We moved into this long 2300 sq ft UBC modular home in 2005 ( gas forced air heat ) and ever since then, the bedroom at the other end of the house from the furnace has always been much colder in winter.
A HVAC guy told us that all the ducts in the house should be open wide all the time, because the system moves a lot of air, so it should all be open, but I put my hand over the vent in that bathroom and bedroom, while the heat is running and can hardly feel any warm air coming out. When the hall outside that room is 60, that room and bath can be 55 if the bedroom door is closed. ( the door has a wall grill above it to let the air flow through )
So I'm wondering if the vents need to be balanced, instead of all open, to force some of the air down to that end of the house?
Otherwise, it may be that the dishonest lying builder could have disconnected the duct down there. I don't know...