Heating and cooling systems tend to decline before they stop, and the signs are usually noticeable months in advance if you know what they look like.
Stop immediately for these
A burning smell that does not clear. Brief dust burn-off when heating starts for the season is normal. A persistent burning smell is not — shut the system down and call.
Any smell of gas. Leave, then call from outside. Do not operate switches.
A carbon monoxide alarm. Get everyone out and call emergency services. Do not attempt to identify the source yourself.
These are not monitoring situations.
Noises
Systems make consistent noise. Changes in that noise are informative.
Grinding or scraping usually indicates a motor or bearing problem. Banging on startup can indicate delayed ignition on gas equipment, which is worth attention. Persistent rattling may be something loose, which is minor, or something failing, which is not. Squealing frequently points to a belt or motor issue.
The useful signal is change rather than the specific noise. A system that has started sounding different is telling you something.
Short cycling
Turning on and off frequently rather than running steady cycles. Causes range from a restricted filter to an oversized system to a control fault.
It matters because short cycling wears equipment faster than steady running and conditions the house poorly. If a system has always done this, it is likely oversized — which is worth knowing before you replace it with another oversized one.
Running longer for the same result
One of the earliest signs and the easiest to miss, because it happens gradually. The system runs longer to reach the same temperature, or never quite reaches it.
Frequently this is airflow — a restricted filter, blocked returns, or leaking ductwork. Sometimes it is refrigerant charge on a heat pump. Occasionally it is a component in decline.
Uneven temperatures
Rooms that were always slightly different are a design issue. Rooms that have recently become noticeably different indicate something has changed — commonly a duct disconnection, which is a genuine possibility where ducts run through Washington crawl spaces.
Rising energy use
Compare against the same period in previous years rather than against last month, since seasons obviously differ. A meaningful year-on-year increase with no change in behaviour or occupancy points at the system.
Humidity behaving oddly
West of the Cascades, an air conditioning or heat pump system in cooling mode should remove moisture. If the house feels clammy while the system runs, that is worth investigating — frequently oversizing, since a system that cools quickly and shuts off does not run long enough to dehumidify.
Repairs clustering
Two calls in a season, or repairs in consecutive years, indicates decline rather than coincidence. Components of similar age that have worked under identical conditions tend to fail in sequence.
Keep a note of what you have spent over three years. That total is often more persuasive than any individual quote.
Ice on a heat pump
Some frost on the outdoor unit in cold weather is normal, and the defrost cycle handles it. Heavy ice that persists, or ice that never clears, indicates a defrost or refrigerant problem and is worth a call.