Checking Heat Pump Charge in Heat Mode: What Works in Winter

Winter calls on heat pumps often end with "is it low on charge?" Answering that in heat mode is harder than in summer, and guessing leads to overcharged systems in July.

Why heat mode is different

In heating:

• The indoor coil is the condenser and the outdoor coil is the evaporator.
• Liquid line subcooling is measured leaving the indoor coil, not the outdoor unit.
• Outdoor coil temperature changes with frost and defrost, so readings move.
• Many systems have separate metering devices or check valves for each mode, so the system behaves differently in each mode.
• The superheat and subcooling targets printed for cooling don't apply.

What manufacturers usually recommend

1. Weigh in the charge. Recover what's in the system, repair any leak, evacuate, and weigh in the nameplate charge plus or minus the line set adjustment from the installation manual. This is the most reliable winter method.
2. Use the manufacturer's heating charge chart if the unit has one, often printed inside the access panel or in the service manual. These charts typically use pressures, subcooling at the indoor coil, or temperature differences at specific outdoor temperatures.
3. Return in cooling weather to verify by the normal cooling method, and note on the invoice that you'll do so.

Signs it might be low on charge in heat mode

• Low suction (outdoor coil) pressure for the outdoor temperature
• Low head (indoor coil) pressure and low supply air temperature
• Outdoor coil frosting unevenly or frosting heavily, and frequent defrosts
• High superheat at the compressor
• A known leak history

Low indoor airflow and a dirty outdoor coil affect readings too. Check those first. Many "low charge" winter calls are actually a stuck reversing valve, a defrost problem or backup heat not coming on. See heat pump blowing cold air and heat pump defrost.

Step by step

1. Confirm mode and airflow: clean filter, correct blower speed, outdoor coil clear of ice.
2. Let it run at least 15 minutes with no defrost in progress.
3. Take readings and compare them with the manufacturer's heating chart, if there is one.
4. If it's low: find the leak first. See how to find a refrigerant leak.
5. Repair, evacuate and weigh in the nameplate charge. See how to pull a deep vacuum.
6. Record what you did on the heat pump heating checklist and the refrigerant service log.
7. Schedule a cooling-season check.

Common mistakes

• Using cooling superheat targets in heat mode.
• Topping off in winter without a leak search, then the system is overcharged in summer.
• Taking readings during or right after defrost.
• Measuring subcooling at the outdoor unit in heat mode instead of leaving the indoor coil.
• Ignoring the line set adjustment when weighing in.

FAQ

Sources

• U.S. Department of Energy: Heat pump systems
• EPA: Section 608 refrigerant management requirements