Safe Checks You Can Do Right Now
Start with the thermostat. Check that the system mode is set to what you expect: heat for a cold day, cool for a warm one, not "fan only" or "off." If your thermostat runs on batteries, replace them regardless of how recently you think you changed them — low battery voltage causes erratic behavior that mimics a broken system. If your thermostat has a programmed schedule, scroll through it and confirm the setpoint is actually higher or lower than the current room temperature. A thermostat calling for 68 degrees in a 69-degree room will not run.
Next, go to your electrical panel. A tripped breaker looks almost on — the toggle sits in a middle position rather than snapping fully off. Reset it by pushing firmly to off first, then back to on. HVAC systems often have more than one breaker: one for the air handler or furnace inside, and a separate disconnect for the outdoor unit. Both need to be on. If the breaker trips again within a few minutes, stop and call a technician — a breaker that keeps tripping is protecting you from something.
Gas furnaces have a door panel over the burner compartment, and most have a safety switch that cuts power when the door is not fully seated. Push it firmly closed until it clicks, then try the system again. While you are there, find the furnace filter. Filters can clog faster than expected in PNW homes, especially during wildfire smoke season. A filter so choked with dust that you cannot see light through it will cause the system to shut itself off on a safety limit. Slide it out and replace it if it looks packed — filters are inexpensive and available at most hardware stores.
Walk through the house and confirm all supply and return vents are open and unobstructed. Closing vents in unused rooms pressurizes the duct system and can cause equipment to overheat or ice over. Move furniture or boxes away from any covered vent. Then check the outdoor unit. In summer, clear leaves or shrubs blocking airflow around it. In winter, a thin layer of frost on a heat pump's outdoor unit is normal; a solid block of ice is not, and the system may have entered a protective fault state.
If you have a high-efficiency furnace or mini-split with a condensate drain, find the drain line near the air handler. A float switch shuts the system down when the condensate pan fills. A puddle nearby or standing water in the pan points to this issue. Note what you see — it is useful information for the technician even if you do not clear the drain yourself. General background on how these systems work is at https://en.wikipedia.org/wiki/HVAC.
What Not to Touch
Leave refrigerant lines, gas connections, and electrical wiring alone. If you smell gas, leave the house without operating any switches and call your gas utility from outside. Refrigerant work requires licensed technicians and specialized equipment. Wiring inside the air handler or furnace controls is live. Do not remove panels beyond the standard access door, and do not attempt to thaw a frozen outdoor unit with a heat gun or hot water.
What to Tell the Technician
When you do call for service, the information you have gathered saves time and money. Note how long the problem has been present and whether it came on suddenly or gradually. Write down any error codes or blinking light sequences on the furnace or air handler — most units have a lookup chart inside the access panel. Describe any sounds: grinding, clicking on startup, short cycling, or complete silence when it should be running. Mention whether the outdoor unit is running, whether the filter was dirty, and whether the thermostat display is lit. A technician with this picture in hand can diagnose faster and often brings the right parts.
One More Thing for Pacific Northwest Homes
Many homes in western Oregon and Washington rely on heat pumps rather than gas furnaces. Heat pumps have a normal defrost cycle where the outdoor unit frosts briefly and then clears itself. If yours has been frosted solid for more than a couple of hours and the system is blowing cool or room-temperature air inside, the defrost function has likely failed. Note how long the frost has been there and the outdoor temperature when you call — that context helps the technician prioritize and arrive prepared. For homeowners in Troutdale, that service area is covered at https://sites.google.com/view/efficiency-heating-cooling/service-areas/troutdale.