anemometer for measuring duct air volume

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Old 11-26-18, 06:58 PM
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anemometer for measuring duct air volume

Heating and cooling the upper level of a 1-1/2 story houses is difficult when a dormer interrupts the knee wall that usually provides a horizontal chase to the east end of the house. We modified the existing HVAC duct-work in our 2012-purchased, 1998-house in the hope that the modifications would solve the problems cooling the east end of the upper level. While the heating in our Indiana location is ok, the cooling is not. The HVAC contractor who installed the retrofit warned me before we started the work that additional modifications might be necessary. We did not want to spend the money or loose any main-floor square-footage to install a large chase to move more air from the basement to the upper level if the chase was not necessary. Four cooling seasons after the modification, we know our current system does not work. It is time to act. I would like to measure the air-flow through the insulated, circular, 8-inch duct that runs through the attic to the east end of the upper level before I add any duct boosters. Please recommend an inexpensive anemometer that might work in this application
 
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Old 11-26-18, 09:00 PM
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The Amazon's Choice Anemometer is only 23 dollars.

An air hood is a better tool but you would have to add a few zeros to the price tag.

If you measure airflow without the grill the formula is (3.14 * radius squared) / 144 * FPM = CFM.

The Anemometer will give you FPM.

You have an 8 inch duct, so .34888 times your FPM = CFM.

The supply air register throws this formula a curve ball. I would pull the grill off to test if I didn't want to use a $2000 air hood.
 
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Old 11-26-18, 09:02 PM
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You will find...... or actually we have found..... you haven't tried them yet..... that duct boosters don't work as well as anticipated. There is no substitute for the correct ductwork.

I have an Extech unit but it was around $150. I don't think you'll need that accuracy.
Amazon looks like they have one that will suffice. Actually they show quite a few units in the $50 and under group. No idea the accuracy of those units.

Anemometers
 
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Old 11-27-18, 05:19 AM
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8inch duct is good for 240 cfm of air (Best practice HVAC) T'hats not a lot of air going to an entire upstairs. Have you thought about an HVAC system just for upstairs?
 
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Old 11-27-18, 12:49 PM
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Airman: 240 cfm = capacity of 8" duct. How many air changes do I need per hour? An 8" duct supplies the east end of the upper level. That is a room 322 sq feet with knee-walls and a typical sloping ceiling going to a flat, 8-ft ceiling. It has a large ceiling fan. The bath is 67 sq ft with sloping, 8-ft ceiling supplied by another 8" duct. No return on the east end. The west end of the upper level is a loft. The loft is 190 sq ft. The open stairway and vaulted ceiling from the main floor to the upper level is 132 sq ft. The loft hallway is 39 sq. The total loft area is 361 sq ft supplied by 2, 8 inch ducts and has a large ceiling fan. There is a 11-12 inch air return on the west end. Am I anywhere close to having enough air capacity if I add 1 duct booster to the west end and 1 duct booster to the bath?
 
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Old 11-28-18, 06:36 AM
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Id suggest doing a heat load for that area so it inst a guess. That is the correct way to do it.
Saying that in residential rule of thumb is 380 Sq-ft/ton -600Sq-ft/ton. So guessing you need a 10inch duct for the minimum or a 14 inch max. Lots of variables that's why a heat load must be done to fix the issue correctly.
 
 

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