Hopefully you get a spot on impeller (or end of the motor?) that you can attach some reflective tape. That should be possible although with so much stuff made of plastic, you’ll need to hold the tach as opposed to magnetic base mounting it. Oh well.
Yup… BTU/hr = CFM x ΔT x 1.08 and the conversion for 1 watt is that it is equal to 3.41 btu per hour
Regarding your idea of using the DC current draw to get to the CFM value… not a bad idea but I think if you don’t do at least a few flow tests, you have no way to ensure that the flow-power relationship is correct or if something else is going on. You mentioned a dirty fan impeller… that’s just one of a few things that can go wrong. Similarly, the filter will get dirty and change the results a bit.
By the way in case you didn’t know, flow varies directly with speed.... it’s a 1 to 1 relationship. If your fan speeds up 20%, the flow goes up by 20%. However, the power goes up by the cube of the speed. Thus if the fan speed increases by 20%, the power increases by 73%.
Thought for you on using your vane anemometer… when you make the cardboard box, put a slot in the side so that you can run the anemometer in and out as well as sideways along the length. That way you can quickly move the vane all over the place to see how even the readings are across the cross section. To do that, you might want to have a second person with a small piece of cardboard to cover the slot so that it can be moved along as the vane is moved… you want to minimize the opening to just around where the vane is inserted. My guess is that if you build the box right, you will find that the readings with the vane anemometer are fairly even… and then you can do the same thing with the hot wire to get a comparison of what that method tells you. After you check what the average is, you can just use one location that is the closest to the average for monitoring purposes. Hopefully the speed holds reasonably steady while you are doing the tests.
Good joke about the Trane! I used to work on them years ago…..
—”By the way in case you didn’t know, flow varies directly with speed.... it’s a 1 to 1 relationship. If your fan speeds up 20%, the flow goes up by 20%. However, the power goes up by the cube of the speed. Thus if the fan speed increases by 20%, the power increases by 73%.”
Good stuff, thank you, I see in the study linked below, they use FAN RPM to determine CFM.
I painted a white dot on the fan but holding a steady aim is not easy, I might have to put a couple of screws in the wall for this one. Also, I have readings from the tach where there is no paint dot???
I acquired a nice selection of cardboard and a working plan...
I shaped a box to fit the front of the head unit and was taping it in place and noticed the unit pulling slightly from the wall???
PROBABLY NOT GOOD.
Pulling down gently on the front of the unit distorts the wall bracket ...
Everything except the bracket is plastic, the unit weighs about 20 pounds, I easily hung it singlehanded. Not made for additional attachments.
And I wanted a long strait trunkline...
Well, I could add a few trapeze hangers?
Looking around I found Figure 13 Flow test setup, on page 13 from:
https://www.nrel.gov/docs/fy11osti/49881.pdf
I have always heard using flex duct is not good for flow.
...Moving on to my floor mount unit with a large flat return grille to attempt a few measurements and compare three different anemometers, a friend loaned me #3.
The grille is dived into six horizontal elements, so one high, one low, and a dozen samples should work.
The meters were surprisingly consistent with each other, the hot wire had a flyer or two; probably from how I held it?
My son-in-law the HVAC guy said “Hold the meter center and multiply by 0.9 “ of what the meter shows in CFM for the opening size.
Doing a second flow survey the numbers dropped and the air cooled???
DEFROST CYCLE; that and a few more variables make hard(or close) numbers difficult to find if not constantly logging input and output. The outside coil will frost up, limiting flow, and the system used compressor heat and maybe steals a bit from the house to defrost.
And the temperature is dropping to minus seven tonight, a good time to be watching what the heat pump does. Typically we have 5 subzero days each year, we had 3 over Christmas... about over.
Looking around the few solutions that come up are expensive and/or junk.
Scope, time, cost-—PICK TWO!
And I’m Cheap Charlie #10