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If you are a HVAC/R technician, you should always check any air-source heat pump (ASHP) for proper airflow across the coils when troubleshooting or during preventive maintenance.

Correct airflow is of crucial importance to the operation of any ASHP. Part of the heat transfer rate is determined by the airflow across the indoor and outdoor coils. If the airflow is incorrect, then the heat transfer rate is incorrect and can drastically affect the equipment's performance.

Accurate airflow measurement is essential in troubleshooting any heat pump system. In fact, no refrigeration test is valid if the air flow volume is not correct!

ASHP manufacturers follow the tenants of the Air Conditioning, Heating, and Refrigeration Institute (AHRI) which has a "Test Stand Value" requiring the measured air volume rate, when divided by the measured indoor air-side total capacity, must not exceed 37.5 SCFM per 1,000 Btu/h [this is a maximum of 450 cubic feet per minute (CFM) of airflow across an indoor coil per 12,000 Btu/h of capacity]. Most manufacturers use an acceptable range of 350 to 450 CFM per 12,000 Btu/h of capacity, and over 750 CFM per 12,000 Btu/h of capacity across outdoor coils (most outdoor fans move approximately 1,000 CFM, up to 1,500 CFM, per 12,000 Btu/h of capacity). In the HVAC/R industry, 12,000 Btu/h of capacity is referred to as a "Ton" of refrigeration. Typically, most manufacturers focus on around 400 CFM per "Ton" when rating their equipment.

Before performing any airflow determination, always make sure that all registers and grilles are open filters and coils are clean, and that blowers and fans are running properly delivering airflow across the indoor coil and the outdoor coil.

There are various methods that help determine the airflow amount across an indoor coil. The indoor coil is typically checked as the airflow must cross this coil to allow the refrigerant to either absorb (ASHP cooling) or reject (ASHP heating) the heat to the appropriate "sink." In summer, the "sink" is the outdoors, and during winter, the "sink" is indoors.

One of the most widely utilized methods when checking ASHP airflow has been the "temperature rise" method across the auxiliary or emergency (back up) heater(s). This method can be performed regardless of outdoor ambient temperature.

When performing this test, you should remember that typically, most air source heat pumps operate with the same airflow regardless of mode of operation. In cooling or heating, or during emergency heat or defrost, the heat pump simply delivers the "same" airflow per ton across the indoor coil. The only change is possible during the cooling mode, as water condensing on the indoor coil increases resistance slightly, lowering airflow amounts somewhat. Of course, you can also change blower speeds and CFM amounts for either cooling or heating on some ASHPs.

To check airflow (CFM) of an ASHP, you have to perform several measurements and use some math. The common formula for calculating CFM is:

Emergency Heat Output(Btu/h)

CFM= -----------------------------

Temperature Rise x 1.08

You must find the CFM per Ton traveling through the indoor coil during cooling or heating modes. To accomplish this, you should place the system in the emergency heat mode and place thermometers in the supply air and return air paths as close to the air handler as possible without being affected by the radiant effect of the heaters.

In the previous formula, CFM equals the emergency heat output in Btu/h. Since you will be checking heaters, you will be calculating electrical data. When finding Btu/h output of the heater, you will simply measure the voltage and the amperage at the disconnect for the heater(s) and record the values. This will require use of a voltmeter and an ammeter.

Supply voltage multiplied by amperage equals wattage. Wattage multiplied by 3.413 (Btus per Watt) equals Btu/h and you will then have the value for the CFM formula numerator.

In the CFM formula denominator, temperature rise comes from the difference in the return air and supply air temperature after the heater(s) has/have stabilized and a difference has occurred. This difference (sometimes called TR or ΔT) is then multiplied by a constant of 1.08 to find the adjusted temperature difference. Always pay close attention to temperature change when finding the difference in temperature.

This adjusted temperature difference is the denominator for the CFM formula.

When the formula is completed with the necessary inputs, the answer is the total delivered airflow (CFM) traveling across the heater in emergency heat mode. You then simply divide this value by the tonnage of the installed outdoor coil to find the CFM per ton. The cooling airflow (CFM) will be near to this value or slightly lower due to increased pressure drop from water condensing on the indoor coil during cooling. The ASHP compressor heating airflow (CFM) per ton will be a similar value to that found from emergency heat. The CFM per ton should be in the AHRI range to be acceptable.

If inaccurate air flow (CFM volume) across the indoor coil (evaporator in cooling, condenser in heating) is determined, this situation must be corrected prior to further analysis of the refrigeration cycle.

Determining ASHP indoor coil airflow (CFM) during emergency heat example:

Two (2) Ton Unit

240 Volts at disconnect

20 Amps at disconnect

Return air = 70ºF

Supply air = 88ºF

CFM = (240 x 20 x 3.413) ÷ (1.08 x 18ºF) = 843

843 CFM ÷ 2 Tons = 421.5 CFM per Ton

(This amount is acceptable per AHRI)

You should also check the outdoor coil for minimal clearance per manufacturer around the unit from shrubs, trees, and decks to allow for proper heat rejection in the summer and absorption in the winter. You can also determine the airflow across the outdoor coil by using the product specifications from the manufacturer as shown below. Most ASHP outdoor coils will typically deliver close to this amount if the coil is installed correctly and unobstructed.

