Bert teaches a practical training class on manometers. In air conditioning applications, we commonly use manometers to measure static pressure, but they may also measure gas pressure in furnaces and pool heaters.
Static pressure refers to the pressure acting on the inner surfaces of the ducts in HVAC systems, and it is affected by airflow resistance; we can think of static pressure as an indicator of airflow. The filter and the coil are also sources of airflow resistance, so we often see static pressure drops across those components. Static pressure may either push against the ductwork (positive, on the supply side) or pull against it (negative, on the return side). Manometers measure this static pressure in inches of water column.
We can find the total external static pressure (TESP) by placing one manometer probe above the filter and another just above the blower and adding the readings to see how much airflow resistance that blower has to overcome. You can compare the measurement with the TESP on the data tag, which shows the static pressure conditions under which the unit was tested by the manufacturer.
The MERV rating on the filter can affect your return static pressure reading; higher MERV ratings can reduce the static pressure on the return side; we can test the actual resistance by measuring static pressure across both sides of the filter: in the return and between the filter and the coil. Dirty coils or filters can reduce the supply static pressure and will make the TESP low if you measure static pressure before the filter in the return, which can be a misleading airflow indicator. Evaporator coils will always cause a pressure drop, but clean coils usually cause a pressure drop of less than 0.1 inches of water column, so a large amount of soil can increase that pressure drop significantly.
Manometers have pressure probes with holes on the side; the probes should point in the opposite direction of airflow to yield the most accurate readings. When using a manometer as a diagnostic tool, you can take readings with and without the filter to see how much it may be contributing to the static pressure drop. We typically want to see a static pressure of less than or around 0.5" WC; once we reach 0.8" WC or higher, we start to see reduced blower motor efficiency, shorter blower lifespans, and more issues. You can also use manometers to locate duct restrictions; collapsed ducts will cause the static pressure to skyrocket as the airflow reaches the restriction, but visual inspections may be more useful if you suspect that the ducts may have collapsed.
If you want to figure out the static pressure drop across the coil and place your manometer probe before the filter or immediately before the blower (above the coil), you will have negative static pressure. Some diagnostic apps may have trouble calculating TESP readings in those locations, so you may have to do the math yourself to figure out the pressure drop across the coil.
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