Tag: airflow

Recommended Duct Velocities (FPM)

Duct Type Residential Commercial / Institutional Industrial
Main Ducts 700 – 900 1000 – 1300 1200 – 1800
Branch Ducts 600 – 700 600 – 900 800 – 1000

As a service technician, we are often expected to understand a bit about design to fully diagnose a problem. Duct velocity has many ramifications in a system including

  • High air velocity at supply registers and return grilles resulting in air noise
  • Low velocity in certain ducts resulting in unnecessary gains and losses
  • Low velocity at supply registers resulting in poor “throw” and therefore room temperature control
  • High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal
  • High TESP (Total External Static Pressure) due to high duct velocity

Duct FPM can be measured using a pitot tube and a sensitive manometer, induct vane anemometers like the Testo 416  or a hot wire anemometer like the Testo 425. Measuring grille/register face velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer

First, you must realize that residential, commercial and industrial spaces tend to run very different design duct velocities. If you have ever sat in a theater, mall or auditorium and been hit in the face with an airstream from a vent 20 feet away you have experienced HIGH designed velocity. When spaces are large, high face velocities are required to throw across greater distances and circulate the air properly.

In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 600 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses.

Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. This helps greatly reduce total system static pressure as well as return grille noise.

Supply grilles and diffusers should be sized for the appropriate CFM and throw based on the manufacturer’s grille specs like the ones from Hart & Cooley shown above. Keep in mind that the higher the FPM the further the air will throw but also the noisier the grille will be.

— Bryan

I am in the midst of testing the accuracy and repeatability of different types of airflow measurements for techs in search of the most practical methods for different applications.

One commonly taught method for measuring airflow is the temperature rise method where you use a heat source that produces a set # of btu/h such as heat strips and using the sensible heat air equation you can “easily” calculate the CFM being produced by the equipment. Let’s use this specific example to illustrate the challenges in getting a truly reliable measurement.

The Typical Equation used for a fan coil with electric heat is –

CFM = (Volts x Amps x 3.41) / (1.08 x Delta T)

  1. Measured Heat Strip Volts x Amps gives you the Watts (This will generally be a reasonably accurate measurement depending on the accuracy of the meter)
  2. 3.41 is a constant watt to BTU conversion
  3. The 1.08 is a combination of factors 1 CFM x 60 minutes per hour = 60 CFH x .075 pounds per cubic foot (Standard Air) = 4.5 x 0.24 Specific heat of air = 1.08
  4. The Delta T is only as accurate as the thermometer used and it’s placement in the air stream

The first trouble comes in when you realize that almost no air is “standard air” which is 70°, 0% RH air at sea level. This leaves us with a .075 lbs per cubic foot standard air # that isn’t very accurate at all. To see how far off it can be, take a look at this calculator

In the case of my test, I found that a 1.05 multiplier was more accurate than 1.08 based on my indoor air conditions during the test.

Before the test, I used a TEC TrueFlow meter to confirm the actual system airflow. The system was a 2-ton Carrier FV ECM air handler and both the fan charts and the TrueFlow confirmed a system airflow of 700 & 718 CFM respectively.

I turned on the 5KW electric heat and ran the system with electric heat only for 5 minutes and then calculated the BTU/h of the heat strips at 12,158. I took a Delta T between the return riser and the supply plenum 24″ above the fan coil with the same pocket thermometer, The delta T was fairly stable at 1°.

It doesn’t take a math major to figure out that a 1° delta T = WAY MORE AIRFLOW THAN 718 CFM

12,158 / 1 x 1.05 = 12,766 CFM

So what went wrong?

In this particular case, we realized that the evaporator coil was still lower temperature than the return air and there was still moisture on the coil and in the drain pan because it had been running in cooling mode before which was decreasing the temperature of the air inside the air handler. Once the system ran in heat another 15 – 20 minutes the delta T came up to 6°.

Still WAY too low!

12,158 / 6 x 1.05 = 1,930 CFM

Next, I moved the thermometer to the side and I was getting a 10° delta T, then to the back and all of a sudden I was reading a 17° delta T which was putting us right in the range, a little low actually (681 CFM). With the pocket thermometer, we were seeing an 11° variation depending on where I placed the probe!

So why was this occurring? Even at 24″ above the air handler the air had not fully mixed and there were areas defined areas of high temperature and low-temperature air in the supply plenum.

I gave up on the pocket thermometer and used a longer Testo probe and a K-type bead probe to get closer to the center of the air stream. I then took three measurements. One in the front, one on the left side and one in the back, added them together and then divided by 3, this gave an average of the supply air temperature from multiple points and the result was a calculation of 755 CFM which is still on the high side but definitely a usable figure.

I then tried it at different blower settings to see if this new averaging method worked under different air flow conditions, sure enough, it was within 10% of the factory fan charts and the TrueFlow.

