Anemometer question

Does anyone use one to check air flow CFMs at supply registers. Trying to get a good benchmark…

A home inspector taking airflow numbers is a great way to get themselves in trouble. There is so much more to an HVAC system than a quick airflow reading. Stick to the tissue test if you want to know if the register has airflow. Going above SOP can bite you in the ass.

Before I became a home inspector, I spent 11 years designing and selling HVAC systems for both residential & commercial properties. We received a call from a customer that said that a home inspector stated that the system that we installed in their home was improperly sized so it would not heat properly. I went out to the home with the service manager, and we looked at the report and the system. The inspector was basing their findings off of the air temperature at the registers. The only problem was is that it was a dual fuel system running on the air-to-air heat pump on a mid 30’s-degree day. Because of this, the temperatures were in the low 90’s. There was nothing wrong with the system. However, the buyers refused to accept that the home inspector could be wrong as he was sticking with his opinion. To satisfy everyone, we had a mechanical engineer from American Standard verify the load calculations and system design. Again, the home inspector refused to accept that he could possibly be wrong because he was taught that the temp should be between 100 to 110 degrees at the register. The buyers walked away from the sale as they did not know who to believe. The inspector wasted the time of everyone involved all because he was trying too hard to impress everyone with his infared thermometer. Sometimes less is more.


Morning, Mark.
I use to during typical home inspections back in the day but now only resurvey that function for axillary inspections.

Last month I inspected a property. My client was confused on several issues and called me saying “I want Answers”. Knowing conversations like that can lead to places I would rather not fight in a court of law or unfounded reviews I went to the home at no expense to the client.

Everything he stated was convoluted. HE does not understand the simple HVAC system in his home and expects everything to self adjust because.
My reported covered it all!

I took pictures throughout incase I have to defend myself.

Benchmarking floor registers.
Temp. and CFM.
1: You need to set an average CFM for registers on every story.
I hold my anemometer and temp prob at either corner of the floor register and take a reading.
Not very professional but that is how I do it.

I concur, Jeff. There is so much more to an HVAC system than a quick airflow reading.
That home inspector that killed the deal was wrong. He was retained to inspect the building and not retained as an expert HVAC consultant.

Sizing a HVAC system for a home. Basics: 1: Sq Ft. 2: Base BTU. 3: Ceiling height.

What Mark is after, I think, I am not speaking for Mark, are basic CFM air flow numbers. *Note: This should only be ascertained, the CFM, when vents are removed.

Many times forced air supply register vents are mixed, closed, inoperable or just the wrong vents for the register boot. So taking reading at vents is a no no.
Most times when I remove forced air registers the boot is not sealed to the subflooring allowing a percentage of conditioned forced air to be delivered in between the flooring assembly. My Infrared Camera defines the space being heated in the flooring assembly.

So I concur, Jeff. There are too many variables for a home inspector to considering HVAC sizing from measuring numbers at forced air registers. Tread lightly if you consider doing this type of measuring.

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Both good discussions thank you. My scenario is: tissue or hand at a supply vent…after 5000 furnaces, very low air flow at a register, low to no air flow. It could be the ducts, the filter, the A coil is plugged, or the evaporative cooler damper is not in place and most of the heat is escaping through the cooler…
Never the less, the Realtor says Im crazy and I dont know what Im talking about…
I m not trying to size anything, Im just trying to have a way / tool to show the Realtor or buyer that I know what Im talking about when I say there is less air flow coming out of these supply registers compared to what I see on other houses…

Did you ask the realtor if they will accept some random number from some device they know nothing about pointed at or placed next to a register as proof of defect? Why do you even engage realtors ? do you work for them or your client? Is it the realtor that has an issue with your findings or your client? Is it the realtor’s job to determine what’s a defect and what is not, in a house?

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It all boils down to the narrative. The above statement is actually a pretty good narrative and it is enough.

I would not try to demonstrate anything. It is an observation (opinion) which is likely followed by a recommendation meant for your client. Simon is spot on. Stick to your guns and move on.

Note: Sounds like the listing agent. Their job is to push the deal thru. If that means pushing you around and see if you will bend, then that is exactly what they will do.

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That is exactly how you should use it as a HI, except that you compare register to register, not another house you have been in.

If you are reporting “Low Air Flow”, how can you say that if you don’t know what the air flow is?

Why is there air flow issues?

You don’t have to mention any of this potential stuff (it’s in the SOP BTW), 90% of the time it is the air duct system itself for this flow rate issue. If you measure all the ducts, they all will have a different flow rate because of the dynamics of the duct design (which is hardly considered in residential). When a TAB Tech comes in to fix the comfort issue, they will rob Peter to pay Paul. The first thing they do is to normalize all flow rates to about the same flow. No two registers have the same static pressure because of length of duct, fittings on the way to the register etc. Then based upon each rooms requirement (size, windows, exterior walls etc.) and the Btu per CFM the unit is producing, CFM is adjusted by closing down duct that has more than needed flow.

