Heat Pump Refrigerant Charging in Cold Weather

Charging an air conditioner in July is straightforward. A heat pump in January is not. Cooling charts do not apply, superheat misleads you, and frost, defrost and the accumulator all change the numbers. Many technicians add refrigerant because the air feels cool, and they leave behind an overcharged system. This is how to choose a charging method that holds up, whatever the weather. It assumes you can connect gauges and read superheat and subcooling. Only EPA-certified technicians may charge a system.

What You Will Be Able to Do

• Rank the charging methods and pick the right one for the conditions.
• Calculate a charge from the rating plate and line set length.
• Explain why a heat pump's readings differ between modes.
• Verify a charge in heating using the maker's data.
• Spot the readings that look like low charge but are not.

Why Heat Pumps Are Harder

A heat pump uses one charge for two jobs. The indoor and outdoor coils are different sizes, so the same charge sits differently in each mode. In heating, the surplus usually sits in the accumulator, which stores liquid so that it does not reach the compressor. An accumulator with liquid in it can make the suction side look strange, and superheat in heating often reads high on a correct charge.

The outdoor coil is the evaporator in heating, so outdoor weather changes every number. Frost makes it worse, because a frosted coil pulls suction down until the next defrost.

Pick the Method

Some makers list a stricter outdoor range for cooling checks, so read the label on the unit. Some manufacturers publish no heating superheat or subcooling targets at all. If you have no data, do not invent a target. A weigh-in is the safe method.

Weighing In a Charge

The rating plate gives the factory charge, and it covers a set line length. Longer lines hold more refrigerant, so you adjust.

Worked example, R-410A heat pump:

1. The rating plate says 7 lb 4 oz, covering 15 feet of line.
2. The line set is 45 feet with a 3/8-inch liquid line.
3. Extra length is 45 minus 15, or 30 feet.
4. A common figure for R-410A in a 3/8-inch line is 0.6 oz per foot. That gives 30 x 0.6 = 18 oz, or 1 lb 2 oz.
5. Total charge is 8 lb 6 oz.

Use the figure and the included length from the installation manual for that model. The included length varies by maker, and a different refrigerant or liquid line size changes the ounces per foot. Use the actual length, not an equivalent length. Mark the final charge on the unit.

On an A2L system such as R-454B or R-32, the manual sets a maximum charge based on the room and the equipment. Do not exceed it to make the numbers look better.

Warning: Weighing in means working with a pressurized cylinder and live equipment. Wear eye protection and gloves, charge blends as liquid, and follow your training. Never heat a cylinder with a flame, and use only a scale you trust.

After a Leak

Find and fix the leak first. Refrigerant does not wear out, so a low system has a leak.

• If you can trust the remaining charge, a technician can usually adjust R-410A, because its glide is very small. Fractionation from a leak is minor.
• If the amount left is unknown, or it is a blend with more glide, recover it and weigh in a full charge. A cold day makes that the only trustworthy method.
• If the system leaked to zero, evacuate it to the usual target and weigh in the charge.

Taking Heating Readings

If the maker provides heating data, follow its setup exactly. Common steps are:

1. Confirm airflow first. Low indoor airflow raises head pressure in heating, because the indoor coil is the condenser.
2. Record outdoor temperature at the coil inlet and indoor return air temperature. Note any strong wind, because it changes the readings.
3. Start with a clean, frost-free coil. Run a defrost if needed.
4. Run at least 15 minutes, with no aux heat on if the data calls for that.
5. Do not read during or just after a defrost.
6. Read the liquid line where the maker says. That may be at the indoor coil outlet or at the outdoor unit's liquid port.

In heating, the condenser is indoors. The refrigerant leaves the indoor coil and travels the line set to the outdoor unit. Subcooling is calculated from high-side pressure and liquid line temperature.

Worked example, R-410A heating subcooling: high-side pressure reads 340 psig, which is 105°F saturated. The liquid line reads 93°F. Subcooling is 12°F. That is in the common 8 to 12°F range for a TXV or EEV system, but the maker's data decides. A long liquid line loses heat on its way outdoors, so subcooling reads higher at the outdoor unit than at the indoor coil. That is why the measuring point matters.

Check Capacity, Not Just Refrigerant Numbers

A strong test in heating is to measure how much heat the system delivers. Use a temperature rise and airflow.

Formula: BTU per hour = 1.08 x CFM x temperature rise.

Worked example: airflow is 1,200 CFM and the rise is 26°F. Output is 1.08 x 1,200 x 26 = about 33,700 BTU per hour. Compare that with the maker's expanded performance data at the same outdoor and indoor temperatures. If they roughly agree, the charge is probably not the problem, whatever the air feels like on your hand. Run this test with backup heat off, and measure airflow properly.

Supply air around 85 to 105°F is normal and feels cool to the hand. That is often the whole complaint.

What Looks Like Low Charge

Real low charge in heating often shows low head, low subcooling, low capacity, high discharge temperature and defrost trouble. Look for several of those together.

What Overcharge Looks Like

Overcharge is common on heat pumps, because topping off is easy. Look for:

• High head pressure, and high amps.
• High subcooling.
• Liquid flooding the accumulator, and low discharge temperature.
• A compressor that sounds rough, or trips on a safety.

An overcharged heat pump can run for years, but it loses efficiency and floods the compressor in cold weather. Recover the excess and weigh the total, rather than bleeding it to the air. Venting is illegal.

Variable-Speed Systems

A variable-speed heat pump changes its speed, so its pressures move around constantly. You cannot read a charge from a moving target. Most makers build in a charging mode that holds the compressor and fan at a set speed. Enter it from the control or service tool, let the system stabilize, and then compare readings with the chart for that mode. On some models, if the readings do not match, the instructions say to remove all charge and weigh it in.

Common Mistakes

Check Your Understanding

A plate charge covers 15 feet, and a coworker weighed in only that amount on a 50-foot line set. What is the result? A short charge from day one. The extra 35 feet needs refrigerant added, using the figure in the manual.

A customer says the air feels cool, and the supply reads 95°F. What does that suggest? Probably normal operation. Air around 85 to 105°F feels cool to the hand, so measure output before you touch the charge.

Airflow is 1,000 CFM and the rise is 28°F with aux heat off. What is the output? 1.08 x 1,000 x 28, or about 30,200 BTU per hour.

Suction is low on a cold morning, and the outdoor coil has a coat of frost. Is it time to add refrigerant? No. Frost pulls suction down. Run a defrost, start with a clean coil and let the system settle before you read it.

A coworker wants to bleed a little extra refrigerant to the air from an overcharged unit. What do you say? Venting is illegal. Recover the excess and weigh the total.

Key Takeaways

• Weighing in is the only exact method, and it works in any weather.
• Adjust the charge for line length using the manual's figures.
• Do not use cooling charts or superheat alone to charge in heating.
• A temperature rise and airflow test shows the heat actually delivered.
• Variable-speed systems need the maker's charging mode.
• Fix leaks first, and recover excess refrigerant rather than venting it.