HVAC Static Pressure

This is what can happen when an HVAC company upsells a customer on a bigger air-conditioner but disregards that the duct system was designed for a smaller system.

An air-conditioner's total external static pressure (TESP) is similar to a human body's blood pressure. The higher the static (or blood) pressure, the harder it is for fluid (blood or air) to flow. In an HVAC system, high static pressure can have serious negative effects, not only on the unit, but also the home's performance, and ultimately, the homeowner's electric bill and comfort.

In the photos below, the A/C unit's test static pressure, or maximum recommended static pressure, is 0.55" w.c., which is calculated by adding the return static pressure to the supply static pressure (disregarding the minus sign).

But this HVAC system is nearly triple that number, even with a clean A/C filter. As the filter gets dirtier, the return static pressure (the negative number) will get even more negative.

The other kicker: This system has 13 percent duct leakage. Unfortunately, most HVAC companies would simply seal the ducts and walk away. But sealing the ducts always causes an increase in static pressures. Sealing more than a half-ton of leakage could increase the supply static pressure to nearly 0.9" and the return static, in this case, to 0.75".

For this system, I recommend against duct-sealing until and unless the static pressure crisis is addressed: Add a return to the master bedroom (an additional breathing tube), and add a "dump duct" in the 2nd-story ceiling above the stairwell to simply bleed pressure into a seldom-occupied area of the home. It's the least-worst compromise in a small home (1400 sq ft) with an A/C system that is too large (4 tons) because of the salesperson's upsell. I noted to the homeowner that the installers didn't even check the static pressure when they installed the unit; I had to drill the tap holes myself.

Be careful who you hire to install your HVAC system. The installers should ALWAYS perform a heat-load calculation, and they should ALWAYS check the static pressure to determine whether the current duct setup can handle an upsized system.