What it is

Heat pumps generate the most head-scratching diagnostics in the trade. A straight-cool system has one mode and a predictable refrigerant flow. A heat pump runs in both directions, has a reversing valve that can fail in ways that partially scramble operation, runs a defrost cycle that briefly reverses things on purpose, and stacks electric aux heat on top. When something's wrong, the symptoms can look contradictory — "weak in heat and cool," "high head in one mode but not the other," "warm air that's not warm enough" — and a tech raised on straight-cool logic gets lost.

The way out is to stop treating the symptoms as separate problems and find the one component condition that explains all of them. On a heat pump, a single fault — especially a reversing valve that bleeds through, or a charge issue that reads differently in each mode — can produce a whole spread of confusing readings.

How it works

Two things make heat pumps confusing, and understanding them is the whole game:

1. The reversing valve can scramble everything. The reversing valve routes hot discharge gas either to the indoor coil (heat) or outdoor coil (cool). When it fails partway — a "bleed-through" or internal leak — hot discharge gas leaks across to the suction side internally. The result: the system can't build a proper temperature difference. You see low head, high suction, low capacity in BOTH modes, warm-ish air that's never quite right, and a telltale temperature difference across the valve body (the lines that should be at very different temps read closer together than they should). One failing valve = a spread of weak, contradictory-looking symptoms.

2. The same charge reads differently in each mode. In heat mode the outdoor coil is the evaporator and the indoor coil is the condenser — the opposite of cooling. Subcooling, superheat, and the "correct" charge all shift between modes. A charge that looks okay in cooling can look off in heating (and charging a heat pump in heat mode has its own method). So a single charge condition can produce a "good in cool, weird in heat" report that looks like two problems but is one.

Layer on defrost (a deliberate brief reversal that, if the board is misbehaving, runs too often or not enough) and aux heat (resistance heat that can mask or mimic a weak compressor), and you get the classic mixed bag. The framework: identify which mode(s) the symptoms appear in, then find the one component whose failure mode covers all of them.

In the field

  1. Verify what mode the system is actually in. Confirm the reversing valve is energized/de-energized as it should be for the call, and that the thermostat's O/B setting matches the equipment (O energizes in cool for most; some are B/energize-in-heat). A simple O/B mismatch makes a heat pump run the wrong mode and looks like a major fault.
  1. Test each mode separately and write down both. Force a cooling call: read head, suction, superheat, subcool, air temp. Force a heating call: read the same. Don't blend them — the pattern across the two modes is the diagnosis. "Weak in both" vs "fine in cool, weak in heat" point at very different roots.
  1. Suspect the reversing valve when both modes are weak. Low head + high suction + low capacity in both directions is the bleed-through signature. Confirm by feeling/measuring the valve body — the discharge and suction tubes on the valve should be at very different temperatures; if they're suspiciously close, the valve is leaking internally. A valve that won't shift fully (stuck, or a bad solenoid) is the other failure — it may run one mode fine and the other not at all.
  1. Check the reversing valve solenoid and its 24V. A valve that won't shift can be a dead solenoid coil or a missing 24V signal in that mode, not a bad valve body. Verify the coil has voltage when it should and the coil isn't open before condemning the valve.
  1. Evaluate charge with the correct mode method. Don't judge a heat pump's charge by cooling-mode habits in winter. Use the heat-mode charging method for the conditions (or weigh it). A "contradictory" charge reading is often just mode confusion.
  1. Separate the compressor from the aux heat. Warm supply air doesn't prove the compressor is working — the strips could be carrying it. Clamp the compressor and the strips independently. A weak/failing compressor hiding behind aux heat is a classic mixed-symptom trap (the house stays warm, the bill explodes, the heat pump's actually limping).
  1. Look at defrost behavior. Too-frequent defrost (board/sensor fault) wastes energy and dumps cold air; never defrosting (failed sensor/board) ices the outdoor coil and kills heat capacity. Both can masquerade as a charge or compressor problem. Check the defrost sensor and board logic.
  1. Find the single root that explains the whole spread. Once you have both modes' readings, the compressor/strip split, and the valve check, ask which one condition accounts for everything. On a heat pump it usually exists.

Normal values & targets

Orientation only (defer to nameplate/conditions and mode):

  • Reversing valve body temps: discharge-side tube hot, suction-side tube much cooler — a clear difference. Tubes reading close to each other = internal bleed-through.
  • Bleed-through signature: low head, high suction, low capacity in BOTH modes.
  • Heat-mode charge: judged by the heat-mode method (or weigh-in); cooling-mode superheat/subcool targets don't directly apply in heating.
  • Aux/strip heat: ~5 kW per ~21A/240V bank — clamp it separately from the compressor to know who's carrying the load.
  • Defrost: initiates on time/temperature/demand logic; terminates on coil temp. Far too frequent or never = board/sensor fault.
  • O/B reversing-valve signal: ~24V to the solenoid in the mode the equipment expects (know whether it's O-energized-in-cool or B-energized-in-heat).

Common faults & what they mean

  • Weak in BOTH heat and cool, low head, high suction: reversing valve bleeding through internally. Confirm with valve-body temperatures.
  • Runs one mode fine, won't do the other at all: reversing valve not shifting — stuck valve, dead solenoid coil, or no 24V in that mode. Check the coil/signal before the valve body.
  • Fine in cool, "weird" in heat (or vice versa): often mode confusion in your reading method, or a charge/airflow condition that reads differently per mode — not two faults.
  • House warm but bill huge, heat pump seems weak: aux strips carrying the load while the compressor limps. Clamp them separately.
  • Cold drafts / iced outdoor coil in winter: defrost running too often (cold air) or never (icing). Defrost sensor/board.
  • Heat pump runs backward from the call (cools when set to heat): O/B thermostat mismatch or reversing-valve wiring. Verify the control setup.

Tech tips & gotchas

  • Read BOTH modes and compare — the pattern across modes is the diagnosis. A single mode's readings can mislead; "weak in both" vs "off in one mode only" splits reversing-valve bleed-through from a valve that won't shift.
  • A bleed-through reversing valve is the great heat-pump impostor — a whole spread of weak, contradictory symptoms from one fault. The valve-body temperature check is your fastest confirmation, and it's nearly free. But before condemning the valve, check the cheap controls: a valve that won't shift is often a $20 solenoid coil or a missing 24V signal, not a $400 valve and a recovery/recharge.
  • Don't judge winter charge by summer rules. Indoor and outdoor coils swap roles between modes; use the heat-mode method or weigh the charge. Most "contradictory" charge readings are just mode confusion.
  • Warm air ≠ working compressor. Aux heat can carry a house while the heat pump dies quietly. Always clamp the compressor and strips separately to see who's actually heating.
  • Find the one root. Heat pumps tempt you to list five problems. Discipline yourself to find the single condition that explains them all — it almost always exists.

Safety / code notes

  • Recover per EPA 608 before replacing a reversing valve or opening the system; brazing a valve in requires care (heat-sensitive solenoid, flow nitrogen, avoid overheating the valve body).
  • A heat pump leaning on aux heat to mask a failing compressor is also an energy/cost hazard to the customer — flag it.
  • Verify defrost and safety logic are intact; don't bypass defrost controls to "test" — an iced outdoor coil and a slugged compressor follow.
  • Live-circuit and pressurized-system safety: rated tools, PPE, recover never vent, bleed capacitors before handling terminals.