It's about the room over the garage that just won't co-operate. You rechecked the heat loss, gave it a little extra, took extra care insulating the trunk line, but the room is still cold. The cooling worked okay last summer, but it just won't heat right. How come? Location, location, location!
It isn't a situation of heat loss, but of heat gain. The other second floor rooms are gaining heat from the room below. The air temperature of the first floor ceilings can easily reach into the eighties, and the air going up the stairways can also be this warm. Both sources of heat reduce the demand of the second floor. Except of course that room over the garage. The room over the garage is losing heat from below instead of gaining. It is an island in the cold with one small bridge, the doorway, connecting it to the main house. Extra outlets won't help. They only function when the system is running, when the thermostat calls.
That playroom in the basement has the opposite problem. It heats fine, but when the cooling comes on, It's freezing down there. Sometimes, it even needs heat when the upstairs rooms are calling for cooling. How come? Again, location. In this case, it's a situation of heat loss all year long. Even in the summer, especially in spring, this room has a heat loss that is usually ignored in calculations.
Both rooms, the room over the garage and the room in the basement, are victims of reverse heat loading. Reverse heat loads are gains or loads that were assigned heat transfer values of zero during calculations instead of being assigned a negative factor.
When the heat gain of the basement was calculated, the floor was given a heat gain value of nothing, because it does not gain heat. That is correct, but inaccurate. The floor in fact loses heat, even during the cooling season.
The best solution in both cases is to isolate these areas by zoning. A special control panel can alternately supply heat to the basement from the same system that provides cooling to the floor above it.