Every furnace and air handler ships with a blower performance table in its installation manual, and it is the most useful document in the equipment.
What the table contains
Rows for blower speed setting — the tap on a PSC motor, or the selection on an ECM.
Columns for external static pressure, usually from 0.1 up to 0.8 or 1.0 in. w.c.
Each cell gives the airflow in CFM the blower will deliver at that speed against that pressure.
Using it
Measure the total external static pressure with a manometer. Note the blower speed setting. Read the cell.
That is what the system is actually moving, and it can be compared against what it should move — roughly 400 CFM per ton in cooling, or the airflow that produces a temperature rise inside the furnace's nameplate range in heating.
The gap between the two is the size of the problem, expressed as a number rather than an impression.
What the table shows about high static pressure
Read across a row and the airflow falls as pressure rises. That is the fan curve, and it is why static pressure matters.
A blower delivering 1,200 CFM at 0.5 in. w.c. may deliver 900 at 0.9. The equipment has not changed; the duct system has taken a third of the air away.
Sizing up the equipment does not fix this, because the larger blower is on the same duct system and lands in the same place on its own curve.
PSC motors and speed taps
A PSC blower has several speed taps, and one is selected at installation by moving a wire.
Two things go wrong. The tap is left at the factory default rather than chosen for the system, and the tap chosen for heating is wrong for cooling — heating and cooling want different airflows and many installations use the same speed for both.
Changing a tap is a few minutes' work, free, and it is one of the more common corrections available on an existing system.
ECM motors
Constant-torque ECMs behave like a PSC with better efficiency: the setting selects a torque and the airflow still falls as pressure rises.
Constant-airflow ECMs hold a target CFM by increasing power as pressure rises. The airflow stays where it should and the restriction shows up as an electricity bill and, eventually, as a motor working near its limit.
That is the important distinction when reading a fan table for an ECM: some tables give CFM against pressure, others give the programmed CFM and a maximum pressure beyond which it can no longer be maintained.
Setting airflow deliberately
Cooling normally wants around 400 CFM per ton, lower in humid climates for better dehumidification, higher in dry ones.
Heating wants whatever produces a temperature rise within the furnace's nameplate range.
Continuous fan wants a low speed, which on an ECM costs very little.
Most equipment allows all three to be set separately, and most installations set none of them.
What to ask for
The blower speed setting selected for heating and for cooling, the measured static pressure, and the resulting CFM read from the table.
Three lines on a commissioning sheet, taken from a document that came in the box, and they describe whether the system is delivering what it was sold as.
Where to find the table
Inside the blower compartment door, on a label, on most residential equipment. Failing that, in the installation manual, which manufacturers publish freely by model number.
It is worth photographing the label when the panel is off, because it is the document a future technician will want and it is behind a screwed panel in a basement.
The temperature rise cross-check
In heating, the fan table and the temperature rise measurement should agree.
Airflow read from the table, the furnace's output BTU and the measured temperature rise are related, so a rise near the top of the nameplate range and a table reading showing adequate airflow is a contradiction — and it usually means the static pressure was measured in the wrong place, most often on the wrong side of the coil or the filter.