Why is my Air Source Heat Pump not reaching the right temperature?

Maintaining a comfortable indoor temperature with an air source heat pump (ASHP) can sometimes present a challenge. If your system isn’t hitting the desired temperature – don’t panic – it’s a common concern for homeowners!

In many cases, issues are easily diagnosed through regular Air Source Heat Pump Servicing, but if your system consistently struggles to maintain the right temperature, it’s worth investigating.

Discover the main problems below – and how to rectify them quickly.

Weather limitations

When temperatures drop near or below freezing, heat pumps become less efficient, as they’re extracting heat from colder air to transfer into your home.

Your system will take longer to heat, struggle to reach higher thermostat settings and may rely more heavily on its defrost cycle, which briefly pauses heating.

Extremely high temperatures reduce your heat pump’s cooling efficiency, particularly with restricted airflow or poor insulation.

What you can do

• Don’t presume something is broken – extreme weather reduces efficiency
• Look at improving your home’s insulation and air sealing
• Ensure your system was properly sized to handle your climate
• If the performance drop is severe or persistent, consider a system assessment

Dirty or clogged air filters

Air filters play a crucial role in keeping your heat pump running efficiently, but if debris builds up in the filter, the system works harder.

When filters are blocked:

• Your home may heat or cool unevenly, or for longer periods
• Airflow through vents can feel weak
• The temperature you set may not be reached

Why is it important?

Poor airflow means harder work for your heat pump, straining internal components and leading to bigger issues.

What you can do

• Check your filters regularly, cleaning or replacing them if they look filthy or clogged
• Keep the areas around vents and units clear
• Book a professional if problems aren’t resolved

Ice or debris on the outdoor unit

The outdoor unit of your air source heat pump needs clear, consistent airflow, or the system’s efficiency is greatly reduced.

What to look for:

• Ice forming on or around the outdoor unit – causing a faulty or inefficient defrost cycle
• Reduced airflow or a “stuck” fan – causing a blocked airflow
• Louder operating noise – leading to low refrigerant levels
• Poor heating performance, especially in cold or damp conditions, which can cause water to freeze on the coils

What you can do:

• Keep the unit and surrounding area clear
• Don’t chip at ice manually – this’ll damage coils and fins
• If the unit repeatedly ices over, call a professional to inspect the defrost system and refrigerant levels

Low refrigerant levels or leaks

Refrigerant is the lifeblood of your system, allowing it to absorb and transfer heat. If levels drop, your heat pump will struggle.

Signs of low refrigerant:

• The system fails to reach the set temperature
• Air from vents is lukewarm or cooler than expected
• Ice forming on coils or refrigerant lines
• Hissing or bubbling noises from the unit
• Higher energy bills for no obvious reason

Why it matters:

Low refrigerant reduces efficiency and can damage the compressor over time, leading to expensive repairs. Act quickly to prevent a potential system breakdown.

What you can do:

CA qualified heating engineer will:

• Locate and repair any leaks
• Refill and re-pressurise the system safely

Routine servicing will pay for itself, as engineers check refrigerant levels during each visit.

Thermostat settings or faults

If your heat pump isn’t hitting the right temperature, the problem could be the unit’s operational settings.

Common thermostat-related issues:

• The temperature is set too low or too high
• Batteries in wireless thermostats are running low
• Thermostat installed in a draughty or overly warm spot
• Compatibility issues with older systems or third-party smart controls
• Sensor malfunctions cause inaccurate readings

What to look for:

• Your heat pump doesn’t respond to temperature changes
• Inconsistent heating or cooling
• The display is blank or flashing error messages
• Your system short-cycles or doesn’t switch off

What you can do:

• Check settings – temperature, schedule, and mode
• Replace batteries if your thermostat is battery-operated
• Reposition the thermostat if near a heat source or window
• Consider upgrading to a smart thermostat
• If you’re unsure, a professional can check calibration and compatibility

Poor home insulation or leaky ductwork

If your system isn’t reaching the desired temperature, consider your home’s insulation.

Common signs:

• Rooms heat unevenly or lose heat quickly after switching off
• Draughts around windows, floors, or loft spaces
• The heat pump runs constantly with little effect
• Temperature fluctuation between rooms

What could be happening:

• Walls, lofts, or floors are poorly insulated
• Windows and doors have gaps or single glazing
• Air ducts are leaking heated air into wall cavities or loft spaces
• Open plan layouts or high ceilings may allow heat to rise too quickly

What you can do:

• Improve insulation in key areas
• Seal draughts around windows, doors, and pipework
• Check your ducting for leaks or poor connections
• Use zone controls or smart thermostats to heat only the rooms you use

Wrong size or poor installation

If your air source heat pump has regularly underperformed, the root cause could be the choice and installation.

Undersized systems:

• Can’t deal with heating or cooling needs
• Run constantly, driving up energy use
• Leave rooms cold or stuffy

Oversized systems:

• Turn on and off too quickly
• Fail to maintain steady temperatures
• Experience more wear and tear

Other installation issues might include:

• Poor positioning of the outdoor unit (e.g. blocked airflow)
• Incorrect refrigerant charge
• Incompatible or outdated controls
• Improperly connected pipework or ducting

What you can do

• Get a full system review to assess sizing, layout, and setup
• Adjustments or system optimisation may resolve the issue without a full replacement

Mechanical or electrical issues

These are less common, but can affect performance.

Possible issues include:

• Faulty or worn-out compressor
• Malfunctioning reversing valve (can’t switch between heating and cooling)
• Electrical faults in wiring, capacitors, or sensors
• Motor or fan problems inside the unit

What to look out for:

• The system isn’t switching modes properly
• You hear loud humming, buzzing, or clicking
• The unit trips the circuit breaker
• Temperature is inconsistent despite good airflow and clean filters

What you can do:

• Don’t attempt electrical or internal repairs yourself
• Switch off the system if it’s making unusual noises or showing warning codes
• Call a qualified engineer to diagnose the fault safely and accurately

 Before calling out a technician, run through these quick checks:

• Is the thermostat set correctly and working?
• Have you cleaned or replaced the air filters?
• Is the outdoor unit clear?
• Are all vents open and airflow unblocked indoors?
• Are doors and windows closed to keep heat in?
• Is your home well-insulated?

If you’ve checked these and the system’s still not reaching temperature, call a professional. This’ll save time, money, and stress, preventing minor issues from becoming major repairs.

The good news? Most issues are fixable and preventable with regular servicing, simple maintenance, and the right system setup.

Regular performance checks help keep your system running at peak efficiency, cutting your energy bills and lowering your carbon footprint. A detailed performance report gives you clear, actionable insights to help you optimise your heat pump’s output and savings.