A furnace's rating plate lists a maximum external static pressure of 0.50 in. w.c. The measured total external static pressure is 0.95 in. w.c. What does this tell the technician?
- The blower is moving more air than the duct was designed for
- The duct system is too restrictive and airflow is below design
- The furnace is operating normally within its allowable range
- The supply ducts are leaking and losing pressure to the attic space
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B. The duct system is too restrictive and airflow is below design
Static pressure is the resistance the blower works against. Nearly double the rated maximum means undersized or blocked ducts, a restrictive filter or a dirty coil, and with a PSC blower airflow falls well below design. Leaky ducts lower static pressure rather than raise it.
Reference: Total external static pressure; blower performance
On a system with a total external static pressure of 0.90 in. w.c., the pressure drop across a 1-inch pleated filter measures 0.35 in. w.c. What does this suggest?
- The filter is bypassing air around its frame
- The filter drop is normal for a residential system
- The filter is too coarse and passing too much air
- The filter is too restrictive or too small for the airflow
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D. The filter is too restrictive or too small for the airflow
Filter pressure drop is usually budgeted at around 0.10 to 0.20 in. w.c. A drop of 0.35 in. w.c. uses most of a typical blower's allowance by itself, pointing to a loaded filter or too little filter area. More filter area or a deeper media filter lowers the drop.
Reference: Static pressure diagnosis; component pressure drops
Static pressure measures +0.55 in. w.c. entering a cooling coil and +0.20 in. w.c. leaving it. What is the pressure drop across the coil?
Answer in in. w.c..
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0.35 in. w.c.
Pressure drop = entering pressure - leaving pressure = 0.55 - 0.20 = 0.35 in. w.c.
Reference: Static pressure diagnosis; component pressure drops