This issue is compounded in Heat Pumps and all Air Conditioners in the cooling mode. The Btu/CFM is never the same. A furnace produces Btu based upon the amount of fuel being used (Btu per Pound). This is a constant. Furnaces run at much lower CFM because of this.

TAB; Testing Adjusting and Balancing is a specialty all of it’s own. It is a separate business from HVAC Contracting, and sure as hell has no place in the Home Inspection field.

Guys like Jeff Spencer who sell equipment know more about this stuff than HVAC contractors. So if your looking for answers, it’s best to call them. I always consulted with them when dealing with issues. As he said, he consulted with AmStan, who knows even more than him. But you will never get one of those guys on the phone as a home inspector!


No, I don’t check airflow CFM.

I do check that every room has a supply feed and that it does indeed flow air. Had one last week in an older Cape Cod, bedroom on the second floor had one vent and after checking it I discovered it was a return air, so that got written up as no heat source for that room.

If you do decide to check for “adequate” air flow, keep in mind everything already mentioned above and also thet there may be dampers installed in the ductwork.

IMO best to steer clear of trying to determine this and report on it.


You need an average of each register also. CFM at the corner doesn’t cut it. Find the highest and lowest reading and average them.

Good Anemometers have have an average setting. You push average and move the meter across the opening. Push again and it displays the average FPM. You need to calculate the CFM.

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Afternoon, David. What I meant by average is that I looked for the highest velocity on that register. The highest CFM was at the corners of registers. Most register where within 5%, or less, of each other while some registers in the center of the home, bathroom, laundry room and walkin clothes closet where under 20% of the average.

As you can see I was using an inexpensive Anemometers with few functions. REED LM-81AM with 4 measurement settings, hi/lo/Hold but no luminary. Hard to read in dimly lit areas.

I am about to purchase an Extech AN200 CFM/CMM Thermo-Anemometer with IR Thermometer.

As for explaining air velocity comparisons, as with other homes you inspected, it is much more complicated than that. Some forced air systems have 2 speed or low/high settings. Other forced air systems have variable speed settings.
As well, some registers may be blocked by furniture, formal draperies or even carpeting, causing velocity imbalances comparisons.

forced air trunk and ducts

Does anyone look for balancing dampers on the duct takeoffs?

If you don’t see a metal lever sticking out of the insulated duct, it can not and has not been “balanced”. This in of itself will result in CFM changes from register to register that you are finding.

Every duct component has a friction loss/equivalent duct length (a 90 degree register boot has an equivalent duct length). Duct type has a friction loss per ft. So performance/CFM can be designed before construction. Then it needs to be checked and balanced to design.
How many times do you see different duct sizes between takeoffs? 6", 4", reducer fittings…?

Air flow, if your going to do it, should be measured at the right location. You put the anemometer down inside the of the boot, not where it enters the room. I used to use one of these types in HI because you can stick it up the duct and actually measure the duct rather than the boot, which is loosing pressure and velocity as it comes out of the duct.

Instead of buying all kinds of bells and whistles you will never use, one of this will do the job, and faster.

Also, if your using one of these things, you are measuring ft/sec, which is velocity not volume.

When air comes out of the duct into the boot/register it slows down as it disperses across the boot. So when you measure different boots in the same house (as Robert showed us in the diagram), you are going to get different speed measurements even if the CFM is the same. Which means your Wrong! CFM is related to Btu delivery.

Again also, ft/min (velocity) is a critical design measurement because it affects airflow pattern across the specific register design into and across the room. Velocity has a huge effect on “comfort” which is your clients sole concern. It also has a direct effect on the CFM Volume coming out of the duct. If velocity is too high, you get turbulent flow rather than laminar flow and the air flow becomes reduced and your design measurement becomes wrong.

None of this is a concern unless you elect to do it.

I know 99% of you do not want to do all the calculations needed to be correct, so save yourselves potential grief and consider sticking with the toilet paper test…


Almost never see them, no money for that here.

Yup! So don’t have to measure any air flow then, Just feel if air is feeding each room requiring heat.


You might consider air entrainment effect when testing like that.

And this subject keeps getting deeper… :crazy_face: :+1: :wink:

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I concur. It is a deep subject.

Every time.
Here is one that perplexed me from last week. All balancing damper ducts takeoffs, 3, were downstream connected to the HRV on the supply side. All ducts were insulated. 1 duck did not have a balancing damper. 1 duct was for makeup air.