Adding a Boost Fan to a Low Flow Duct

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  #1  
Old 07-22-20, 12:18 PM
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Adding a Boost Fan to a Low Flow Duct

I have a second story bedroom that has low air flow in the only register in the room. I would like input on adding a boost fan that would increase the airflow in the 4 inch duct and control the fan with my furnace for both heat and ac .

The furnace is Trane XL 80 Gas Induced Draft 2 stage heat model TUD120R960A1 and Trane XL 80 Air Con. My plan would be to tap into the furnace power circuit for the boost fan (specs are 110v , .65 Amp, 210 cfm) and use a 120 v , 24v coil relay to control to the boost fan. The AC is controlled by a 24 V circuit from the furnace control board.

A few questions are:
Anything particular about a Trane xl80 control board too look out for ?
Should I look for an 24 v output from the fan on/off to switch for both heat and ac ?
Recommendations for the best relay, boost fan and general setup.
 

Last edited by edge10; 07-22-20 at 12:36 PM.
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Old 07-22-20, 03:51 PM
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Before doing that have you shut all your basement ducts and closed or partially closed unused first floor ducts to force a little more air upstairs?

Even removing the metal grate will make a big difference in the room!
 
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Old 07-22-20, 07:09 PM
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Unfortunately many members have tried those booster fans but I can't recall any that said they were satisfied with the performance.
 
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Old 07-22-20, 11:53 PM
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I used booster fans once in 30+ years, in a $$$ condo building where the homeowners didn't want their walls cut to correct duct problems, and were willing to pay for the additional energy to run the fans. We spec'd three fans per four story condo and they greatly improved comfort throughout.

Always fix the ducts, and balancing is the first step as M recommended. If that doesn't help, identify and correct duct issues.

Is the duct supplying the room metal or flex? And are you sure the duct is 4" diameter? I only spec 4" for 20 CFM or less: Walk in closets, small rooms with little exposure to outdoors, etc. A bedroom with outside wall(s) and window(s) beneath an attic requires 6" or 7".
 
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Old 07-23-20, 10:32 AM
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All room ducts are 4 inch except to the new addition which are 8 in. oval . The ducts have been balanced and two of the lower flow registers are closed at night to force more flow.

The problem is this low flowing bedroom duct was located poorly and required two extra 90 bends which reduces the flow. Also the inlet to this duct from the main trunk, about 3 ft downstream two other ducts. which are 16 apart. The builder made this mistake with the 2nd story bath duct also,

The 2 other 2nd story larger bedrooms have 2 duct registers,fewer bends and have much better airflow/ temp control for both heat and AC.

The duct cannot be relocated in the basement to remove the bends because the space and obstructions. There might be a chance of replacing 4 inch bends with a transition 6 elbow or 6 inch flex . Too long a flex might reduce the flow also.

I found a higher quality boost fan with 210 cfm, but of course you cannot tell how it works until it is installed
 
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Old 07-24-20, 07:24 PM
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My kitchen ceiling AC louver was fed with 12 foot of flexible 8" line from main duct. Air flow was never adequate.

Installed duct fan, Kitchen temperature is now balance with rest of house. Highly recommend duct fans.

https://www.homedepot.com/b/Heating-...1248,1212,1261
 
  #7  
Old 07-26-20, 12:40 AM
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Doubt you'll get 210 CFM through a 4" duct. You probably need a fan capable 100 CFM at 0.5" ESP.

Do what you can to reduce duct restrictions first, though. If there are space restrictions, you might look into replacing the accessible 4" with larger oval ducts (should be able to find 7" or 8" oval locally). That'll reduce much of the PD. Just saying.

Oh yes: A duct can't be too big, the air "Won't Get Lost In There". I actually saw a well-known HVAC guy say that on a popular TV home remodeling show back in the day.
 
 

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