Air-source heat pump: cycle and reverse

Hardware looks like a conventional split: indoor coil and blower, outdoor coil, compressor, and fan, plus metering device(s). The heat pump adds a four-way reversing valve, defrost logic, and often auxiliary heat.

Your job: confirm correct mode, airflow on both sides, and pressures/temperatures that match the active mode.

Familiar behavior:

• Indoor coil evaporates — absorbs heat from return air.
• Compressor raises vapor pressure and temperature.
• Outdoor coil condenses — rejects heat to outdoor air.
• Metering device drops pressure into the evaporator.

Touch cues: suction line to the indoor coil is cold; liquid line behavior follows normal A/C expectations. Outdoor air leaving the condenser is warmer than ambient.

Roles reverse:

• Outdoor coil evaporates — absorbs heat from outdoor air (even when air feels “cold,” it still contains usable heat until conditions get extreme).
• Indoor coil condenses — rejects heat into the supply air.
• The large vapor line to the indoor unit runs hot in heat mode (it can approach very high temperatures — use care when touching).

Capacity falls as outdoor temperature falls. That is physics, not always a failed compressor.

The reversing valve redirects discharge gas so the “hot” side becomes the indoor coil in winter and the outdoor coil in summer. Metering may use dual devices, check valves, or bi-flow TXVs so liquid can feed the correct coil in each mode.

• COP (coefficient of performance): heat delivered divided by energy input (as heat). Higher is better.
• Seasonal ratings such as HSPF (heating) and SEER (cooling) compare systems under standardized test conditions — useful for sales talk, not a substitute for field temperatures and airflow.

The outdoor temperature where heat-pump capacity equals building heat loss is the balance point. Below it, auxiliary heat (usually electric strips or fossil backup) must help. Selling a heat pump without explaining that curve creates “it never heats” callbacks.

A window A/C bolted in reverse would heat the room by rejecting condenser heat indoors. A heat pump does that on purpose with a valve instead of flipping the cabinet.

Many heat pumps use a suction-line accumulator to protect the compressor when liquid can return during mode changes or defrost. Metering may be a single bi-flow device or a pair of devices with check valves. If someone replaced “the TXV” without understanding the heat-pump piping, cooling or heating can fail alone.

Split systems separate indoor and outdoor sections with a line set. Package air-to-air units keep both coils in one outdoor cabinet and duct conditioned air through the roof or wall. Cycle physics is the same; only access and airflow paths change.

In heating, the large vapor line toward the indoor coil carries hot gas after the valve shift. Careful touch (it can be very hot) is a fast mode confirmation. In cooling, that same physical line behaves differently. Mode awareness prevents wrong gauge interpretations.

Heating mode often needs the same or carefully specified airflow as cooling. Restrictive filters and undersized ducts raise liquid pressure on the indoor condenser in winter and cut capacity. Measure static pressure; do not assume the blower tap from the cooling install is automatically correct for heat-pump heating.

Heat pumps modulate and defrost; they do not sound like furnaces. Prepare owners for outdoor unit sound signatures and occasional defrost whooshes so “noise complaint” does not become a false sealed-system call.

### 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. Customer says “heat pump blows cold.” Outdoor is 42°F. Supply air is only modestly warm. Thermostat is in heat; outdoor unit runs; reversing valve is energized per OEM (or not — verify that brand’s convention). Auxiliary heat is off.

What to think. At 42°F many air-source units deliver supply air that feels cooler than a gas furnace. Check mode, airflow, and OEM temperature-rise expectations before condemning the sealed system.

Conclusion. Compare against heating-mode charts, not cooling-mode habits.

### 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.