Why does my AC unit struggle with both heating and cooling?

Why does my AC unit struggle with both heating and cooling?

Let me tell you straight up – if your AC unit is failing to heat properly in winter and also can’t cool down a room in summer, you are not alone. I talk to HVAC dealers and distributors every single week, and this is one of the most common complaints from end‑users. The problem is almost never a single, simple cause. It’s usually a combination of factors that compound over time. And for you, as a B‑2‑B importer or exporter, understanding these root causes is crucial – not just for troubleshooting but for educating your own customers and avoiding returns, warranty claims, and reputation damage.

Let’s break this down into the real meat of the issue. Forget the textbook answers. Let’s talk about what actually happens in the field.

The compressor is the heart, and the refrigerant is the blood – both can be half‑dead

The first thing I check when a unit struggles with both modes is the compressor’s health and the refrigerant charge. These two are inseparable. A compressor that is worn, has internal bypass, or is simply undersized for the system will struggle to build enough pressure for both heating and cooling. In heating mode (reversing valve engaged), the system needs even higher discharge pressure to deliver warm air into the room. If the compressor is weak, the pressure differential drops, and you get lukewarm air instead of hot air. In cooling mode, the same weak compressor means insufficient cooling.

Now, refrigerant charge. This is a classic. Many installers or DIY users add refrigerant without checking for leaks, or they overcharge. Overcharge causes high head pressure, which in cooling mode leads to poor condensation and low efficiency – but in heating mode, it can cause the system to short‑cycle or trip on high‑pressure protection. Undercharge, of course, reduces both cooling and heating capacity. A 10% undercharge can drop heating capacity by 15‑20%. And the worst part? You can’t just guess. You need to measure subcooling and superheat, but most end‑users never do that.

For you as a distributor, I’d recommend pushing for units that have factory‑installed TXV valves (thermal expansion valves) instead of capillary tubes. Capillary tubes are sensitive to charge variations – a slight undercharge kills performance. TXV valves can adapt to a wider range of conditions, but they are not bulletproof either. A bad TXV can cause flash gas, which shows up as poor heating and cooling.

Real‑world data from the field (2023‑2024 service reports)

These numbers come from aggregated service data of 5,400 residential split‑system units across 12 countries (2022–2024). Notice that refrigerant charge error is number one. That’s a training opportunity for your dealer network.

The reversing valve – the most misunderstood component in your system

You probably know that a heat pump uses a reversing valve to switch between cooling and heating. But what you might not realize is that this valve is a pain point in many budget‑level units. The valve slides a spool that changes the flow direction of refrigerant. If the spool gets stuck in a mid‑position, the system will try to run in both modes simultaneously – or worse, it bypasses part of the refrigerant, reducing capacity in both directions.

Common causes for a stuck reversing valve:

• Rust or debris inside the valve body. This is especially common in units that sit idle for months (like in moderate climates where heating is only used a few days a year). The refrigerant oil can degrade and form a varnish that gums up the spool.
• Low pressure differential. The valve requires a certain pressure difference to shift. If the system has a refrigerant leak or the compressor is weak, the valve may never fully seat.
• Electrical coil failure. The solenoid that moves the spool can fail open or shorted. I’ve seen many cases where the coil provides just enough magnetic force to start the shift but not enough to hold it, causing intermittent operation.

When the reversing valve is partially stuck, the temperature swing from setpoint can be huge – like 5–6°C difference between indoor temperature and supply air. Your customer feels it’s “not working” for both modes. The fix is often valve replacement, which is expensive and time‑consuming. As a manufacturer, you can minimize this by using high‑grade reversing valves with stainless steel spools and PTFE seals. Also, include a filter/drier with solid‑core desiccant to trap contaminants before they reach the valve.

Airside issues – dirty coils, undersized ductwork, and restricted airflow

You can have the perfect compressor, perfect charge, and perfect reversing valve – but if the airflow over the condenser (outdoor coil) or evaporator (indoor coil) is restricted, you get poor performance in both modes. This is the most overlooked problem, especially in commercial or large residential installations.

In cooling mode, dirty outdoor coil means high head pressure and reduced heat rejection. The compressor works harder, draws more current, but delivers less cooling. In heating mode, the outdoor coil acts as the evaporator (since the refrigerant is now extracting heat from outside air). A dirty coil reduces heat absorption, so the system can’t pick up enough heat from the cold air. The result: weak heating and longer defrost cycles.

Similarly, indoor coil blockage (dirty filter, closed registers, or undersized ductwork) starves the evaporator. In cooling, the coil runs too cold and freezes. In heating, the coil runs too hot and the system trips on high discharge temperature. Both conditions lead to reduced capacity.

Data from a 2023 field study by a European HVAC research group shows that 43% of residential split‑systems have an airflow deficiency of at least 15% from design specification. That’s massive. For every 10% reduction in airflow, cooling capacity drops 3–5% and heating capacity drops 4–7%. The Energy Efficiency Ratio (EER) and Coefficient of Performance (COP) also take a hit, increasing operational costs.

Airflow impact on capacity (typical split system, 3.5 kW rating)

These are rough numbers but they match what you’ll see in real service calls. So if a dealer calls you complaining that a recent batch of units is “weak” in both modes, first ask: what’s the airflow situation? Are they using the factory‑supplied filters? Are the coils clean? In many cases, units that are shipped with a factory charge for a specific line length are installed with longer copper lines (e.g., 15 meters instead of 5 meters) without adding extra refrigerant. That’s a whole separate problem, but it ties back to both heating and cooling performance.

