Static pressure calculator
Enter your manometer readings to get total external static pressure, how it compares with the equipment rating, and the friction rate for sizing duct.
Inputs
Result
How to take the readings
- Drill two 3/8 in. test holes: one in the supply plenum between the air handler and the coil, one in the return before the blower but after the filter.
- Run the system on the speed it uses for cooling.
- Read each hole with a manometer and a static pressure probe.
- Add the two readings as absolute values. The return reads negative because the blower pulls on it, but the magnitudes add: 0.42 supply and −0.38 return make 0.80 total.
Compare the total with the maximum external static pressure on the equipment nameplate. Most residential furnaces and air handlers are rated 0.5 in. w.c., and field surveys repeatedly find installed systems running well above that.
What a high reading costs
The blower moves less air than it was designed to. That shows up as rooms that never reach temperature, a coil that freezes in cooling, a heat exchanger that runs hotter than intended, higher bills and a shorter equipment life. It is the single most common fault in residential duct systems, and the equipment usually gets blamed for it.
Whichever side reads higher is where to start. A high return usually means the return is undersized, there are too few return paths, or the filter is restrictive. A high supply usually means undersized trunks, crushed or sagging flex, closed registers or a dirty coil.
Friction rate, for sizing duct
The friction rate is what the duct sizing calculation needs, and it comes from the pressure left over once the equipment and its components have taken their share:
Total effective length is the longest supply run plus the longest return run, with each fitting counted at its equivalent length in feet rather than its physical size. Take the result to the duct size calculator.
Questions
What is a good static pressure reading?
At or below the maximum on the equipment nameplate, which is 0.5 in. w.c. for most residential systems. Under about 0.45 leaves useful margin. Anything above the rating means the blower is moving less air than designed.
Why do I add the return reading if it is negative?
The two readings measure pressure on either side of the blower, and the blower has to overcome both. The return is negative because it is under suction, but the work adds up, so the magnitudes are summed.
Where exactly do I drill the test holes?
Supply side: in the plenum between the air handler and the evaporator coil. Return side: before the blower and after the filter. Taking the return reading before the filter leaves the filter's drop out of the total.
My static pressure is 0.9. What now?
That is roughly double a typical rating. Check the filter first, since a restrictive one-inch filter alone can account for 0.2 to 0.3. Then look at return sizing, which is undersized far more often than supply.
What is total effective length?
The longest supply run plus the longest return run, where each fitting is counted at the equivalent length in straight duct that would cause the same pressure loss. Manual D publishes those equivalent lengths.
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