Charging a heat pump in heat mode means adding refrigerant while the system is actively heating your home
When you charge a heat pump in heat mode, the system is running and pulling heat from the outdoor air (or ground) to warm your indoor space. This is different from charging in cooling mode because the refrigerant flow direction is reversed, and the pressures inside the lines are different. You measure and add refrigerant based on the system's operating pressures while it is actively heating, not while it sits idle.
The core reason to charge in heat mode rather than cooling mode is accuracy. A heat pump's refrigerant charge is correct when the pressures and temperatures match the manufacturer's specifications for the mode you are using. If you charge it in cooling mode but the system spends most of its time heating, it will be overcharged or undercharged when heating actually happens. Charging in the mode the system will operate in gives you the right amount.
Key Takeaways
- Heat mode charging requires the system to be running and actively heating, so outdoor temperature must be cold enough that the heat pump turns on automatically.
- You will need a manifold gauge set rated for heat pump service, a thermometer, and the manufacturer's charging chart specific to your outdoor temperature.
- The suction line (low-pressure side) and liquid line (high-pressure side) pressures must both fall within the range printed on your system's data plate or service manual for the current outdoor temperature.
- If outdoor temperature is above 60°F, the heat pump may not run long enough to reach stable pressures, so you may need to force the system into heating mode using the service switch or wait for colder weather.
- Overcharging or undercharging in heat mode reduces heating output and can damage the compressor, so take pressure readings multiple times before adding any refrigerant.
Gather the right tools and information before you start
You will need a manifold gauge set designed for heat pump service, not just air conditioning service. A standard AC gauge set may not read accurately on the low-pressure side when the system is in heat mode because the pressures are inverted. The gauges should have a range that covers both positive and negative pressures, or separate scales for heating and cooling.
Get the manufacturer's charging chart from the data plate on the outdoor unit or from the service manual. This chart shows the correct suction-line and liquid-line pressures for different outdoor temperatures. Write down the outdoor temperature at the time you will be charging, because the correct pressure range shifts with temperature. You will also need a digital thermometer to measure the outdoor air temperature accurately—do not rely on a weather app, because the air temperature at your unit may differ from the reported temperature a few miles away.
Have a refrigerant recovery machine on hand if you need to remove refrigerant. If the system is overcharged, you cannot straightforward let it out into the air; you must recover it into a tank. A recovery machine is expensive to buy but can be rented from HVAC supply shops by the day.
Confirm the system will run long enough to reach stable pressures
Heat pumps in heating mode cycle on and off based on indoor temperature. If your home is already warm or if outdoor temperature is mild, the system may run for only a few minutes before the thermostat shuts it off. You need the system to run continuously for at least 10 to 15 minutes so that pressures stabilize and you can take an accurate reading.
If outdoor temperature is above 60°F, the heat pump may not run at all, or it may run so briefly that pressures never settle. In this case, you have two options: wait for colder weather, or use the service switch (also called the reversing valve switch) on the outdoor unit to force the system into heating mode. The service switch is a small lever or dial on the side of the compressor housing. Set it to the heating position and leave it there during charging. This overrides the thermostat and keeps the compressor running continuously. Do not forget to switch it back to auto after you finish charging.
Lower the indoor thermostat below the current room temperature so the system will call for heat and run continuously. Set it 5 to 10 degrees below the actual indoor temperature. Once the system is running steadily, wait another 5 minutes before you attach your gauges, so pressures have time to stabilize.
Attach gauges and take initial pressure readings
Locate the service ports on the outdoor unit. The suction line (low-pressure side) is the larger copper line coming from the indoor coil; the liquid line (high-pressure side) is the smaller line. Each has a service port—a small valve with a cap. Remove the caps and attach the low-pressure gauge hose to the suction port and the high-pressure gauge hose to the liquid port.
Open both gauge valves slowly so you do not introduce air into the system. Watch the gauges for 2 to 3 minutes as they settle. Once the needles stop moving, write down both readings and note the outdoor temperature at that exact moment. Do not take a single reading and assume it is correct; take three readings over the next 5 minutes and average them. Pressure fluctuates slightly as the compressor cycles, so averaging removes that noise.
Compare your readings to the manufacturer's charging chart. Find the row that matches your outdoor temperature (or the closest temperature listed). The suction pressure should fall within the range shown for that temperature, and the liquid pressure should fall within its range. If both are within range, the charge is correct and you are done. If either is outside the range, you need to add or remove refrigerant.
Add refrigerant if pressures are too low
Low suction pressure and low liquid pressure together mean the system is undercharged. You will add refrigerant to the suction line (low-pressure side) only, never to the liquid line. Adding to the liquid line can cause liquid slugging, which damages the compressor.
