What it is

Charging a heat pump is harder than charging a straight AC because a heat pump runs in both directions, and you often need to add or verify charge in the dead of winter when it's in heat mode. The problem: the standard cooling-mode charging methods (target superheat for a fixed orifice, target subcooling for a TXV) are designed for cooling-mode operation at reasonable outdoor temps. In heat mode, the indoor coil is the condenser and the outdoor coil is the evaporator, conditions are all over the place, and the manufacturer's normal charging chart usually assumes cooling mode. So "just hook up the gauges and chase a number" doesn't work cleanly.

How it works

In heat mode the roles flip:

  • The indoor coil is the condenser (subcooling happens here).
  • The outdoor coil is the evaporator (superheat happens here, in cold ambient).

That means if you wanted to charge to subcooling, the relevant subcooling is now at the indoor coil, and the conditions affecting it (indoor airflow, indoor temperature, the cold outdoor evaporator) are not what a cooling-mode chart was built around. Outdoor ambient is low, the outdoor coil may be frosting and defrosting, and pressures are very different from summer. All of that makes a gauge-based charge in heat mode unreliable unless the manufacturer specifically provides a heat-mode charging procedure.

This is why weigh-in is the gold standard for heat pumps, especially in winter.

In the field

Preferred: weigh in the charge.

  1. Recover the existing charge fully (or start from a known-empty system after service).
  2. Read the nameplate factory charge and the line-set adjustment (manufacturers specify a charge for a baseline line-set length — commonly the first 15 or 25 feet — plus an add/subtract per foot beyond that).
  3. Weigh in the calculated total with an accurate scale. This is independent of outdoor temperature, so it works in any weather. For any system where you've opened it up, recovered, or replaced components, weigh-in is the right answer.

If you must verify by gauges in heat mode:

  • Only do it if the manufacturer provides a heat-mode charging chart/procedure — some do, with target subcooling (at the indoor coil) referenced to indoor and outdoor conditions. Follow that specific chart; don't apply cooling-mode targets.
  • Make sure the system is at steady state and not in or near a defrost — defrost completely scrambles the readings. Let it run a stable heating cycle.
  • Be aware a low outdoor charge reading can be normal in cold weather; don't add refrigerant to hit a cooling-mode number you shouldn't be using.

Best practice: verify in cooling when you can. If conditions allow (mild day, or you can get the system into cooling), confirming the charge in cooling mode against the standard subcooling/superheat target is more reliable, then trust the weigh-in for the winter. Some techs set/verify charge in cooling during install season specifically to avoid winter guesswork.

Normal values & targets

  • Weigh-in: total = nameplate factory charge ± line-set adjustment (per the data plate; e.g., baseline for first 15 ft, then add a fraction of an ounce per additional foot of liquid line — use the plate's exact figure).
  • Cooling-mode verification (when usable): TXV systems → target subcooling (commonly in the ~8–12°F range, per the chart). Fixed-orifice → target superheat from the wet-bulb/dry-bulb method. Always defer to the unit's chart.
  • Heat-mode subcooling: only per a manufacturer heat-mode chart; no universal number — it depends on indoor/outdoor conditions and the equipment.
  • R-410A runs higher pressures than R-22; use the right gauges/PT values for the refrigerant in the system.

Common faults & what they mean

  • Overcharged heat pump (added in winter chasing a number) — high head, can slug the compressor, poor efficiency, possible high-pressure trips on milder days. A super common way to wreck a system is adding charge in heat mode to hit a wrong target.
  • Undercharged — low capacity, the outdoor coil frosts heavily and defrosts constantly, low suction, high superheat at the outdoor coil. But confirm it's actually low (weigh-in) before adding.
  • Readings won't settle — system keeps going into defrost, or you're reading during the transient after a mode change. Wait for stable steady-state heat.
  • Looks low on gauges but weighs correct — that's the trap. Cold-weather gauge readings can look "low" by summer standards. Trust the weighed charge.

Tech tips & gotchas

  • When in doubt, weigh it in. It's the only method that doesn't care what the weather is doing. For any system you've opened, recovered, or that has unknown charge history, recover and weigh.
  • Don't apply a cooling-mode subcooling/superheat target in heat mode. That's the number-one cause of overcharged heat pumps. The chart matters.
  • Stay out of defrost. A reading taken right before, during, or right after defrost is garbage. Confirm the unit is in a stable heating cycle.
  • Account for the line set. A long line set with no adjustment to the factory charge is a quiet undercharge (or overcharge). Read the plate.
  • Subcooling in heat mode is at the INDOOR coil — if you do use a heat-mode procedure, your liquid-line reference point isn't where it is in cooling. Don't probe the wrong line.
  • If you can possibly get it into cooling for a sanity check (mild afternoon), do — then lock in the weighed amount for winter confidence.

Safety / code notes

  • Recovery is required by EPA Section 608 — never vent refrigerant when recovering to weigh in a fresh charge.
  • After opening the system, pull a proper vacuum (deep vacuum verified with a micron gauge and a standing decay test) before charging — non-condensables and moisture wreck capacity and cause false pressure readings.
  • Use scales and gauges rated/calibrated for the refrigerant; R-410A and other higher-pressure refrigerants require appropriately rated equipment.
  • For A2L refrigerants (e.g., R-454B, R-32), follow the additional handling, leak-detection, and equipment requirements that apply to mildly flammable refrigerants.