I have a wall mounted mini split (Fujitsu 9,000 BTU) installed in my bedroom over my bed, intended to heat and cool my upstairs (there has no suitable location in the hallway). It is a standard sized bedroom, and the heat pump does an excellent job cooling and heating the room (I can get the room down to 60F on a hot day). However, here is the issue - the cooled or heated air will not leave the room. The doorway is about 10 feet away and you can feel it blowing if you stand in the threshold, but the airflow makes a dead stop there. I have tried pointing the air at different heights and even tried putting a fan in the door way to push it along, but nothing seems to work (heat seems to flow a bit better than the AC air). The installer has ensured me that the heat pump has plenty power for the job and it is an air pressure issue?? Any help would be greatly appreciated.

1 Answer 1


Consider the flow of air to be similar to the flow of water (because it is) enter image description here

You are trying to force air from one room into another through a large open space. The problem is, you're not actually creating any pressure differential. The air in your room is being moved around by the unit but not being pressurized. The unit in your room creates a low pressure point and a high pressure point and the air moves around the room stirring up the air and creating an equalization of temperatures within the room.

So the air in your room is blown towards the door at an extremely low pressure. The air in your hallway is stationary (for the most part) and at the same pressure as the air in your room. There's really no force behind the air in your room to dislodge the air in the hall, and the air in the hall also has nowhere to go.

Forced air systems work because they have both air outputs and air returns - the returns serve to carry air needing heating/cooling back but doing so also creates that critical pressure differential needed for air to move from room to room.

Otherwise it's like the picture above the air sits next to each other, mixing only very slowly.

  • Great answer! I really like the beach analogy. Is there any way to solve, or at least help, the pressure differential (without replacing the heating system)? Jun 29, 2017 at 18:34
  • You need to pull air from the other rooms into the room being conditioned, and that will allow air to move from the conditioned room to the non conditioned rooms. These units like you got are meant to deal with SINGLE ROOMS - your best solution is buy more units. Jun 30, 2017 at 0:30
  • Thanks for the reply. Excuse my ignorance of this question, but would a more powerful unit help? The only reason I ask is because I have a 15,000 BTU version on my main floor and it cools and heats the entire level of the house with no pressure issues. Also, would ceiling fans in the hallway and two bedrooms help at all? Thanks for all your help on this - I am learning a lot. Jun 30, 2017 at 12:00
  • Is your downstairs more open? Most are, which means that natural airflow will cause hot air to rise, cool air to sink, and the cooler, heavier air will seek it's own level around the floor - it's slow and gentle but it does happen. Along those same lines, the little cool air upstairs that does make it into the hallway (some will, of course) is likely flowing down the steps and helping to cool the downstairs while the hot air from downstairs is rising up to the hallway to replace the cold. Jun 30, 2017 at 13:54
  • Anyway a more powerful unit will cool the air more quickly, but it probably will not help with the redistribution of the air. Jun 30, 2017 at 13:55

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