Control board logic and sensor failures – the hidden gremlin

Modern inverters and even some fixed‑speed units rely on a control board to manage everything: compressor speed, fan speed, reversing valve, defrost cycle, and superheat targets. If a sensor goes bad – typically the outdoor ambient sensor or the indoor coil sensor – the board gets wrong data and adjusts the system to the wrong operating point.

For example, an outdoor ambient sensor that reports 15°C when it’s actually 0°C will cause the system to think it’s in mild weather. In heating mode, it might not engage the full defrost cycle, causing the outdoor coil to ice up. Then the unit struggles to heat, and once it ices completely, it can’t cool either because the defrost cycle can’t complete. Or vice versa: in cooling mode, a bad indoor coil sensor might prevent the unit from reducing compressor speed, leading to over‑cooling or freezing.

Also, many budget units have a “memory” feature that stores error codes and operating hours. A glitchy board can lock the system into a derated mode (e.g., lower compressor frequency) to protect itself, but that derated mode kills both heating and cooling capacity. I’ve seen units that run at 60% capacity because the board thinks the compressor current is too high – but it’s actually a bad current sensor.

For B‑2‑B buyers, I recommend asking your suppliers about the grade of control boards they use. Cheap boards with potted electronics are prone to thermal stress. Look for boards with conformal coating and automotive‑grade relays. Also, ensure the unit has a “quick‑test” mode that allows a technician to force‑run the reversing valve and compressor at full speed for diagnostics. That saves hours of troubleshooting.

Installation mistakes that kill dual‑mode performance

Finally – and this is the biggest one for your customers – installation errors. The unit itself might be perfect, but if the installer makes any of these common mistakes, both heating and cooling will suffer.

• Wrong line set size. Using a larger diameter line than specified reduces refrigerant velocity, causing oil return issues. Over time, the compressor runs dry and fails. Both modes degrade. Using a smaller line increases pressure drop, which reduces efficiency and capacity.
• Excessive bends and long line sets. Every 90‑degree bend adds about 1 meter of equivalent length. Long lines beyond the factory charge limit without additional refrigerant cause both high‑ and low‑side pressure issues.
• Improper vacuum and dehydration. Moisture left in the system reacts with refrigerant and oil to form acids. These acids eat the compressor windings and plug the metering device. Result: gradual loss of capacity in both modes.
• Incorrect wiring of reversing valve. Some installers wire the reversing valve to energize in cooling mode instead of heating mode (or vice versa). The unit will appear to heat or cool, but the pressure difference is wrong. You’ll get about 30% less capacity in one mode.
• Placing outdoor unit in a confined space. If the outdoor unit is in a courtyard or under a deck with poor airflow, it recirculates hot exhaust air in summer and cold exhaust air in winter. In cooling, the outdoor unit sees elevated ambient temperature, causing high pressure and poor cooling. In heating, it sees lower ambient temperature (because it’s pulling cold air from the shaded area), causing low suction pressure and weak heating.

As a manufacturer, you can help by providing clear, visual installation guides (not just diagrams) and offering training for importers’ installation teams. Many Chinese manufacturers now include QR codes on units that link to installation videos. That’s a good start, but also include a checklist that the installer must sign – covering line length, vacuum level, and sensor placement.

Q&A – For professional HVAC distributors and importers

Q1: A customer reports that a 2‑ton split system can’t maintain 22°C in summer or 20°C in winter. Temperature difference between supply and return is only 6°C in both modes. What’s the most likely cause?
A: Low refrigerant charge is the first suspect. With a 6°C ΔT, the system is likely undercharged by 20‑25%. Check the subcooling and superheat. Also verify that the reversing valve is fully shifted – if it’s stuck in mid‑position, the ΔT will be low in both modes. A quick field test: measure the compressor current draw – if it’s 60% of rated value, you have either low charge or a weak compressor.

Q2: We imported a batch of 500 heat pump units. After one year, 15% of them have reduced heating capacity in winter. What could be the manufacturing defect?
A: This sounds like a systematic issue, not random. Check the filter/drier – if the desiccant is loose or the molecular sieve dust is clogging the capillary tube, you’ll get poor heating. Also, review the compressor oil – some cheaper units use mineral oil instead of POE oil, which can lead to wax formation at low temperatures. Ask your supplier for the batch’s oil analysis report. Another possibility: the outdoor coil fins are too thin (aluminium 0.1mm instead of 0.15mm) – they bend easily during shipping, restricting airflow.

Q3: What percentage of dual‑mode failures can be prevented by proper commissioning?
A: Based on field data from China’s HVAC service alliance (2023 report), about 62% of “both modes weak” complaints are linked to installation or commissioning errors. That means if your distributors train installers properly, you can slash warranty claims by more than half. Prioritize vacuum gauge use, proper line sizing, and charging according to line length.

Q4: Should we recommend inverter or fixed‑speed units for markets where both heating and cooling are essential?
A: Inverter units, especially those with DC inverter compressors and brushless fan motors, adapt better to partial load conditions. In heating mode, a fixed‑speed unit cycles on/off, causing temperature swings and high defrost frequency. Inverters can run at low speed to maintain steady temperature and avoid icing. However, the control board cost is higher. For B‑2‑B, the lower return rate and higher customer satisfaction often justify the price premium. Data from 2024 shows inverter models have a 4.2% first‑year failure rate versus 9.8% for fixed‑speed models in the same capacity range.

Q5: What is the single most important thing to check when diagnosing a unit that fails in both modes?
A: The pressure difference between liquid line and suction line. If the difference is less than 150 psi (for R410A), the compressor is not generating enough compression. Then check the reversing valve – manually apply voltage to the solenoid and see if the pressure difference changes. If not, the valve is stuck. If it does change but the ΔT is still low, the refrigerant charge is off. This method works in 90% of cases.