Why Is My Heat Pump Fine In Autumn But Struggles In Winter?
Why Is My Heat Pump Fine In Autumn But Struggles In Winter?
Why Is My Heat Pump Fine In Autumn But Struggles In Winter?
Why Is My Heat Pump Fine In Autumn But Struggles In Winter?
Why Is My Heat Pump Fine In Autumn But Struggles In Winter?
UK Heat pump Help Technical Team
Independent Heat Pump Engineer
Why Is My Heat Pump Fine In Autumn But Struggles In Winter?
One of the most common concerns we hear from UK homeowners every year, usually from December onwards, is some version of the same thing: the heat pump worked perfectly well in October and November, but now that proper winter has arrived, it simply cannot seem to keep up. The house is not reaching temperature. The system seems to be running constantly. Something feels different.
This often leads homeowners to conclude that the heat pump has developed a fault, or that the unit is no longer capable of heating the property. In the majority of cases, neither of those things is true.
Winter Does Not Usually Create Heat Pump Problems. Winter Exposes Them.
This is one of the most important things to understand about heat pump performance, and it is something that causes a great deal of confusion. When a heat pump suddenly appears to struggle in winter, most people assume something has gone wrong with the equipment. The assumption is understandable — the system was working, and now it is not. But the reality is usually more straightforward and more fixable than a hardware failure.
What actually happens is that winter places the system under a fundamentally different level of demand compared to autumn. As outdoor temperatures fall, two things happen simultaneously and they work against each other. The heat pump's available output gradually reduces as there is less thermal energy in the colder air for it to extract. At exactly the same time, the property's heat loss increases because the greater the difference between indoor and outdoor temperatures, the faster heat escapes from the building fabric.
The result is that the heat pump has less heat available to deliver, whilst the house needs more heat than it has ever needed before. A system that appeared perfectly adequate during a mild autumn can begin to struggle noticeably during a cold January or February, not because anything has broken, but because the margin that was hiding the underlying problem has been removed.
Why Does A Heat Pump Produce Less Heat In Winter?
All air source heat pumps produce less output as outdoor temperatures fall. They are still extracting heat from the air, but there is less available energy to collect at lower temperatures. Modern heat pumps are designed to continue operating effectively well into sub-zero conditions, but their output at minus five degrees is meaningfully lower than their output at ten degrees. This is a fundamental characteristic of how refrigerant-based heating works, not a fault.
The problem occurs when a system was already operating close to its design limits during autumn. A heat pump that was just about adequate at ten degrees outside may be noticeably inadequate at zero degrees. Our article on is my heat pump undersized — 5 signs to look for covers the specific signs that a heat pump may not have been correctly sized for the property from the outset, including the pattern of autumn performance followed by winter struggle that we are describing here.
Why Does My House Need More Heat In Winter?
At the same time as the heat pump's output is reducing, the property's demand is increasing. Heat loss from a building is driven by the temperature difference between inside and outside. The greater that difference, the faster heat escapes through walls, windows, the roof, and the floor. At twelve degrees outside, a well-insulated home may need a relatively modest heat input to stay comfortable. At zero degrees, that same home needs considerably more. At minus three or below, the system is working under its most demanding conditions of the entire year.
This is why problems that were invisible in October or November only become apparent in January. The house was never truly tested until the coldest weeks arrived. Our article on heat loss in a house — what it means and why it matters for heat pumps explains how heat loss is calculated, why getting those calculations right matters so much for heat pump sizing, and what happens when they are underestimated.
Common Reasons Heat Pumps Struggle In Winter
Incorrect Heat Loss Calculations
If the original heat loss calculations carried out before installation were incorrect, the heat pump may have been undersized from the very beginning. The system appeared to work during autumn because demand was relatively low and the gap between what the heat pump could deliver and what the house needed was small. Winter simply closed that gap and exposed what was always there. This is one of the most common findings in our technical reviews, and it is entirely preventable with a proper independent design check before installation. If you are planning an installation and want to avoid this outcome, our pre-installation design and heat loss review service checks the heat loss calculations and system design before any money is committed.
