Why Is My Heat Pump Not Cooling on Hot Days?

A heat pump not cooling on hot days usually means airflow is restricted or the system is at its capacity limit in extreme heat. The most common culprit is a dirty outdoor coil or a clogged air filter that chokes airflow and makes the unit fall behind. Thermostat settings, low refrigerant, or duct issues can also reduce cooling, and even a healthy, properly charged unit can struggle during peak afternoon spikes. We see a surge of these calls in the Seattle area during short, intense heat waves. On many midsummer visits, I find the outdoor coil packed with cottonwood fluff that looks clean until you shine a flashlight through it.

• Check these quick items before scheduling service:
• Set thermostat to Cool and fan to Auto; lower the setpoint 3–5° and wait 15 minutes.
• Replace a dirty filter; gently rinse the outdoor coil from the inside out if accessible.
• Clear 2 feet of debris around the unit and keep sun load off the condenser where possible.
• If you see ice on lines, frequent breaker trips, or no temp drop at vents, stop and call a pro.

Turn the system off to thaw if you see ice and avoid opening electrical panels yourself. <br/> Updated: 2026-04-04 <br/> Need help in Seattle, WA? Call (425) 616-3318 or book online. “Brandon is a very knowledgeable and skilled professional… Highly recommend Brandon and the entire Fast team..” — C. Wolf.