System Performance Checks

Some technicians cannot perform effective system performance checks because they don’t know the ins and outs of their tools. These technicians fail to maximize their tools’ potential by not knowing where to measure, how to measure, or what their tools can do.

Static pressure measurements are vital for system performance checks because of static pressure’s relationship with airflow. Static pressure is NOT airflow; it is the force exerted in all directions in the ductwork. However, misuse of the manometer is common and leads to inaccurate airflow calculations. Technicians must understand what they must compare their static pressure measurement to. They need access to the blower chart from the manufacturer AND the specifications. Airflow should be measured BEFORE the technician checks the charge.

According to RSES, your typical A/C unit will have an airflow of 400 CFM/ton. You can expect 450 CFM/ton for a heat pump or a high sensible heat load, and you can expect 350 CFM/ton when there is lots of outdoor air coming in or when the latent heat load is high. The best way to take a measurement is to use a flow hood, vane anemometer, TrueFlow grid, or manometer and blower chart to measure the airflow and use these CFM/ton values to verify the airflow. However, these values vary with humidity, altitude, and barometric pressure.

When it comes to understanding airflow values, a good starting point is for technicians to understand how the basics of heat transfer work with airflow, especially when it comes to the intricacies of sensible and latent heat loads.

A system measurement of airflow differs from a distribution/point method of measuring airflow. Total system airflow measures the total volume going through the air handler (or furnace). You can add up the airflow of all the supply registers, but accuracy can be an issue. You also cannot account for leakage if you add up the supply registers.

External static pressure is usually measured with a diaphragm-type pressure gauge. These come in electronic or Magnehelic varieties. When it comes to the tools that measure static pressure, you must know how to use them. If the tool says that it measures a percentage of the full scale, you must account for that. If you take the measurement at face value without accounting for resolution or scale, you will get a highly inaccurate measurement. Also, due to the nature of pitot tubes, you will need to take multiple duct measurements. You can also use manometers for blower door tests to measure duct leakage, and you need a very high-quality manometer for that.

On the refrigerant side, “beer can cold” is an outdated way to measure when you have tools that can take accurate superheat and subcool readings. Superheat and subcool readings help you determine your charge and can give you some insight into the unit’s efficiency.

While the refrigerant charge is important for system performance checks, technicians generally seem to have a better grasp of it than airflow.

Remember, just because an instrument gives you a value, that doesn’t mean that the value is your exact measurement. Applications such as MeasureQuick also help technicians interpret their readings and collect more accurate data during performance checks.

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