Why Your Air Conditioning Cools But Doesn’t Heat

The sudden drop in temperatures puts household climate control systems to the test, often revealing a fault that is as uncomfortable as it is baffling. Discovering that your split unit is blowing an icy breeze just when you need to warm the room is one of the most frequent technical queries we face as industry professionals at the start of the winter season. This phenomenon, where the system seems to have lost its heating capacity whilst maintaining its ability to cool, is usually down to specific blockages in the thermodynamic cycle that require precise diagnosis.

At JAG Alcaide, we have an in-depth understanding of the internal architecture and behaviour of these systems—knowledge backed by over 70 years of experience in the installation sector since we were founded in 1954. Throughout these decades of continuous work in Barcelona, we have found that a lack of warm air rarely means the machine is completely useless. Instead, key components such as the reversing valve or the temperature sensors are often compromising the process.

To understand the root cause of this issue, it is essential to look at the mechanics of heat pumps. These appliances do not generate heat using traditional electrical resistance elements; rather, they absorb thermal energy from the outside air and transfer it indoors. According to the technical standards and efficiency guidelines of the Institute for the Diversification and Saving of Energy (IDAE), the performance and heating capacity of a unit drop drastically if there are leaks in the refrigerant gas circuit or if the outdoor heat exchanger is blocked by frost. Identifying the exact factor preventing the correct reversal of the refrigeration cycle is the fundamental step to restoring thermal comfort in your home.

Operating mode configuration and filter maintenance

It is quite common that, after months without using the heating, the remote control is the first hurdle to enjoying a warm environment at home. The first step is to verify that the unit is set to heat mode, usually represented by a sun icon (Heat). It is crucial to understand that upon activating this function, the indoor unit will not blow air immediately. There is a technical standby period, usually lasting between three and five minutes, required for the indoor heat exchanger to warm up sufficiently. This safety protocol is vital to ensure the fan doesn’t blow out room-temperature air before the compression cycle becomes truly effective.

The setpoint temperature selected on the remote should be at least 3 degrees higher than the current room temperature. If the difference is smaller, the unit’s sensor might interpret that the target comfort level has already been reached and will not send the start signal to the compressor. In Barcelona, during the winter months, a suitable setting is usually around 21°C or 22°C to balance comfort and energy efficiency without straining the machinery unnecessarily.

Accumulated dirt is the primary enemy of heating performance. While in summer a dirty filter simply makes the unit cool less effectively, in winter it can cause total system blockages. The indoor unit needs to move a large volume of air to transfer heat from the refrigerant to the room; if the filters are clogged with dust, the heat exchanger quickly overheats, and the safety sensors will shut down operation to prevent major faults.

Keeping these components in optimal condition is an investment in the lifespan of the appliance. A professional filter cleaning and sanitisation costs between €60 and €90, ensuring the unit heats correctly while eliminating bacteria from the airflow. If, after these checks, the air is still cold, the problem could lie with internal components such as the temperature probe or the capacitor (€90 – €180) or, in more complex cases, a fault in the four-way reversing valve (€250 – €500), which is responsible for reversing the refrigeration cycle.

Four-way or reversing valve failure

A system’s ability to switch between cooling and heating relies on a critical component known as the reversing or four-way valve. This technical device is responsible for changing the direction of the refrigerant flow within the circuit. In cooling mode, the hot gas is sent outside, whereas in heating mode, the flow is reversed to harness that thermal energy indoors.

When this mechanism malfunctions, the unit becomes stuck in one function—usually cooling. There are two main aspects to this fault:

The solenoid is an electromagnetic coil that receives the command from the control board to activate the valve. If the board fails to supply voltage or if the coil itself is electrically open, the plunger will not move. Measuring the continuity of this part or verifying whether it receives the corresponding 230V when heat mode is activated from the remote is standard technical practice. A defective solenoid is a relatively simple and inexpensive repair, with component prices typically ranging from €50 to €90, depending on the brand.

In other cases, the problem is purely physical. The internal piston of the valve can get stuck due to sludge, impurities in the circuit, or degraded compressor oil. A telltale symptom is a constant hissing or a metallic noise when the unit tries to switch cycles without success. Unlike an electrical fault, a mechanically seized valve usually requires a complex intervention involving gas recovery, pipe welding, and a fresh refrigerant charge, which often pushes the repair estimate above €300.

