advance your help.