Connect a refrigerant cylinder to the low-pressure port using a charging hose with a ball valve. Make sure the cylinder is upright so you are drawing vapor, not liquid. Open the ball valve slowly and let refrigerant flow into the suction line while the compressor is running. Add a small amount—typically 0.5 to 1 ounce at a time—then close the valve and wait 2 minutes for pressures to stabilize. Take new readings and compare them to the chart again.
Repeat this process in small increments until both pressures fall within the correct range. Do not rush or add large amounts at once. Overcharging is easier than undercharging and causes more damage. Once pressures are correct, close the ball valve on the refrigerant cylinder, then close the gauge valves, and remove the hoses from the service ports. Replace the caps on the service ports when ready.
Remove refrigerant if pressures are too high
High suction pressure and high liquid pressure together mean the system is overcharged. You must use a refrigerant recovery machine to remove the excess. Do not open a valve and let refrigerant vent into the air; this is illegal and harms the environment.
Connect the recovery machine to the low-pressure port using a hose. Follow the machine's instructions to begin recovery. Most machines pump refrigerant out of the system and into a storage tank. This process takes 20 to 45 minutes depending on how much refrigerant you need to remove and the machine's capacity. Once the machine shows the system is empty, close the valves and remove the hoses.
Now you can recharge the system from scratch using the method described above, adding refrigerant in small increments until pressures are correct. This is slower than adding a small amount, but it is the only safe way to fix an overcharged system.
Return the system to normal operation and verify the charge
Once pressures are correct, close both gauge valves and remove the gauge hoses from the service ports. Replace the caps on the service ports and tighten them by hand—do not use a wrench, because you can crack the valve body.
If you used the service switch to force heating mode, switch it back to auto. Raise the indoor thermostat back to your normal setting. Let the system run for 15 to 20 minutes in normal operation, then check the pressures one more time to confirm they are still in range. Pressures may shift slightly once the system returns to normal cycling, so a final check confirms the charge is stable.
If pressures drift outside the correct range during normal operation, the charge is still not right, and you will need to adjust again. This is rare if you took your time during the initial charging, but it can happen if the outdoor temperature changed significantly between your first reading and your final check.
Common mistakes that lead to incorrect charging
The most common error is taking a pressure reading too soon, before the system has run long enough for pressures to stabilize. If you read pressures after only 2 or 3 minutes of running, they will still be climbing, and you will think the system is undercharged when it is actually fine. Always wait at least 10 minutes after the system starts running, and take multiple readings over several minutes.
Another frequent mistake is charging in cooling mode when you meant to charge in heating mode, or vice versa. The correct pressures are completely different between the two modes. If you use a cooling-mode chart to charge a system that will operate in heating mode, the charge will be wrong. Always confirm which mode the system is in before you open the gauge valves, and use the chart that matches that mode.
Adding refrigerant to the liquid line instead of the suction line is dangerous and can destroy the compressor when ready. The liquid line is the smaller of the two service ports. If you are unsure which is which, trace the lines back to the indoor coil; the line that comes from the coil is the suction line, and the line that goes to the coil is the liquid line. Mark them with tape before you attach gauges.
Frequently Asked Questions
What if my outdoor temperature is 65°F and the heat pump won't run?
At 65°F, the heat pump may not run because the outdoor air is warm enough that the system does not need to heat. Use the service switch on the outdoor unit to force the system into heating mode. This keeps the compressor running continuously so you can take stable pressure readings. Switch it back to auto when you finish charging.
Can I charge a heat pump in cooling mode and expect it to be correct in heating mode?
No. The correct refrigerant charge is different for heating and cooling because the refrigerant flow direction reverses and the pressures change. Charging in cooling mode will result in the wrong charge for heating. Always charge in the mode the system will operate in most of the time.
What happens if I add refrigerant to the liquid line by mistake?
Liquid refrigerant entering the compressor can cause liquid slugging, which means the compressor tries to compress a liquid instead of a vapor. This can crack the compressor housing or break internal parts when ready. Always add refrigerant to the suction line (low-pressure side) only. If you accidentally added to the liquid line, stop when ready and call a technician.
How long does it take to charge a heat pump in heat mode?
If the system is only slightly undercharged, charging takes 30 to 45 minutes from start to finish, including the time to wait for pressures to stabilize. If the system is significantly undercharged or overcharged, it can take 1 to 2 hours because you must add or remove refrigerant in small increments and verify each time.
Do I need to recover refrigerant if I am only adding a small amount?
Yes. Federal law requires recovery of all refrigerant removed from a system, even if the amount is small. You cannot vent refrigerant into the air. If the system is overcharged, you must use a recovery machine to remove the excess, even if you are only removing 1 or 2 ounces.