I installed an AirScape3.5e whole house fan. I don't think the fan is operating at its max potential. It just doesn't seem to cool the house down very much when running even when the outdoor temperature is much colder.

To add specific measurements to this question, I bought an anemometer to measure velocity of airflow (and have calculated some CFM readings by multiplying velocity by area of the vent/window). I measure 4150 CFM going into the attic vs 1794 CFM coming in from the window (with that being the only open window). These were measuring the air velocity at the center of the window/vent where I think velocity would be greatest. So I suspect actual flow to be a bit lower.

So it seems like of the air being pulled into the attic only about half of it is coming in from the window. How can I find out where the other air being pulled into the attic coming from? More importantly how can I improve the cooling effect of the fan? Would adding ventilation to the attic help? And side question, what kind of pro would I hire to help with this (e.g., roofer, HVAC, other)?


  • The fan says that at its max speed it can move 3500 CFM of air. The manual says the attic needs 7 sq feet of net vent area. Manual is here.
  • My attic measures 23' by 48' meaning it should have at least 23*48/150= 7.3 sq feet of net vent area (if it was built to the 1 sq foot of vent area per 150 sq feet of floor area).
  • Attic has soffit vents along probably 3/4 of the two 23' sides. I think it also have a ridge vent (23' of ridge vent). No other vents.
  • House built in 2001. NY Area. It's a town home, but attics are completely separated.
  • I don't understand your 23*48/150= 7.3 sq feet calculation. You need to measure the actual vents in your roof and add them up to see what you get. Also, incorrectly installed insulation can block soffit vents, so that's something to look at as well.
    – JPhi1618
    Commented May 22, 2019 at 4:13
  • 1
    Each soffit vent will be spec'd for a certain area of free vent; can you find out what it is? And, are the vents completely open behind them? Do you feel air coming out the vents when the whole house fan is running? Commented May 22, 2019 at 10:29
  • Also, do you know what kind and how much ridge vent you have? Commented May 22, 2019 at 14:37
  • I dont believe insulation is blocking any of the soffits. I'm not sure the type of soffit or ridge vent. The ridge vent in particular is so high up I would never be able to reach it. I edited the question to add some measurements (and be more be more in line with my goal of cooling the house better). I do suspect that poor attic ventilation may be part of the issue still, by made the question less directed at that. Thanks Commented May 23, 2019 at 2:32
  • Pretty good that a 3500 cfm fan is moving 4150 cfm into attic. If only 1794 cfm is coming into house through window there is a lot of air leaks in your home envelop. the fan may be pressurizing the attic and recycling air through ceiling penetrations. have any can lights? Ceiling registers and returns? Poorly sealed attic access trap door?
    – Kris
    Commented May 23, 2019 at 3:43


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