#design conditions

Tech Tips:

I’ve been designing a system for a home here in Dallas and noticed a couple of interesting (and disturbing!) things about the equipment’s advertised capacity vs. the actual capacity at design conditions. Equipment is rated at AHRI conditions (95 degrees outside and 80-degree dry bulb, 67-degree wet bulb inside). These conditions make equipment performance LOOK […]

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In this short podcast episode, Bryan covers some common mistakes when measuring static pressure. Static pressure is balloon pressure, not CFM or velocity pressure, but it can be an airflow indicator. The best way to start learning about static pressure is to start measuring total external static pressure.

Gas furnaces and heat pumps are different; static pressure is rated at the furnace in the former (not the cased coil, which is additional), and the static pressure includes the coil in a heat pump. The highest static pressure will always be at the blower, but restrictions on either side of the blower will raise static pressure on both sides of the blower.

Dirty components can drop the total external static pressure, which is a common point of confusion that can lead to misdiagnosis. You’ll want to determine static pressure at the blower and across components. If you want to determine if a cased evaporator coil is clogged, you can measure the static pressure drop across the coil (by measuring below and above the coil) and compare it to the manufacturer’s specifications. Dirty blower wheels can also cause problems, as they don’t move much air in the first place.

Lower static pressure at the blower is the best for amp draw and airflow, and our system designs and accessories can support low static pressure. Dialing in airflow, putting your probes in the right positions when you measure static pressure, proper register and grille selection, and minimally restrictive filters all go a long way.

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