Picture in your mind’s eye a sprinkler head hooked up to a garden hose. The sprinkler is getting a good supply of water and is reaching all parts of the lawn — that is, until little Bobby comes along.

Little Bobby has discovered that when you poke a hole in a garden hose, you can begin sprinkling other things. When you poke 50 holes, you can sprinkle a lot of other things. And when you place a rock on the hose, you don’t get as much water coming out of all the holes.

Now, just what do you suppose is the net effect of little Bobby’s experiments on the sprinkler head that is trying to water the lawn? It’s not getting much water, is it?

Now picture little Bobby taking his show on the road. This time, instead of a garden hose, Bobby has worked his magic on residential ductwork systems throughout the country. And he has done a bit more than just poke holes.

Here are some of the items we would encounter on little Bobby’s destructive tour of the states:

  • Undersized/oversized ductwork;
  • Dirty air delivery systems;
  • Ductwork air leakage;
  • Broken/smashed ductwork;
  • Plugged evaporator coils;
  • Mismatched air conditioning equipment;
  • Poor ductwork design;
  • Excessive temperature swings throughout the house;
  • Blockage/restriction in ductwork;
  • Improper air filters.

    Amazing, isn’t it? But just how busy could Bobby have possibly been? He couldn’t have gotten to that many houses.

    You might be surprised. According to the National Comfort Institute, nearly 80% of residential hvac systems have problems affecting comfort and efficiency — and these problems have gone undiagnosed.


    Air-side diagnostics are systematic procedures taken to ensure that the proper amount of air is reaching its intended destination and the proper amount of air is returning to the system fan side.

    Here are some of the test instruments and tools employed in these procedures:

  • Balometer — Commonly known as a flow hood, this instrument measures the cubic feet per minute (cfm) of air at diffusers and registers.

  • Anemometer — Measures the velocity of airflow. The measurement is read in feet per minute (fpm) of air.

  • Hygrometer — Measures relative humidity.

  • U-tube manometer — Liquid-filled, U-shaped instrument that measures a pressure differential.

  • Pitot tube — Tubing inserted into ductwork to collect pressure measurements and transfer them to a manometer or air velocity gauge.

  • Magnehelic® gauge — A gauge type of manometer (made by Dwyer Instruments) used to measure air pressure.

  • Inclined manometer — A manometer that measures in the lower pressure ranges with an increased degree of sensitivity.

  • Static pressure tips — Attach to a manometer or differential pressure gauge to measure static pressures.

  • Thermometer — An instrument that measures temperatures.

  • Manual J — The Air Conditioning Contractors of America’s (ACCA’s) residential load calculation manual.

  • Manual D — ACCA’s residential duct design manual.

  • Manual S — ACCA’s residential equipment selection manual.


    Load calculations, correct ductwork and equipment sizing, and air balancing, in addition to equipment maintenance, are all necessary procedures. For our discussion today, we will look at arguably the most important factor in air-side diagnostics — airflow pressures.

    Three types of pressure readings are usually taken:

    1. Velocity pressure — The difference between total and static pressures.

    2. Static pressure — The force of pressure against the inside of an air duct. Measured in inches of water column (in. wc), the source of this pressure is the system fan.

    3. Total pressure — A combination of static and velocity pressures.

    System static pressure is a combination of air pressure measured at the outlet side of the fan (positive pressure) and air pressure measured at the inlet side of the fan (negative pressure). Typically, manufacturers design their equipment to run at 0.5 in. wc.

    For the system to operate correctly, the components (which consist of the air filter, dampers, registers, evaporator coil, ductwork, and fittings) cannot have a total pressure drop of more than 0.5 in. wc. When the total system static pressures exceed 0.5 in. wc, there is usually a blockage or restriction.

    If the total system static pressure is too low, often leaking or incorrectly sized ductwork is the problem. (See Table 1.)

    Let’s return for a moment to little Bobby. If he did not properly install an air filter, unfiltered air would pummel the evaporator coil with dirt. Eventually the coil would plug up, creating high static pressures. If sneaky little Bobby cut holes in the ductwork, airflow would diminish, leaving a low static pressure condition.


    Little Bobby has been busy indeed. The next time you approach your customer’s system, think about little Bobby — an arch nemesis of the industry — and the places on which he has wreaked havoc.

    Rothacker is a director for area51hvac.com. For questions or comments on the Tech Page, contact Rothacker at ewizaard@hotmail.com (e-mail).

    Publication date: 04/22/2002