Phillip A. Rains

Copyright Phil Rains

Phil is the Master Trainer/Technical Developer for HVACReducation.net. He has over 35 years of HVAC and Refrigeration experience in installation, service, and training.
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Teaching Children About The Four Kinds Of Kinetic Energy

Your child comes home with questions about the different kinds of kinetic energy. So how do you distinguish between the many different types? You can use common examples to help your child understand the kinds of kinetic energy.
Kinetic energy, or the energy of motion, is the ability to do work. If you use a force across a distance, you do work in a scientific sense. If you slide a book across a table, you have used kinetic energy. The many kinds of kinetic energy include mechanical, electrical, radiant, and sound.

Four Kinds of Kinetic Energy #1- Mechanical kinetic energy is the most familiar and common example of kinetic energy. As an object moves, it has kinetic energy. In order to lift an orange over your head it takes a joule of work. Kinetic energy is measured in joules (rhymes with cool) or Newton-meters, with force measured in Newtons and distance measured in meters. A car accelerating at the same rate as a bicycle has more kinetic energy because it has more mass.

Four Kinds of Kinetic Energy #2- Electrical energy that moves through wires is measured in watts. A joule of work done in one second is a watt, which is named for James Watt, the inventor of the steam engine. Trains used to run using steam engines. If you lift a quarter-pound hamburger with cheese vertically, a distance of one meter in one second, you use a watt of power. Take a close look at a light bulb that is not in a lamp. You will see a number followed by "kw", or kilowatts, on most light bulbs. Electric motors also have kilowatt ratings. A kilowatt is one thousand watts. Electrical power can also be described as the product of current, measured in amperes or amps, and potential, measured in volts.

Four Kinds of Kinetic Energy #3- Solar energy is the heat and light, or radiant energy produced by the sun. The sun does not burn like a fire does. Nuclear fusion creates the sun's energy. Helium atoms are formed from lighter hydrogen atoms, in the process creating heat, light, and other particles.

Light energy is a form of radiant energy emitted by accelerating electric charges or the electrons of atoms. Light we see, or visible light, is a small portion of electromagnetic waves, which are partly electric and partly magnetic. Light moves through the vacuum of space or through a material as waves. A wave can be described as a back and forth motion or a vibration. 

Light can also move as particles, or photons. Frequency measures how many complete vibrations occur in a given time period, usually a second. Hertz is the unit for frequency, named from Heinrick Hertz who demonstrated radio waves, a type of electromagnetic radiation in 1886. Electromagnetic waves range from the longest, radio waves, to the shortest, gamma rays. The lowest frequency of light that we can see is red and the highest is violet. AM radio waves are broadcast in thousands of hertz, or kilohertz. FM radio waves are broadcast in millions of hertz, or megahertz. Radar and microwave ovens operate in the gigahertz, or billions of hertz range. X-rays and even shorter gamma rays are used in chemotherapy. In a vacuum, all electromagnetic waves travel at the speed of light which is 300,000 km/s.

Heat energy is another form of radiant energy. If you have ever seen a shimmering effect above a paved road or parking lot on a hot day, you have seen heat waves. You can feel heat radiating from a candle flame, a fireplace, or a space heater. Most light bulbs heat up while they operate because the tiny wire inside each bulb, called the filament, heats up and glows.

Four Kinds of Kinetic Energy #4- Acoustic, or sound, energy begins with the vibration of an object. A guitar has strings that vibrate to produce sound. A clarinet has a vibrating reed while a trumpet has a vibrating column of air in the mouthpiece. The human voice has vocal chords. These vibrations start waves in the air. However, sound also needs something to move through, or a medium. There is no sound in outer space since there is nothing for it to travel through. Astronauts outside the space shuttle or the International Space Station use radio waves, a form of electromagnetic radiation, to communicate.

You can use common examples to explain the various kinds of kinetic energy to your child. The many forms of kinetic energy include mechanical, electrical, radiant (heat and light), and sound.

About the Author

I earned my B.S. degree from Penn State in Earth Sciences and my M.S. degree plus teacher's certification from Drexel University. I have 20 years of teaching experience in both public high school and science museum settings.

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can this amp meter be used on a car??

Can you run this on a car to read the amps across an electrical component? If you make the picture big it says 50mv on the face. The component will be running at about 25 amps.
http://www.allelectronics.com/make-a-store/item/PMD-50A/50-AMP-DC-PANEL-METER/-/1.html

The meter itself measures 50 millivolts (0.05 volts). If you read the description, you will see "Shunt required." The shunt is a precision resistor that has a very low resistance but can safely carry 50 amps. When the current through the shunt is 50 amps, the voltage across it is 50 millivolts. Click the "Related Items" tab to see the required shunt. If you look carefully at the picture, you will see a large screw and a small screw on each end. Cut the wire that carries the current that you want to measure. Connect one end of the cut wire to one of the large screws and the other end to the other large screw. Connect the meter to the two small screws. The positive meter lead connects to the terminal that is at the end of the shunt that is in the direction of the positive terminal of the car battery.

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