Radiators Are Too Small
Radiators that appear to be heating a room adequately during mild autumn weather may be genuinely undersized for heat pump operation. Heat pumps run at lower flow temperatures than gas boilers typically between 35 and 50 degrees rather than 60 to 80 degrees. At those lower flow temperatures, a radiator produces significantly less heat output than it would on a gas system. If the radiators were not correctly sized for low-temperature operation during the design stage, they may keep up during October but fall behind during the coldest months. Our article on do heat pumps need bigger radiators explains how radiator output changes at different flow temperatures and what to look for if you suspect yours are not performing adequately. Our case study on house not reaching temperature — incorrect heat loss and undersized radiators is a real example of exactly this problem, where the system appeared to work during milder weather before winter revealed the true extent of the design shortfall.
Weather Compensation Is Not Set Correctly
Weather compensation is one of the most important settings on any heat pump. It controls how the flow temperature adjusts as outdoor temperatures change raising it when it is colder outside and lowering it when it is milder, which keeps the system efficient without sacrificing comfort. A weather compensation curve that is set too shallow will not raise the flow temperature aggressively enough as winter sets in, leaving the property unable to reach its target temperature on the coldest days. Our article on what does weather compensation actually do explains how the curve works in plain language, and our article on how to set weather compensation on a heat pump gives practical guidance on adjusting it correctly for your property.
Flow Temperatures Are Set Too Low For Winter Conditions
There is a common belief that heat pumps should always run at the lowest possible flow temperature. Whilst lower flow temperatures do improve efficiency, the temperature still needs to be high enough to actually heat the property. A flow temperature that delivers comfortable rooms at ten degrees outside may be completely inadequate at zero degrees. Our article on what flow temperature should my heat pump run at explains how to find the right balance between efficiency and performance, and why a fixed low flow temperature is often not appropriate across all winter conditions. Our case study on the family home in Birmingham running at 55 degrees all winter is a good illustration of the other extreme — a system running at an unnecessarily high fixed flow temperature that was costing far more than it needed to, when weather compensation would have delivered better efficiency and the same comfort.
More Frequent Defrost Cycles In Cold Weather
During cold and damp winter weather, heat pumps spend more time in defrost mode. This is completely normal and is not a fault. However, every defrost cycle is a period during which the heat pump is not delivering heat into the property. For most well-designed systems this is not a significant issue, but if the system was already operating at its limits, the additional time spent in defrost can tip the balance. Our article on why does my heat pump defrost so often explains what is normal in terms of defrost frequency and what patterns suggest the system is working harder than it should be. If you have ever looked out of the window and been alarmed by what looked like smoke coming from the outdoor unit during defrost, our article on why does my heat pump look like it is on fire explains exactly what you are seeing and why it is almost always nothing to worry about.
Commissioning Settings Were Never Optimised For Winter
Many heat pumps are commissioned during summer or early autumn when outdoor temperatures are mild. Settings that produce acceptable results during commissioning may not have been reviewed or adjusted before winter. If the installer did not return to check and optimise the settings before the cold weather arrived, there is a reasonable chance the system is not configured for its most demanding operating conditions. Our article on common commissioning mistakes with air source heat pumps covers the most frequent errors made at this stage and how they affect performance through the winter.
How Do I Know If There Is Actually A Problem?
A properly designed and correctly commissioned heat pump system should be able to maintain comfortable temperatures throughout a normal UK winter, including during the coldest periods. If your home is consistently failing to reach its target temperature, or if the system is running constantly without ever quite getting there, it is worth investigating the following areas before assuming the heat pump unit itself is faulty: the original heat loss calculations and whether the heat pump was correctly sized, radiator output at the actual flow temperatures the system is running at, the weather compensation curve and whether it is raising flow temperature sufficiently as outdoor temperatures fall, the flow temperatures themselves and whether they are high enough for current outdoor conditions, and the system flow rates and whether sufficient water is circulating through all emitters.
