High Static Pressure and Closed-Door Bedrooms
A 2,000 sq ft single-story house has a 3-ton air handler rated at 1,200 CFM. Measured external static pressure is 0.90 in. wc: 0.65 in. wc supply and 0.25 in. wc return. The filter is a 20 by 25 by 1 in. MERV 13 in a return grille. Several bedrooms have no return and doors are normally closed. The homeowner complains of dust and hot bedrooms. A blower-door test shows 4.5 ACH50.
1. Interpret the static pressure readings and identify the most likely air-distribution problems.
Read the worked answer
A total external static of 0.90 in. wc is high for a typical residential air handler; many blower tables show 0.50 to 0.80 in. wc as the usable range, and 0.90 is at or beyond the edge. The split shows the supply side is the larger problem at 0.65 in. wc, but the return side at 0.25 in. wc is also significant. A 1 in. MERV 13 filter in a single return grille can add substantial pressure drop and should be checked against the blower table. High static reduces airflow, raises supply-air temperature, and can cause the evaporator to run cold or freeze.
The closed-door bedrooms with no return path are a separate problem: supply air enters but cannot return, pressurizing the room and forcing air out through envelope leaks. That contributes to dust, poor mixing, and comfort complaints.
2. What corrective measures would you recommend, and in what order?
Read the worked answer
First verify airflow and static with a clean filter and all registers open. Then reduce restriction: increase return capacity, add a properly sized filter grille or media cabinet with lower pressure drop, and correct undersized or restrictive fittings on the supply side. Check the blower speed against the equipment table after duct changes; do not simply raise speed to overcome a bad duct system. Provide a return path for each closed bedroom using a transfer grille, jump duct, or return duct sized for the airflow. After changes, re-measure static and room pressures to confirm the system is within the blower table and the bedrooms are not strongly pressurized.
3. What ventilation and filtration recommendations are reasonable for this house?
Read the worked answer
A 4.5 ACH50 blower-door result suggests a relatively tight house, so the system should not rely on uncontrolled infiltration for outdoor air. Recommend mechanical ventilation sized to the applicable residential ventilation standard, such as a properly sized exhaust or balanced ERV/HRV, and verify that it does not create unacceptable depressurization. Keep MERV 13 filtration only if the pressure drop is acceptable to the blower and the filter is serviced; otherwise use a media filter with adequate face area. Correcting return paths and static comes before adding more filtration, because a high-static system will not deliver the intended airflow.