Is Your AC Constantly Running? Duct Blockages Could Be the Culprit

At Air Duct Cleaning Spokane in Spokane, WA, we routinely diagnose air conditioners that seem to run forever—and we find that restricted duct airflow is one of the most common, fixable causes.

AC running nonstop? Start with airflow and static pressure

When ducts are clogged or undersized, the blower can’t deliver the design airflow across the evaporator coil. Most central systems are engineered around ~350–450 CFM per ton of cooling depending on climate and latent load, with ~400 CFM per ton as a nominal target. Too little airflow spikes static pressure, lengthens run times, and can even cause coil icing.

Why this matters: air conditioning already accounts for about 12% of household electricity use, so airflow losses translate directly into higher bills and hotter rooms.

What creates duct blockages (and why they overwork your AC)

• Loaded filters and smoke residue. During wildfire season, fine PM2.5 rapidly loads filters and pleated media. Upgrading to MERV 13 (if your system supports it) improves smoke particle capture, and using recirculation helps protect indoor air.

• Crushed or kinked flex duct, closed/broken dampers, collapsed liners. Any of these raise total external static pressure (TESP) above the equipment’s rating plate. Many residential air handlers are rated around 0.50 in. w.c. maximum external static pressure.

• Matted coils and blower debris. Dust and construction particulate choke fins and blades. Industry standards call for cleaning when performance is compromised due to contamination build-up or dirt is discharging into occupied spaces.

Typical homes lose roughly 20–30% of the air that moves through the duct system because of leaks, holes, and poor connections. That shortfall forces longer AC cycles and uneven room temperatures.

Spokane realities: smoke season accelerates airflow problems

Wildfire episodes in Eastern Washington make filter maintenance mission-critical. Regional health and air agencies advise MERV 13 or higher where compatible and running HVAC on recirculate to keep smoke out.

How we pinpoint the culprit—no guesswork

1. Static pressure test (TESP). We drill test ports and compare measured TESP to the unit rating (commonly ~0.50 in. w.c. max). Exceedances indicate restrictions or undersized ducts; we also check filter and coil pressure drops.

   
   

2. Airflow verification. We confirm supply airflow against the 350–450 CFM/ton design range (adjusted for latent load and climate).

   
   

3. Duct leakage testing. With a duct blaster at 25 Pa, we measure CFM25. Programs aligned with energy-efficient new homes often require leakage limits of ≤4 CFM25 per 100 sq ft (or 40 CFM25, whichever is larger), and many codes enforce similar thresholds.

   
   

4. Visual and particulate inspection. Following NADCA ACR criteria, we document contamination that compromises performance or emits visible debris, then prescribe corrective cleaning.

   
   

Fixes that stop the endless run time

• Remove blockages and restore coil/blower cleanliness. Clean based on ACR thresholds; verify post-clean static pressure and airflow.

• Seal and insulate leaky ducts. Sealing can improve HVAC efficiency by as much as ~20%, while typical homes lose 20–30% of conditioned air through duct faults.

• Correct design bottlenecks. Replace crushed flex runs, add/resize returns, and balance branches to bring TESP back within the rating (often ~0.50 in. w.c.).

• Use higher-efficiency filtration without starving airflow. Where equipment can handle it, MERV 13 captures finer particles; during smoke events, keep the fan set to ON/recirculate. Always verify pressure drop and change filters more frequently in wildfire season.

Cost and incentives snapshot (Spokane, WA)

• Local rebates: Utility programs in the Spokane area (such as Avista) periodically offer residential efficiency rebates; offerings vary by year. Check current Spokane-area energy rebates.

• Federal tax credits: The Energy Efficient Home Improvement Credit (25C) provides 30% credits (annual caps apply) for qualifying air-sealing/insulation upgrades that reduce leakage and loads.

Maintenance schedule that prevents repeat issues

• Filters: Inspect monthly in summer and during smoke advisories; install the deepest pleat your rack allows and aim for MERV 13 if the blower can maintain airflow.

• Annual test-in/test-out: Check TESP and coil/filter pressure drops at the start of cooling season; verify airflow after any filter upgrade.

• Ducts: Re-inspect after renovations or any dust-heavy project; clean when contamination compromises performance.

CONCLUSION

When an AC won’t cycle off, airflow is the first place we look. By measuring static pressure, verifying CFM/ton, testing for duct leakage, and correcting blockages or leaks, we restore design airflow—and with it, shorter run times, lower bills, and better comfort.

Get practical tips to keep your indoor air healthy during Spokane fire advisories in this guide.

FAQs

1) How do I know if duct blockage—not low refrigerant—is making my AC run nonstop? 

Tell-tales include weak supply airflow, unusually high static pressure, hot/cold rooms, and filters that load rapidly (especially during smoke events). A quick TESP reading against the rating plate and a duct leakage test isolate airflow faults before refrigerant work.

2) Will sealing ducts really make a difference if my equipment is old? 

Yes. Even with older systems, sealing and insulating ducts commonly recovers up to ~20% efficiency and improves comfort; many homes also lose 20–30% of conditioned air through duct faults.

3) Is a MERV 13 filter safe for my system? 

Often, yes—but only if the blower can maintain target airflow and TESP stays within the unit’s max (commonly ~0.50 in. w.c.). We measure TESP and adjust fan speed or return area as needed; during smoke, MERV 13 (where compatible) and recirculation are recommended.