The fact that the heat pump appeared to work during autumn does not confirm that everything was correct. It may simply mean the weather had not yet placed enough demand on the system to reveal where the shortfall was. Our article on why your heat pump is not reaching target temperature works through the most likely causes in order and is a useful reference for anyone going through this process.
Related Case Studies
The house not reaching temperature — incorrect heat loss and undersized radiators case study is one of the clearest examples of a system that worked adequately during mild weather but failed during winter because the original design calculations were wrong. The installer returned and upgraded the system at no cost once the issue was properly documented.
In the why this £1.5 million home still felt cold with a heat pump case, a large new-build property with a heat pump that appeared correctly sized on paper could not maintain comfortable temperatures during cold weather. Thermal imaging revealed hidden insulation failures and air leakage that the original calculations had not accounted for a reminder that winter is often the first genuine test a system faces.
The family home in Birmingham running at 55 degrees all winter with higher than expected bills shows how incorrect flow temperature settings can cause a system to run inefficiently throughout the entire winter season, costing significantly more than necessary whilst delivering comfort that could have been achieved at a lower temperature with weather compensation properly configured.
Related Articles
Our article on what does weather compensation actually do is essential reading for anyone whose heat pump struggles as temperatures drop, as it explains the single most important setting for maintaining comfort efficiently across a variable UK winter.
Our article on is my heat pump undersized — 5 signs to look for covers the specific indicators that a heat pump was never correctly matched to the property's actual heat demand, including the autumn-to-winter performance drop described throughout this article.
Our article on what flow temperature should my heat pump run at explains how to find the right flow temperature for your system across different outdoor conditions, balancing efficiency with the actual heating output the property needs.
Need Help With Your Heat Pump?
If your heat pump worked well in autumn but is struggling now that winter has arrived, we may be able to identify the cause. Whether the issue is in the system design, the commissioning settings, the flow temperature configuration, or the radiators, most problems like this can be diagnosed during a remote technical review without an engineer visit. Visit our Fix My Heat Pump page to find out what is involved, or contact us directly to describe your situation and we will let you know if a review would be worthwhile.
Why Is My Heat Pump Fine In Autumn But Struggles In Winter?
One of the most common concerns we hear from UK homeowners every year, usually from December onwards, is some version of the same thing: the heat pump worked perfectly well in October and November, but now that proper winter has arrived, it simply cannot seem to keep up. The house is not reaching temperature. The system seems to be running constantly. Something feels different.
This often leads homeowners to conclude that the heat pump has developed a fault, or that the unit is no longer capable of heating the property. In the majority of cases, neither of those things is true.
Winter Does Not Usually Create Heat Pump Problems. Winter Exposes Them.
This is one of the most important things to understand about heat pump performance, and it is something that causes a great deal of confusion. When a heat pump suddenly appears to struggle in winter, most people assume something has gone wrong with the equipment. The assumption is understandable — the system was working, and now it is not. But the reality is usually more straightforward and more fixable than a hardware failure.
What actually happens is that winter places the system under a fundamentally different level of demand compared to autumn. As outdoor temperatures fall, two things happen simultaneously and they work against each other. The heat pump's available output gradually reduces as there is less thermal energy in the colder air for it to extract. At exactly the same time, the property's heat loss increases because the greater the difference between indoor and outdoor temperatures, the faster heat escapes from the building fabric.
The result is that the heat pump has less heat available to deliver, whilst the house needs more heat than it has ever needed before. A system that appeared perfectly adequate during a mild autumn can begin to struggle noticeably during a cold January or February, not because anything has broken, but because the margin that was hiding the underlying problem has been removed.
Why Does A Heat Pump Produce Less Heat In Winter?
All air source heat pumps produce less output as outdoor temperatures fall. They are still extracting heat from the air, but there is less available energy to collect at lower temperatures. Modern heat pumps are designed to continue operating effectively well into sub-zero conditions, but their output at minus five degrees is meaningfully lower than their output at ten degrees. This is a fundamental characteristic of how refrigerant-based heating works, not a fault.
