I am trying to lower the fan speed on blower fan. It is currently set to factory default setting. My AC is 3 ton unit. Model no is 4A7A3036H1000NA. The fan settings on IFC are like TAP 1, 2, 3.... and currently it is set to Tap 7 which is the last setting. But when I look at the chart for CFM vs External Static Pressure Tap 7 comes around 2000 for all the numbers. My question is should I lower the fan speed to get closer to the required CFM instead of letting it be on factory default setting? I know that first static pressure need to be measured but again the factory default setting seems to be high for 3 ton unit.enter image description here

  • It has more to do with your duct size and coil size you may have a 3 ton compressor but what is the rating on the evaporator coil and the duct sizes if you drop the flow two low across the coil it will freeze up I have seen this when all the components are sized based on price some flows need to be measured I can make your static pressure be high by damping a few ducts.
    – Ed Beal
    Commented Aug 10, 2020 at 19:46
  • Do you have a 7 speed fan or a 9 speed fan? Lowering the fan speed will reduce the noise level, but the temperature of the air coming out of the coil will drop as the fan speed is reduced. You don't want it to get too low. Just as an example, in my system the air going in is at 75 F and coming out of the closest duct is 59 F. Commented Aug 10, 2020 at 20:56
  • Jim, I am not sure about 9 Tap speed. But I am just going off the std 300-450 CFM/ ton statement that I found on quite a few website. So in my case Tap 7 is still higher than what is actually needed right? On the product manual it states Designated Gas heating air flow taps for my model is 1-7. Not sure if it will apply to AC air flow as well.
    – Mayur
    Commented Aug 10, 2020 at 22:54
  • @jim, I checked my AC fan speed can go to Tap 9. To my surprise I found that some HVAC companies don’t do static pressure testing.I called three reputed companies in my area and all of them said We do not measure the static pressure. However for technician to come out it will be 89$ fees. I adjusted the fan speed by my self to Tap 4 which still is higher but my unit got quieter so will see how it performs in terms of cycling. And as far as icing of the coil goes I am not sure Tap 4 is low enough to ice it up. Because Tap 4 across the air flow table is still higher than avg 1200 cfm for 3 ton.
    – Mayur
    Commented Aug 12, 2020 at 0:08

1 Answer 1


When I was still doing residential work the normal temperature drop across the Coil was between 15F. and 20F. A temp difference above 20F meant that more air flow was needed and a temp below 15F meant that there was too much air. Unless things have changed recently or you have a special A/C system you can use these temp drops as a guide line. Just buy a temperature thermometer or a probe type and measure the air temp difference across the coil. my 2 cents

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    This unit is installed recently. But I was not at home so not sure if the contractor did all the calculations or set the speed correctly. But just going by std 300-450 CFM/ ton. It looks higher than what is needed. I could be wrong but any suggestions are welcome.
    – Mayur
    Commented Aug 10, 2020 at 22:56
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    Air flow must be "right". Too low and the coil will freeze. Too fast and not enough moisture will be condensed and humidity will be high; if extreme it can cause ducts to drip water. Commented Aug 11, 2020 at 0:24
  • @d.george I lower the speed to Tap 4. Can I check the temp at the return and supply vent closets to unit and take the difference. And if it is in 15-20F range am I ok with the speed I set?
    – Mayur
    Commented Aug 12, 2020 at 0:12
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    I like to take the temperatures close to the air handler. I always used a digital pocket thermometer that costs around $10.00 which could be inserted through a canvas connector if present or through a small hole in the duct within a couple feet of the coil. Using the access points you described would be OK . Remember, this is just a "rule of thumb" or a quick way to get the temp difference. Try to get closer to the 15F than the 20F drop.
    – d.george
    Commented Aug 12, 2020 at 11:29

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