Air-source heat pump diagnosis

Confirm heat vs cool vs emergency heat at the thermostat. Measure control voltage at the outdoor unit. Many split heat pumps use dedicated conductors (O/B for reversing, W for aux, Y for compressor, G for fan). A miswired O/B leaves the valve in the wrong season forever.

Indoor filter, coil cleanliness, blower speed, and duct static kill capacity in both modes. Outdoor coil dirt or cottonwood fluff destroys winter evaporation. Fix air first; pressures lie when coils cannot breathe.

Pull OEM capacity tables. At low outdoor temperatures, reduced capacity is design. Compare building loss (from earlier lessons) to unit capacity: if you are below the balance point, strips should stage. Absence of aux when needed feels like a broken heat pump.

Do not take steady-state gauges during active defrost. Note ice pattern (even sheet vs wind-driven one-sided), steam, fan status, and whether indoor heat is supplemented.

Charging methods differ by OEM and by mode (weigh-in, subcooling, heating charts, etc.). A system charged “by cooling superheat habit” in winter can be wrong. Check for:

• noncondensables or restriction symptoms that appear differently per mode,
• reversing valve bypass (temperatures on false ports),
• compressor mechanical issues only after the layers above are clean.

Package heat pumps put indoor and outdoor sections in one cabinet — same physics, fewer line-set variables, still need clear condenser air and correct duct connections. Variable-speed and scroll designs change sound and staging; use OEM sequences, not 1990s fixed-speed folklore alone.

In cooling you compare to familiar A/C charts. In heating, the outdoor coil is the evaporator, so suction pressure tracks a cold outdoor coil — often much lower than summer suction. Using summer “normal” numbers in January creates false low-charge diagnoses.

Prefer recovery and weigh-in when the history is messy. If OEM allows heating-mode charts or subcooling methods, follow them exactly, including required indoor fan speed and outdoor temperature windows. Weigh the cylinder; do not “add until the vapor line sweats.”

Verify capacitor health on outdoor fan and compressor, contactor faces, and defrost board outputs. A weak outdoor fan in winter looks like a refrigerant problem because the outdoor evaporator cannot absorb heat without air.

Record outdoor dry-bulb, mode, supply/return temperatures, which OEM chart you used, and strip amps. Heat-pump callbacks shrink when the file shows facts instead of “added gas.”

Print the OEM sequence: what energizes in heat stage 1, stage 2, defrost, and cool. Test the unit against that script. Random jumper tests without a sequence create new faults.

Some systems lock out the heat pump below a chosen outdoor temperature and fire a furnace. A “heat pump not running” at 5°F may be intentional dual-fuel lockout. Read the control strategy before parts cannon.

### Field memory hooks

Write the key temperatures, flows, or mode checks for this topic on your job notes. Teach the helper beside you to repeat the layered order aloud before tools come out. Most callbacks shrink when the first visit follows a written order instead of a parts guess.

When OEM data exists — capacity tables, wiring diagrams, water-flow charts — photograph the rating plate and the page you used. Future you (or the next tech) should not have to rediscover the same facts under pressure.

Situation. Winter no-heat call. Outdoor 18°F. Compressor runs, supply ~85°F, strips never energize, house falls through the night. Prior tech “added a pound” in heat mode without scales.

What to think. 85°F supply can be plausible for the heat pump alone at 18°F, but the building likely needs aux. Why is W not calling or strips open? Also verify whether the random charge add masked or created a problem — recover/weigh to factory when unsure.

Conclusion. Controls and balance-point expectations first; sealed-system last with OEM procedure.

### Teach-back for the helper

Before leaving the topic, have the helper explain in their own words: what is normal, what is measured first, and what must never be skipped for safety. If they cannot teach it back, review the main “How it works” blocks again with a sketch on paper.

Connect this lesson to the previous one and the next one as a chain. Field skill is sequence memory — not isolated trivia. Keep OEM charts and job photos with the work order so the next visit starts smarter.

### Common measurement habits

Use the same meter locations every time so readings compare day to day. Steady the system before judging pressures or temperatures — mid-shift and mid-defrost lie. Write ambient conditions on every note page.

If a reading disagrees with the control display, believe the calibrated handheld until sensors are proven. Controls cannot fix what they cannot see accurately.

### Safety and professionalism

Apply LOTO where energy can hurt you. Wear eye protection around refrigerants, chemicals, and coil fins. On roofs and at window units, respect fall hazards. Explain findings to the owner without jargon soup — clear next steps beat impressive vocabulary.

Dispose of oil, filters, and refrigerant legally. Cutting corners on recovery or tower chemicals is not “saving the customer money”; it is transferring risk.