The problem occurs when a system was already operating close to its design limits during autumn. A heat pump that was just about adequate at ten degrees outside may be noticeably inadequate at zero degrees. Our article on is my heat pump undersized — 5 signs to look for covers the specific signs that a heat pump may not have been correctly sized for the property from the outset, including the pattern of autumn performance followed by winter struggle that we are describing here.
Why Does My House Need More Heat In Winter?
At the same time as the heat pump's output is reducing, the property's demand is increasing. Heat loss from a building is driven by the temperature difference between inside and outside. The greater that difference, the faster heat escapes through walls, windows, the roof, and the floor. At twelve degrees outside, a well-insulated home may need a relatively modest heat input to stay comfortable. At zero degrees, that same home needs considerably more. At minus three or below, the system is working under its most demanding conditions of the entire year.
This is why problems that were invisible in October or November only become apparent in January. The house was never truly tested until the coldest weeks arrived. Our article on heat loss in a house — what it means and why it matters for heat pumps explains how heat loss is calculated, why getting those calculations right matters so much for heat pump sizing, and what happens when they are underestimated.
Common Reasons Heat Pumps Struggle In Winter
Incorrect Heat Loss Calculations
If the original heat loss calculations carried out before installation were incorrect, the heat pump may have been undersized from the very beginning. The system appeared to work during autumn because demand was relatively low and the gap between what the heat pump could deliver and what the house needed was small. Winter simply closed that gap and exposed what was always there. This is one of the most common findings in our technical reviews, and it is entirely preventable with a proper independent design check before installation. If you are planning an installation and want to avoid this outcome, our pre-installation design and heat loss review service checks the heat loss calculations and system design before any money is committed.
Radiators Are Too Small
Radiators that appear to be heating a room adequately during mild autumn weather may be genuinely undersized for heat pump operation. Heat pumps run at lower flow temperatures than gas boilers typically between 35 and 50 degrees rather than 60 to 80 degrees. At those lower flow temperatures, a radiator produces significantly less heat output than it would on a gas system. If the radiators were not correctly sized for low-temperature operation during the design stage, they may keep up during October but fall behind during the coldest months. Our article on do heat pumps need bigger radiators explains how radiator output changes at different flow temperatures and what to look for if you suspect yours are not performing adequately. Our case study on house not reaching temperature — incorrect heat loss and undersized radiators is a real example of exactly this problem, where the system appeared to work during milder weather before winter revealed the true extent of the design shortfall.
Weather Compensation Is Not Set Correctly
Weather compensation is one of the most important settings on any heat pump. It controls how the flow temperature adjusts as outdoor temperatures change raising it when it is colder outside and lowering it when it is milder, which keeps the system efficient without sacrificing comfort. A weather compensation curve that is set too shallow will not raise the flow temperature aggressively enough as winter sets in, leaving the property unable to reach its target temperature on the coldest days. Our article on what does weather compensation actually do explains how the curve works in plain language, and our article on how to set weather compensation on a heat pump gives practical guidance on adjusting it correctly for your property.
Flow Temperatures Are Set Too Low For Winter Conditions
There is a common belief that heat pumps should always run at the lowest possible flow temperature. Whilst lower flow temperatures do improve efficiency, the temperature still needs to be high enough to actually heat the property. A flow temperature that delivers comfortable rooms at ten degrees outside may be completely inadequate at zero degrees. Our article on what flow temperature should my heat pump run at explains how to find the right balance between efficiency and performance, and why a fixed low flow temperature is often not appropriate across all winter conditions. Our case study on the family home in Birmingham running at 55 degrees all winter is a good illustration of the other extreme — a system running at an unnecessarily high fixed flow temperature that was costing far more than it needed to, when weather compensation would have delivered better efficiency and the same comfort.
More Frequent Defrost Cycles In Cold Weather
During cold and damp winter weather, heat pumps spend more time in defrost mode. This is completely normal and is not a fault. However, every defrost cycle is a period during which the heat pump is not delivering heat into the property. For most well-designed systems this is not a significant issue, but if the system was already operating at its limits, the additional time spent in defrost can tip the balance. Our article on why does my heat pump defrost so often explains what is normal in terms of defrost frequency and what patterns suggest the system is working harder than it should be. If you have ever looked out of the window and been alarmed by what looked like smoke coming from the outdoor unit during defrost, our article on why does my heat pump look like it is on fire explains exactly what you are seeing and why it is almost always nothing to worry about.