Out of curiosity, I ran the system in cooling mode to see if I would get a similar level of variation and while I did see a few degrees difference it was only 2° due to the mixing in the blower after the coil.

For those of you who work on gas furnaces will coils on top you know how much the supply air temperature measurement can vary based on where you place your probe due to poor air mixing and radiant cooling near the coil, this is the same effect I was seeing above the heat strips.

Here are some good rules for accurate measurement in critical test circumstances –

  • Probe placement matters. When possible take readings 6′ away from the appliance and out of “line of sight” from the cooling/heating surface (Heat strips, heat exchanger, coil)
  • Use the same probe to take the return and supply readings to improve accuracy if the device isn’t highly accurate (more than +/- 1°)  
  • The speed of the reading matters, a K-type bead is not the most accurate measurement but the quick read they will help reduce the impact of radiant heat on the probe if you are “line of sight” to the heat source. 
  • When performing any temperature rise calculation the entire inside of the appliance must be the same temperature as the return air and completely dry.  
  • When a temperature measurement is critical it is best to take multiple measurements and average them vs. trusting a single measurement.  

In this case we are looking for an accurate differential temperature, the absolute values don’t matter as much. I found that the quick read and insertion depth of a K-type bead probe (Shown below) was a good tool for this measurement, even though it isn’t the MOST accurate reading.

On another note –

I also neglected to add in additional BTUs of heat to the equation to compensate for what is added by the blower if we wanted to get really crazy.

Motor heat added is

Volts x amps = watts

Watts x (1 – efficiency) = watts of heat added

Watts x 3.412 = BTU of heat added

Just in case you wondered

— Bryan

Measuring airflow is easy… measuring airflow accurately is quite a bit more difficult. In many cases when we as technicians measure airflow we are trying to get to the almighty CFM (Cubic Feet per Minute) volume measurement. You can take CFM readings fairly easily with a hood like the Testo 420 shown above, but even a hood has some limitations when the goal is to measure total system CFM vs. register / grille CFM.

In this series of videos Bill Spohn from Trutech tools demonstrates all of the tools you can use to measure airflow from hot wire and rotating vane anemometers, to flow hoods, to smart grids and pitot tubes, all the way down to using a GARBAGE BAG.

I had the privilege of seeing this presentation in person (I am the one behind the camera) and I wanted to share it with you. It is well worth your time.

— Bryan

When you start talking airflow, it can get pretty in-depth pretty quick. There is a big gap between what is useful for the average tech to apply every day and the whole story so let’s start with the simplest part to understand, Static Pressure.

Static pressure is simply the force exerted in all directions within any contained substance, or in this case air. This means it’s not the directional force of air moving or blowing (that is called velocity pressure), it is simply to force pushing out on the positive side of the air system and pulling in on the negative side.

Measuring static pressure helps a tech know whether or not the system has excessive resistance to air flow overall or at a particular point.

Static pressure is measured in inches of water column (“WC) and is the amount of pressure needed to displace one inch of water in a water manometer.

 

A Magnehelic is a brand name for a high-quality Dwyer analog pressure gauge that comes in many different scales. Many techs will already have a high-quality digital differential manometer (like the Testo 510) for reading gas pressure, which makes getting a separate Magnehelic largely unnecessary.

When using a manometer or a Magnehelic, you will first zero it out to room pressure (for a Magnehelic make sure it is level). Next place the negative side probe in the return side of the unit after the filter but before the blower and place the positive probe in the supply duct. Keep the negative side probe away from the side of the blower and insert the probes in as straight and square as possible. It is advised to use a static pressure tip like the one shown below to prevent air velocity pressure or air currents from interfering with the static pressure reading.

With a static pressure tip point the tip against the direction of airflow (points opposite the airflow) in both the return and supply. DO NOT confuse a static pressure tip with a pitot tube tip. A pitot tube tip is designed to measure velocity pressure or total pressure (velocity + static = total)  NOT static pressure, and it will have an open end.

Total external static pressure is return plus supply, positive plus negative and in general, you would like to see it be 0.5″ or less…

If you see 0.9″ or higher that is when you start to see trouble on most residential systems, but as always, each piece of equipment is different depending mostly on motor design. Whenever possible design your equipment / duct system so the result is 0.4″ – 0.6″ of total static (Once again talking general residential / light commercial here).

If you do find it to be high, then read the return and supply separately to see which is higher which is just a matter of removing the hoses to your manometer or Magnehelic alternately. Whichever reads higher is the greater cause of the issue.

I could keep going on this, but instead, I will just link to some more in-depth articles if you want to do more reading.

— Bryan

Epic airflow write up from Dwyer 

Measuring Airflow from TruTech

Troubleshooting Ductwork by ACHR News

 

 

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