Static pressure calculator

How it works

Total external static pressure (TESP) is the pressure the blower works against outside its own cabinet. The tool adds the size of the two readings and ignores the sign:

TESP = |supply static| + |return static|

Goodman’s furnace instructions (IOG-2023C) give the sign convention: the return reading at the furnace inlet is negative, the supply reading is positive, and TESP is the difference. Their example is −0.1 in. w.c. return and +0.3 supply, so 0.4 in. w.c. total. Adding absolute values gives the same answer.

The inputs:

• Supply static (in. w.c.), in the supply plenum on the leaving side of the equipment.
• Return static (in. w.c.), between the filter and the blower. Enter it with or without the minus sign.
• Rated external static from the rating plate or blower table.
• Filter drop and coil drop (optional), each measured across the component.

Where to put the test ports

Manuals differ, so follow the one for the unit in front of you.

• Gas furnace with add-on coil. Goodman counts cooling coils, filters, ducts, grilles and registers as external. It reads the negative side between the filter and the blower, and says that when a coil or electronic air cleaner is installed the readings must include it. Its text places the supply port at the coil’s A-shaped block-off plate.
• Air handler. NCI’s David Richardson, writing in ACHR News, uses the “as-shipped” rule: the factory coil is part of the air handler, so ports go on the leaving side of the filter and in the supply duct after the unit. He treats the filter as external and warns against measuring across the electric heater.
• General practice. BPA’s PTCS guide says most manufacturer fan tables call for return static between the filter and air handler and supply static in the plenum far enough from the unit to avoid turbulence. It requires a static pressure probe pointing into the airflow and says not to drill into the cabinet.

Filters in the table. Carrier’s 59TP7A air delivery table is “with filter”, based on 3/4 in washable media; without that filter, Carrier says to assume another 0.1 in. w.c. available. BPA warns that a filter drop printed by the manufacturer is often lower than the filter you actually use.

Coil drop. Lennox’s CHX35 coil instructions measure the drop from the air entry side to the air exit side of the coil and list it dry and wet. For the -36A cabinet at 1,200 CFM it is 0.30 in. w.g. dry and 0.39 wet.

Worked example

The tool opens at +0.45 supply and −0.35 return with a 0.50 rating.

1. TESP = 0.45 + 0.35 = 0.80 in. w.c.
2. 0.80 ÷ 0.50 = 160% of the rating.

Add the filter and coil drops from ACCA’s Manual D example: filter 0.18 and DX coil 0.23. Those add to 0.41 in. w.c., 51% of the total, leaving 0.39 for the ducts, grilles and registers.

Reading airflow works the same way at any TESP. BPA’s example: return 0.28 (downstream of the filter) plus supply 0.34 = 0.62 in. w.c., rounded to the 0.60 column. On the medium speed row of that table the blower moves 1,078 CFM, which is 359 CFM per ton on a 3-ton heat pump.

Reading the result

Within the rating. Read the airflow from the blower table at your TESP and speed tap, then check it against the manufacturer’s airflow target. Lennox, for example, asks for 400 ± 50 CFM per 12,000 Btuh with a wet coil. BPA notes many tables are for a dry coil, so with a wet coil airflow may be lower than the table shows. Cross-check with temperature rise or drop in the airflow calculator.

Above the rating. Goodman says too much external static causes insufficient air, which can mean excessive temperature rise, limit tripping and heat exchanger failure. If TESP exceeds the rating-plate maximum, it says to check for closed dampers, registers, and undersized or poorly laid out duct. Carrier’s temperature-rise procedure says to check for restrictions causing more than 0.5 in. w.c. BPA says most manufacturers suggest 0.5 in. w.c. or less; PTCS allows up to 0.8.

Charging. CEC’s superheat correction rule assumes indoor airflow is right, so fix static problems before you use the target superheat calculator.

Design link. Manual D starts from the blower’s external static at design CFM, subtracts device losses (coil, filter, and 0.03 in. w.c. each for supply outlet, return grille and balancing damper) to get available static pressure, then divides by total effective length to get the friction rate used in the duct sizing calculator.

Common mistakes

• Subtracting the readings as signed numbers the wrong way. 0.45 and −0.35 make 0.80, not 0.10.
• Ports on the wrong side of the filter or coil, so a component is counted twice or not at all. Enter a filter or coil drop only if that component sits between your two ports.
• Measuring without a static pressure probe. BPA says readings without one are not valid.
• Reading with the blower door off. Carrier says this changes duct static and airflow.
• Comparing a wet-coil reading with a dry-coil table and assuming the airflow is right.
• Mixing units. Use the pressure converter for pascals (BPA lists 0.5 in. w.c. as 125 Pa).