Commissioning Settings Were Never Optimised For Winter
Many heat pumps are commissioned during summer or early autumn when outdoor temperatures are mild. Settings that produce acceptable results during commissioning may not have been reviewed or adjusted before winter. If the installer did not return to check and optimise the settings before the cold weather arrived, there is a reasonable chance the system is not configured for its most demanding operating conditions. Our article on common commissioning mistakes with air source heat pumps covers the most frequent errors made at this stage and how they affect performance through the winter.
How Do I Know If There Is Actually A Problem?
A properly designed and correctly commissioned heat pump system should be able to maintain comfortable temperatures throughout a normal UK winter, including during the coldest periods. If your home is consistently failing to reach its target temperature, or if the system is running constantly without ever quite getting there, it is worth investigating the following areas before assuming the heat pump unit itself is faulty: the original heat loss calculations and whether the heat pump was correctly sized, radiator output at the actual flow temperatures the system is running at, the weather compensation curve and whether it is raising flow temperature sufficiently as outdoor temperatures fall, the flow temperatures themselves and whether they are high enough for current outdoor conditions, and the system flow rates and whether sufficient water is circulating through all emitters.
The fact that the heat pump appeared to work during autumn does not confirm that everything was correct. It may simply mean the weather had not yet placed enough demand on the system to reveal where the shortfall was. Our article on why your heat pump is not reaching target temperature works through the most likely causes in order and is a useful reference for anyone going through this process.
Related Case Studies
The house not reaching temperature — incorrect heat loss and undersized radiators case study is one of the clearest examples of a system that worked adequately during mild weather but failed during winter because the original design calculations were wrong. The installer returned and upgraded the system at no cost once the issue was properly documented.
In the why this £1.5 million home still felt cold with a heat pump case, a large new-build property with a heat pump that appeared correctly sized on paper could not maintain comfortable temperatures during cold weather. Thermal imaging revealed hidden insulation failures and air leakage that the original calculations had not accounted for a reminder that winter is often the first genuine test a system faces.
The family home in Birmingham running at 55 degrees all winter with higher than expected bills shows how incorrect flow temperature settings can cause a system to run inefficiently throughout the entire winter season, costing significantly more than necessary whilst delivering comfort that could have been achieved at a lower temperature with weather compensation properly configured.
Related Articles
Our article on what does weather compensation actually do is essential reading for anyone whose heat pump struggles as temperatures drop, as it explains the single most important setting for maintaining comfort efficiently across a variable UK winter.
Our article on is my heat pump undersized — 5 signs to look for covers the specific indicators that a heat pump was never correctly matched to the property's actual heat demand, including the autumn-to-winter performance drop described throughout this article.
Our article on what flow temperature should my heat pump run at explains how to find the right flow temperature for your system across different outdoor conditions, balancing efficiency with the actual heating output the property needs.
Need Help With Your Heat Pump?
If your heat pump worked well in autumn but is struggling now that winter has arrived, we may be able to identify the cause. Whether the issue is in the system design, the commissioning settings, the flow temperature configuration, or the radiators, most problems like this can be diagnosed during a remote technical review without an engineer visit. Visit our Fix My Heat Pump page to find out what is involved, or contact us directly to describe your situation and we will let you know if a review would be worthwhile.
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If you're unsure whether your heat pump problem can be diagnosed remotely, send us a short description of the issue and we’ll let you know if a technical review is worthwhile. No obligation.
If you're unsure whether your heat pump problem can be diagnosed remotely, send us a short description of the issue and we’ll let you know if a technical review is worthwhile. No obligation.
If you're unsure whether your heat pump problem can be diagnosed remotely, send us a short description of the issue and we’ll let you know if a technical review is worthwhile. No obligation.