Refrigerant gas charge and temperature sensors

Maintaining thermal comfort in winter requires the climate control system to operate under significantly higher pressure parameters than during the summer months. A partial leak of refrigerant gas is often the most common technical reason behind this anomalous behaviour. Although the remaining volume of gas might be sufficient to absorb heat from the indoor environment and cool reasonably well, the heat pump needs a full charge to reach the high pressures required for the heating cycle. Without the correct gas density, the unit is unable to generate enough heat to warm the room, merely circulating air at room temperature.

Temperature probes, or NTC sensors, act as the eyes of the electronic board. These components continuously measure the temperature of the coil and the return air to regulate the appliance’s operation. If a sensor becomes miscalibrated, it can send false signals indicating that the unit has already reached the set temperature or that there is a risk of freezing in the outdoor unit. This false information causes the electronics to block the flow of hot gas or stop the compressor prematurely, preventing the heating cycle from stabilising.

Forcing an appliance that shows these symptoms poses structural dangers to the machine. The compressor uses the refrigerant itself to cool down and to move lubricating oil throughout the circuit. When operating on a low charge, the internal motor overheats and lubrication becomes inadequate. Ignoring that your air conditioning cools but doesn’t heat and leaving it running for hours can result in a seized compressor—a breakdown so costly to repair that it often forces the complete replacement of the outdoor unit.

If your air conditioning unit has stopped heating or you suspect a gas leak, our specialised HVAC technical team will travel to your home in Barcelona and the metropolitan area to perform an accurate diagnosis and a fully guaranteed gas recharge.

The defrost cycle and outdoor temperatures

One of the most baffling situations during the colder months occurs when the unit suddenly stops blowing warm air. This phenomenon is usually a programmed protection measure known as the defrost cycle or Defrost mode. In environments with high relative humidity, the outdoor unit’s coil can reach negative temperatures while performing the physical process of extracting heat from the outdoor air. Upon contact with environmental moisture, a layer of frost forms on the aluminium fins, eventually blocking the airflow.

To resolve this physical issue, the system temporarily reverses its operation: it stops heating the room to send that heat to the external unit to melt the ice. During this process, it is normal for the indoor unit to stop and for the outdoor unit to emit light steam or gas expansion noises. This is standard operation and typically lasts between 5 and 10 minutes.

To identify whether we are dealing with a normal operational pause or a genuine fault, we should observe the behaviour of the outdoor fan. If, after the waiting period, the unit resumes normal operation, the system is healthy. However, if the outdoor fan never spins, you hear a persistent electrical hum with no blade movement, or the indoor split displays an error code, it is highly likely that the fan motor or the starting capacitor has failed.

Furthermore, it is worth bearing in mind that lower-end units or older models without advanced Inverter technology have critical limitations. When the outdoor temperature drops below 0°C or 2°C, their performance (COP) falls dramatically. In such cases, the machine consumes a vast amount of energy but is unable to generate enough heat, entering endless defrost cycles that prevent maintaining a comfortable temperature in the home.

Restoring thermal comfort at home during the coldest months requires a thorough understanding of why a machine designed for climate control stops fulfilling its heating function. Throughout this text, we have detailed the most common causes of this dysfunction. The spectrum of solutions is broad, ranging from basic adjustments—such as correctly selecting the heat pump mode on the remote and regularly sanitising filters—to highly complex technical interventions.

At this point, it is clear that internal faults do not disappear on their own. A shortage of refrigerant gas due to unnoticed leaks, faulty readings from temperature probes, or a mechanically jammed four-way valve invariably require the expertise of a specialist. Tampering with the closed circuit or electrical components without the proper qualifications poses a risk to both the user and the integrity of the machinery, significantly increasing the final repair costs.

For this reason, the smartest long-term strategy is always continuous prevention. Scheduling annual preventive maintenance is key to ensuring maximum energy efficiency, optimising air quality, and detecting any worn parts in good time. A well-maintained and regularly serviced unit rarely fails unexpectedly, avoiding nasty surprises exactly when you need its heating power the most.

Do not let low temperatures compromise your family’s daily comfort. Request a visit from an expert technician at JAG Alcaide to inspect, diagnose, and repair your climate control system. We travel throughout Barcelona and its metropolitan area to restore your air conditioning unit to peak performance quickly, safely, and with a full guarantee.