Welcome! Here are the website rules, as well as some tips for using this forum.

# Converting Knots to CFM

Options
Member Posts: 748
I want to measure CFM from each of our poorly installed high velocity air conditioning system's 2" dia. supply outlets and then compare the aggregate CFM to the air handler's rated CFM. The purpose is to detect leakage / inadequate airflow.

I purchased a TurboMeter, an electronic wind speed indicator made by Davis Instruments. It can read wind speed in knots which is equal to FPM x 100.

How do I convert knots to CFM? I know that the TurboMeter is what high velocity AC firms use for this purpose. Please note that I am a homeowner and not a contractor.

• Member Posts: 306
Options
Knots to CFM

How about furlongs per fortnight. Cross sectional area of the duct in square feet times average velocity in feet per minute=CFM.
• Member Posts: 2,343
Options
Knots to CFM

For all round outlets multiply knots by 2 to obtain cfm. For the slotted outlet multiply reading by 4 then add 4 to get CFM. Example: round outlets with knots reading 16.3 has 32.6 CFM. Normal range is between 20 and 40 CFM. Slotted outlets with 7.1 knots has 32.4 CFM. Normal range is between 15 and 35 CFM.

There is a meter similar to what you have that will automatically convert this for you. Hope this helped. One additional thought,..before you even check these check the external static pressure in the supply plenum,...should be around 1.5" of water.

Mike T.
• Member Posts: 67
Options
Wow

Did I read that right 1 to 5 inches?No wonder they only need one return.
• Member Posts: 2,343
Options
Sorry don,......;-)

Was early in the morning and eyes and fingers weren't working yet....;-) I corrected it just for you... Actually it's 1.8" max and no less than 1.0". Recommended is 1.5" wc

Mike T.
• Member Posts: 67
Options
i knew that.

Just messing with ya.Good Morning!

• Member Posts: 2,343
Options
:--)

• Member Posts: 2,322
Options
Sounds like

Sounds like you have a hi velocity A/C system ala Unico or Space Pak. If so, your manual will provide you with info on setting the outlet shutter to acheive correct CFM based on amperage draw. PROVIDED of course that your installer used the correct number of outlets per ton of cooling. Find your manual and look it up. It'll tell you how many outlets you need along with how many additional you need to compensate for long runs.

I was in a new house that we had done the heating in checking out the boiler. I noticed the condensing unit cycling on/off frequently and asked the HO about it. (The A/C was installed by the builder's regular F/A guy) The HO stated that it had cycled like that since new and he thought that was normal. In the next breath, he said the A/C couldn't keep the house cool. I was curious, always wondering how our competition does their work, and asked if I could look in the attic.

• Member Posts: 168
Options
1.0 to 1.8\" WC

Thanks to those who explained that knots x outlet area in s.f. = CFM.

What does a WC figure actually represent in layman's terms? Pressure? What would be the reasons for an unacceptable reading if correct dia. tubing was used for line sets?
• Member Posts: 306
Options
W.C.

WC is inches of water column. It's an accurate way to measure small pressures. 27.72 inches of water column equals one pound per square inch. The 1.0 to 1.8 inches WC is a pressure inside the duct not the line set.
• Member Posts: 2,343
Options
S Ebels...

I have seen that to where the trunk line is really short. Especially with unico, they would rather you run more trunk line as close to termination as possible. 10' @ 40 cfm per outlet and 10% reduction for every 5' over 10'.

My .02

Mike T.
This discussion has been closed.