I have purchased a MORI HR 150, which is an extractor vent. It has an "alternate flow" and "extract only" setting. I want to set it on extract only, which means it will take the air from inside and move it outside at a pace of 7 litres/second.

However, the bedroom has no other vent hole, so I am a little concerned that the air pressure will decrease as the vent just removed the air from the bedroom, with no entry point for new air, except for the space below the door.

Should I be concerned by that, from a health point of view?

  • What's the health concern? That this little fan will suck your brains out your ears? :) Are there no HVAC vents in the room?
    – isherwood
    Apr 6, 2017 at 21:04
  • If the opening under the door isn't large enough, the "make up air" coming in under the door, may "whistle" (make a noise) in order to draw enough air into the room. If that happens, which I doubt it will, you could trim 1/4" off the bottom of the door to decrease or eliminate the sound. There are no health risks, like lake of oxygen, etc. However, you will have higher heating costs because you'll be heating or cooling the makeup air.
    – Lee Sam
    Apr 7, 2017 at 5:02

2 Answers 2


The fact is that no home is sealed 100%. There are many sources of makeup air, from electrical outlet penetrations to imperfect windows.

Even so, the airflow under and around your door is more than adequate for that little fan. No worries.

  • You may have to trim the bottom of the door if it was installed with very little free space under it.
    – d.george
    Apr 6, 2017 at 22:36

Pressure is not a problem. Even the smallest space around the door will allow enough air to get in that you will not notice any pressure difference. You might find closing the bedroom door a tiny bit more difficult if it seals up fairly well.

Depending on where you live and the season, you might want to rethink the idea of using it in extract only mode. The "Integral Regenerative Heat Exchanger" is only going to do it's thing when allowed to flow both ways. Any air vented out is replaced by outside air leaking in somewhere else in the house which may need to be heated or cooled depending on the temperature difference. It is sort of a hidden cost of operating this unit. (It might still be well worth it)

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