Part IX: Tools, Calculators, and TestingChapter 14. Measure It: Testing Your Builds
Measuring Airflow and Collector Efficiency
How to turn temperature rise and airflow into a rough efficiency number.
- Difficulty
- Intermediate
- Cost
- $30–100
An air collector's useful heat is the airflow times how much that air warmed up. Its efficiency is that heat divided by the sun hitting the aperture. Neither half is a guess you are allowed to round upward. Airflow is the hard measurement. Sun on the glazing, not sun on a flat roof next door, is the honest denominator.
For ordinary air, heat in BTU per hour is about 1.08 times cubic feet per minute times the temperature rise in degrees Fahrenheit. A watt per square meter on the collector is about 0.317 BTU per hour on each square foot. Those two factors are the sketch below. They are good enough to compare a change you made. They are not a laboratory rating, and a result over 100 percent means the test is wrong.
Temperatures come from the logging guide. Sun, if you build a comparison meter, is the pyranometer guide. Fan and duct sizing for a collector you are building, rather than testing, is the duct guide. Methods linked at the bottom are other people's. This is the arithmetic.
Difficulty
- Level
- Intermediate
- Time
- A clear hour with the fan already running
- Skills
- Measuring flow instead of hoping the fan's sticker is the flow
Cost estimate
$30–100
A small anemometer, or a manometer if you are using a known duct restriction, fits this range along with the temperature probes. A real pyranometer is more. A free efficiency number with an invented airflow is worth what you paid.
Materials
| Item | Qty | Notes |
|---|---|---|
| Inlet and outlet temperatures at the same moment | 1 pair | In the airstream. A glazing temperature is not the outlet air. |
| Airflow in the duct | 1 | An anemometer traverse, or a fan curve at the pressure you actually have. The free-air sticker on the fan is neither. |
| Sun in the collector plane | 1 | Watts per square meter on the glass, or a clearly labeled stand-in from a clear-sky hour you do not pretend is exact. |
| Aperture area | 1 | The glazed opening, not the absorber plus the frame plus the wall around it. |
Heat out, sun in
Read inlet and outlet together. A rise you computed from this morning's inlet and this afternoon's outlet is a weather report. Flow and sun should be from that same stretch of minutes. Clouds make all three jump. Average a steady period, or log them and throw out the minute a cloud passed, and say which you did.
The fan's "CFM" on the box is the flow with almost no duct. Your collector has a duct, a hood, and restrictions of its own. Measure in the duct, in a straight section if you can, at more than one point across the pipe if the anemometer is small. One reading in the center of a round duct is high. The airflow calculator is for choosing a fan. This page is for finding out what the fan is doing now.
Divide heat by sun on the aperture. A useful collector at a useful temperature rise is well under 100 percent. Very hot, slow air loses more out the glass and the efficiency falls, which is a real result, not a failed test. Very cold, fast air can look efficient and deliver lukewarm heat you cannot use. Report the rise and the flow beside the percentage or the percentage is an advertisement.
If you do not have a sun measurement, you can still compare two hours by the heat delivered, and you should say that is heat, not efficiency. Borrowing "full sun is 1000 watts per square meter" on a hazy day inflates the denominator: you would compute a low efficiency and blame the collector for the haze. Note the sky or measure it.
Composite: the sticker CFM
Invented test. The fan box says 200 cubic feet per minute. The duct is long, small, and squashed at one bend. Someone multiplies 200 by a 40 degree rise and announces a marvelous efficiency against a guessed sun. A traverse in the duct would have shown far less air. The sticker describes the fan in somebody else's test tunnel, and your number is the flow in your pipe. The composite is not a real bench or an experiment copied from the links.
Log inlet and outlet in the airstream
Same minutes. Not the glass temperature. Not yesterday's room temperature.
Measure flow in the duct you have
Anemometer or a real fan curve at the real pressure. Do not type the free-air sticker.
Put sun and aperture in the denominator
Plane of the collector. Glazed area. 1.08 and 0.317 are the sketch factors.
Throw out anything over 100 percent
Then check the flow, the probes, and the sun. Report the rise and the flow with the percent.
Further reading
These links go to outside sources. This book cites them by link and does not copy their words or pictures.
- https://www.builditsolar.com/References/Measurements/measurments.htm
- https://www.builditsolar.com/References/Measurements/CollectorPerformance.htm
- https://www.builditsolar.com/References/Measurements/KitPyr/KitPyr.htm
- https://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm
- https://www.builditsolar.com/Experimental/SunSim/SunSim.htm
- https://www.builditsolar.com/Experimental/HeatTransferCoef/HeatTransferCoef.htm
General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.