Tech Page: Air-Side Diagnostics

April 21, 2002
/ Print / Reprints /
ShareMore
/ Text Size+
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.

    DIAGNOSTIC PROCEDURES

    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.

    WHAT TO TEST

    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.

    BEWARE

    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

  • Did you enjoy this article? Click here to subscribe to The NEWS Magazine

    You must login or register in order to post a comment.

    Multimedia

    Videos

    Image Galleries

    2014 MCAA Annual Convention

    Scenes from the 2014 MCAA Annual Convention in Scottsdale, Ariz.

    Podcasts

    NEWSmakers: Julian Scadden

    Training is an ongoing process. Julian will discuss how you can generate maximum return on time and energy invested training by following a three part process. Listen to this podcast to get expert tips on training, tracking and follow up. 

    More Podcasts

    THE MAGAZINE

    ACHRNEWS

    NEWS 04-14-14 cover

    2014 April 14

    Check out the weekly edition of The NEWS today!

    Table Of Contents Subscribe

    SERVICE CALLS POLL

    Which statement on service calls best applies to your business?
    View Results Poll Archive

    HVACR INDUSTRY STORE

    plumbing-hvac.gif
    2014 National Plumbing & HVAC Estimator

    Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

    More Products

    Clear Seas Research

     

    Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

    DON'T MISS A THING

    Magazine image
     
    Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

    STAY CONNECTED

    facebook icontwitter iconyoutube iconLinkedIn i con