Why Is My AC Not Cooling Properly? Common Causes

Why Is My Air Conditioning Not Cooling Properly? Common Causes in Essex Homes

When your air conditioning stops cooling effectively, the cause is usually one of a handful of specific issues that you can identify quickly. This guide walks you through the most common problems, what you can check yourself, and when to call an engineer.

Before You Panic: What Qualifies as “Not Cooling Properly”?

First, understand what normal AC performance looks like. A working system reduces room temperature by 3 to 5 degrees compared to outside temperature. If outside temperature is 28°C and your system is working correctly, your room will cool to around 23 to 25°C.

If your room stays at 26°C or higher even when the system runs constantly, something is wrong. If the system runs but doesn’t cool at all, that’s also a clear fault.

Minor variations happen (a very hot day, a room with lots of windows, equipment heat load), but significant cooling failure is always a sign to investigate.

The Thermostat is Set Incorrectly (Most Common)

Before assuming your AC is broken, check the thermostat. This solves about one third of “not cooling” calls.

Verify:

• The mode is set to “Cool” or “AC” (not “Heat” or “Off”)
• The target temperature is lower than the current room temperature
• The fan is set to “Auto” or “On” (not “Off”)
• The thermostat batteries aren’t dead (if it’s wireless)

Some homeowners accidentally switch to Heat mode during summer, or set the target temperature higher than the room’s current temperature. The system won’t cool if the target is set to 28°C and the room is already 24°C.

Spend 30 seconds checking this before assuming there’s a fault.

The Air Filter is Blocked

A clogged air filter restricts airflow, reducing cooling performance dramatically. If your system is running but barely any cool air comes from the vents, check the indoor unit’s filter.

Filters should be replaced every 1 to 3 months during heavy use. A blocked filter looks grey or brown instead of white. Air barely moves when you hold your hand near the vents.

Replacing a filter is simple and cheap (£5 to £15). Most homeowners can do this themselves. Open your indoor unit’s access panel (usually a plastic cover), remove the old filter, and slide in a new one matching the same dimensions.

If the filter was very blocked, clear it and wait 15 minutes for the system to resume normal cooling. Frozen evaporator coils (see below) sometimes develop if a blocked filter runs too long, which requires professional attention.

The Outdoor Unit is Blocked or Dirty

The outdoor condenser unit needs airflow to release heat. If the unit is covered in dust, leaves, plant growth, or debris, cooling performance suffers.

Check your outdoor unit for:

• Dust and dirt buildup (normal, especially near roads or in summer)
• Leaves, plant growth, or nesting material inside the fins
• Boxes, bins, or objects blocking airflow around the unit
• Leaves clogging the fan inlet

If it’s dirty, turn off the system and gently rinse the outdoor unit with a garden hose. Use low pressure to avoid damaging the fins. Clear any debris by hand (wearing gloves). Never use high-pressure water, as this damages internal components.

A seriously blocked outdoor unit prevents proper cooling and can cause damage if left unaddressed. If the unit is very dirty or you’re unsure how to clean it safely, call an engineer.

Low Refrigerant Charge

If the filter is clean, the outdoor unit is clear, and the thermostat is correct, refrigerant charge is likely the problem.

Refrigerant is the substance that actually removes heat from your room. If the air conditioning system is low on refrigerant, it can’t cool properly. Low charge happens due to a slow leak, usually at connection points.

Signs of low refrigerant include:

• System runs but produces barely any cool air
• Ice or frost forms on the indoor or outdoor unit in warm weather (counterintuitive, but a sign of the fault)
• Outdoor unit sounds like it’s working hard but nothing happens

You cannot fix this yourself. Low refrigerant is a job for a qualified engineer. Don’t add refrigerant without diagnosing the leak first, as this masks the underlying problem and wastes money on top-ups.

A professional will locate the leak, repair it, test the system, and recharge it properly. This typically costs £300 to £600.

The Compressor Isn’t Running

The compressor is the heart of your AC system. It’s located in the outdoor unit and does the heavy lifting to compress and circulate refrigerant.

If the compressor doesn’t run, the system won’t cool. Check whether the outdoor unit makes any sound when you turn on the system. A working compressor produces a steady humming or buzzing noise.

If the outdoor unit is completely silent, several things could be wrong:

• The power supply to the outdoor unit is disconnected or tripped
• An internal electrical fault has occurred
• The compressor has failed (expensive, but rare in modern systems)

Check that the outdoor unit’s power cable is connected and the circuit breaker hasn’t tripped. If power is present but the unit is silent, call an engineer. Compressor issues require professional diagnosis and repair.

Evaporator Coil is Frozen

In rare cases, the indoor evaporator coil freezes over, blocking airflow and preventing cooling. This usually happens if:

• A blocked air filter has restricted airflow for an extended period
• Refrigerant is very low
• The outdoor temperature is very cold (unusual in Essex but can happen during freak cold spells)

Signs of a frozen coil include ice visible inside the indoor unit or no airflow despite the system running.

If you see ice, turn off the system immediately and don’t run it again until the ice has melted (usually several hours). Running a frozen system risks serious damage.

Once melted, check and replace the air filter, ensure the outdoor unit is clear, and turn the system back on. If it freezes again, call an engineer. Repeated freezing indicates a deeper fault (low refrigerant, coil blockage, or compressor malfunction).

The Room is Too Large for the System

Occasionally, a system that was correctly sized at installation simply isn’t adequate because the room’s heat load increased. This happens if:

• You’ve added large windows or skylights
• You’ve installed powerful equipment (additional lighting rigs, projectors, heat-generating devices)
• Insulation has degraded
• The room is now used more intensively than originally planned

A properly working system can only cool to a certain limit. If demand exceeds the system’s capacity, it runs constantly but can’t reach the target temperature.

Solutions include upgrading to a larger system (costly), reducing heat sources in the room, or improving insulation and shading to lower cooling load.

External Conditions Are Extreme

During extreme heat (temperatures above 35°C), air conditioning systems work at their maximum limit. A system rated for normal cooling may struggle to cool your room by the expected 3 to 5 degrees during a heatwave.

This isn’t a fault. It’s the system working as designed at its limits. Once the external temperature drops, normal cooling resumes.

If your system fails completely during normal temperatures, something is wrong. If it simply doesn’t cool as much during a heatwave, that’s expected performance under stress.

The System Needs Professional Servicing

If you’ve checked the thermostat, cleaned the filter, cleared the outdoor unit, and the system still doesn’t cool, servicing may be needed.

Annual servicing includes cleaning internal components, checking refrigerant levels, testing electrical connections, and ensuring the system is performing at peak efficiency. A system that hasn’t been serviced in over a year may develop reduced cooling performance due to dust buildup on internal coils or electrical contact corrosion.

Professional servicing costs £100 to £200 and often improves cooling performance significantly.

When to Call an Engineer

Call an engineer immediately if:

• The outdoor unit makes no sound when you turn on the system
• You suspect low refrigerant or a refrigerant leak
• Ice forms on the indoor or outdoor unit
• Electrical sparking or burning smells occur
• You’ve checked the thermostat, filter, and outdoor unit but cooling hasn’t improved
• The system is more than 1 year without professional servicing

A service call typically costs £100 to £150 for diagnosis. If a fault is found, repairs vary in cost from £200 for simple fixes to £800 for component replacement. Emergency calls outside business hours may incur additional fees.

Getting Back to Cool Comfort

Most cooling problems have simple explanations and quick fixes. Start with the basics: check your thermostat, replace the filter, and clear the outdoor unit. If those don’t work, call a professional. Prompt action prevents small issues from becoming expensive failures.

Frequently Asked Questions