Air Flow

Does anyone check airflow from the registers? On the side I also do duct leakage tests and I figured out how to use my manometer and a simple box to get a CFM reading from the registers. Before I started reporting on it, I wanted to figure out some sort of baseline to measure against and looked at a chart that had the CFM based on duct sizing. On the chart, an 8 inch round duct was listed at around 200 CFM, but in all the houses that I tested, I was only getting around 75. I asked a couple of HVAC contractors about the discrepancy and they couldn't give me a good answer. Any thoughts?

We don't. . I do note leaking ducts when found in the wall with our thermal

Bensoelberg wrote:Does anyone check airflow from the registers? On the side I also do duct leakage tests and I figured out how to use my manometer and a simple box to get a CFM reading from the registers. Before I started reporting on it, I wanted to figure out some sort of baseline to measure against and looked at a chart that had the CFM based on duct sizing. On the chart, an 8 inch round duct was listed at around 200 CFM, but in all the houses that I tested, I was only getting around 75. I asked a couple of HVAC contractors about the discrepancy and they couldn't give me a good answer. Any thoughts?

Why would you want to do this? I don't think even a/c contractors do it when commissioning an ordinary house system.

On some levels of commercial jobs a full test and balancing service is required with a detailed report.

Bensoelberg wrote:Does anyone check airflow from the registers? On the side I also do duct leakage tests and I figured out how to use my manometer and a simple box to get a CFM reading from the registers. Before I started reporting on it, I wanted to figure out some sort of baseline to measure against and looked at a chart that had the CFM based on duct sizing. On the chart, an 8 inch round duct was listed at around 200 CFM, but in all the houses that I tested, I was only getting around 75. I asked a couple of HVAC contractors about the discrepancy and they couldn't give me a good answer. Any thoughts?

That sounds like too much work!

Yes, a lot of work for I don't think very much benefit.

Air flow out of the registers is not needed for a home inspection, are you checking in the heating cycle or cooling. In the heating the fan is on low speed and the only thing you might want to know is the temperature rise for the furnace to make sure the proper amount of airflow is going across the heat exchanger and there is adequate return air. On the cooling cycle the fan is on high speed or the fan speed is set for 400 CFM per ton of cooling, if you have a 2-Ton A/C you need 800 CFM of air across the evaporator for the system to operate correctly. The reason you don't see 800 CFM from an 8" pipe is because of the pressure drop across the register, the 4x10 register puts out 110 CFM at .01 static pressure. As you can see, this is too much figuring for a home inspection. Just check the temperature rise for heating and the temperature drop across the evaporator for cooling. Along with checking for CO from the gas furnace and gas water heater.

I do these Duct Leakage Tests as well but I never thought of that idea. Seems to me the fan on the duct blaster is much smaller then a air handler. Some times when issue arise I pressurize the ducts and am pretty good at finding the air leaks but I have to really cranck up the fan on the duct blaster. Now when drywall is up that another story.

Wow. This was a long time ago. I've learned a lot since I initially asked this question. My original assumption was wrong, but now I do offer HVAC system efficiency testing as an additional service, if clients want it.

BTW Orirosen, an easier way to pinpoint which duct run is leaking is to use vent cap duct covers. Get an extra and drill a small hole that you can add a second static pressure probe to. Pressurize the ducts to 25 pascals like during a standard test. Then with your extra cover, go to each register and replace the cover with the one with the test probe attached. If the pressure is still 25 pascals, that duct run is fine. When you find a register that the pressure is less than 25 pascals, that is the duct run that is leaking. Hope that helps!

https://ventcapsystems.com/collections/vent-cap-systems

Bensoelberg wrote:Wow. This was a long time ago. I've learned a lot since I initially asked this question. My original assumption was wrong, but now I do offer HVAC system efficiency testing as an additional service, if clients want it.

BTW Orirosen, an easier way to pinpoint which duct run is leaking is to use vent cap duct covers. Get an extra and drill a small hole that you can add a second static pressure probe to. Pressurize the ducts to 25 pascals like during a standard test. Then with your extra cover, go to each register and replace the cover with the one with the test probe attached. If the pressure is still 25 pascals, that duct run is fine. When you find a register that the pressure is less than 25 pascals, that is the duct run that is leaking. Hope that helps!

https://ventcapsystems.com/collections/vent-cap-systems

Genius!