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:
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.
Here are some of the test instruments and tools employed in these procedures:
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.
Rothacker is a director for area51hvac.com. For questions or comments on the Tech Page, contact Rothacker at firstname.lastname@example.org (e-mail).
Publication date: 04/22/2002