Why Is My Heat Pump Not Heating My House?

Why Is My Heat Pump Not Heating My House?

A heat pump that runs continuously but never gets the house warm is one of the most common and most frustrating problems reported by UK homeowners. The outdoor unit is operating, the controller shows the heating is on, and yet rooms never reach the set temperature, some areas of the house feel consistently cold, and the electricity bills keep climbing.

The important thing to understand is that this situation rarely points to a heat pump fault. In the large majority of cases, the heat pump unit is working correctly. The problem lies in how the system has been configured, how the emitters are sized, how the heating circuit is balanced, or how the controls have been set up. These are all fixable problems, and most do not require replacing any major components.

This guide works through the most common causes in order of how often we encounter each one, explains what each cause looks like in practice, and describes what needs to be checked.

Understanding the actual cause is the essential first step. Acting on the wrong assumption, for example upgrading the heat pump when the real issue is radiator sizing, wastes money and leaves the underlying problem in place.

Is The Heat Pump Actually Running?

Before investigating the cause of poor heating performance in detail, it is worth confirming what the heat pump is actually doing. A system that is not running at all has completely different causes from a system that is running but failing to heat the property.

Work through these checks on the controller and around the property before going further:

• Is the outdoor unit running? It should be audible and, during cold weather, you may see steam rising from it during defrost cycles

• Are any fault codes or warning symbols shown on the controller display? A fault code that has not been investigated is a common reason for no heating output

• Is the controller actively calling for heating? Check that the schedule is enabled and that the thermostat set point is above the current measured room temperature

• Are the radiators or underfloor circuits getting warm? Walk around and touch each radiator to confirm that heat is circulating to all parts of the property

• Is the system within its normal pressure range? Most UK domestic heat pump systems should show between 1.0 and 2.0 bar when cold

If the outdoor unit is not running, the cause is likely a fault code, a disabled heating schedule, a tripped circuit breaker, or a control configuration issue that has prevented the system from starting.

If the unit is running and the controls are calling for heat but the house still feels cold throughout, the causes below cover the most common explanations.

The Flow Temperature Is Too Low

Flow temperature is one of the most significant variables in how much heat a heat pump delivers to the radiators. If the flow temperature is lower than what the system needs to meet the building’s heat demand, rooms will feel cold regardless of how long the heating runs.

Heat pumps operate at lower flow temperatures than gas boilers by design. A typical domestic system aims for between 35°C and 50°C depending on outdoor conditions and emitter sizes. The consequence is that radiators must be larger to deliver the same heat output, and the system needs to run for longer steady periods rather than short intense bursts.

This is not a fault. It is how the technology works. The challenge is ensuring the flow temperature is high enough to heat the property during cold weather while remaining low enough to sustain good efficiency in mild conditions. Getting this balance right is the role of weather compensation. Our guide on what flow temperature your heat pump should run at explains the appropriate ranges for different property types and emitter configurations.

Flow temperature problems typically arise in these specific situations:

• Weather compensation settings that use too flat a curve, so flow temperature does not rise sufficiently as outdoor temperature falls

• A system that was never properly commissioned, with the flow temperature left at a factory default that does not reflect the property’s actual heat demand

• Flow temperatures that have been reduced by an installer trying to improve efficiency, without checking whether the emitter output at the new lower temperature is sufficient to meet peak winter demand

The Radiators Are Too Small

Radiator output is not a fixed value. The amount of heat a radiator emits depends directly on the temperature of the water flowing through it. Manufacturers publish output ratings based on a condition called Delta T 50, which assumes the mean water temperature is 50 degrees above room temperature. Under a gas boiler running at 70°C, many radiators operate close to this rated output.

Under a heat pump running at 40°C to 45°C, the same radiator operates at Delta T 20 to Delta T 25, producing roughly 40 to 50% of its rated output. A radiator that was marginal under a boiler will fall well short of what is needed under a heat pump at the same water temperature. The heat pump is not underperforming; the emitters simply cannot release enough heat at lower water temperatures.

The symptoms of undersized radiators in a heat pump system are consistent and recognisable:

• Specific rooms that never reach their set temperature, even after several hours of continuous heating

• Bedrooms, north-facing rooms, and rooms at the end of the pipe circuit consistently colder than the rest

• The system running throughout the day without the house ever feeling properly warm

• Higher electricity consumption than expected from a system that appears to be working but is struggling to transfer enough heat

Not every radiator in the property will need replacing. In many cases, upgrading a small number of the worst-performing emitters in critical rooms is sufficient to resolve the problem. Our article on whether heat pumps need bigger radiators explains the Delta T calculation in detail. Our case study on a house that failed to reach temperature due to an inaccurate heat loss calculation and undersized radiators shows what this issue looks like in a real installation and how it was resolved.

The Heating System Is Poorly Balanced

System balancing is the process of adjusting the flow through each radiator so heat is distributed evenly across the whole property. In an unbalanced system, radiators close to the heat pump receive too much flow and overheat quickly, while those further away on the circuit receive too little and may barely warm up at all.

The practical signs of a poorly balanced heating system are consistent:

• The closest radiators to the heat pump get very hot, while those furthest away remain cool or cold

• Some rooms heat up quickly and overshoot their temperature while others take hours and never arrive

• The heat pump reaches its high-temperature cutoff in part of the circuit before the rest of the system has received adequate heat

• The homeowner raises the flow temperature to compensate for cold rooms, which drives up running costs without fixing the root cause

Balancing involves adjusting the lock-shield valves on each radiator, working progressively outward from the heat pump. It is frequently performed incorrectly at installation and rarely revisited during service visits, yet it has a significant impact on comfort and efficiency. Our article on how heat pump system balancing works and why it matters explains the process and what correct balancing should achieve.

Too Many TRVs Are Restricting Flow

Thermostatic radiator valves reduce flow to a radiator once the room reaches its set temperature. Used correctly, they allow individual rooms to be maintained at different temperatures. The problem for heat pumps is that they have a minimum system flow rate requirement: if total circuit flow drops too far, the heat pump will short cycle, fault, or run inefficiently.

When TRVs close across multiple rooms simultaneously, the cumulative effect on total flow can be significant:

• The heat pump short cycles repeatedly because the reduced load causes it to reach its target temperature and shut down faster than it should, which reduces efficiency and increases wear

• Rooms with closed TRVs may never receive adequate heat because flow has effectively been diverted away from them

• In more extreme cases, the heat pump generates a low flow fault and locks out entirely, leaving no heating at all

• The homeowner adjusts the thermostat to try to compensate, which can worsen the cycling pattern further

Heat pump systems generally perform better with fewer zone restrictions than gas boiler systems. Keeping the majority of radiators fully or nearly fully open provides the stable flow and consistent load the heat pump needs to run efficiently in long steady cycles. Our article on what happens when too many TRVs are turned down at once explains the mechanism and how to identify whether this is a factor in your system.

The House Loses Heat Faster Than Expected

Every heat pump installation should begin with a heat loss calculation: a room-by-room assessment of how much heat the building needs to maintain a target temperature on the coldest expected day. The heat pump is sized to meet that figure, and the radiators are sized to deliver it. If those calculations are correct and the system is set up properly, the property should be comfortable.

If the actual heat loss is higher than the calculation assumed, the system will struggle regardless of how well it has been commissioned. The heat pump may be operating correctly and delivering exactly what it was designed to deliver. The building is simply losing heat faster than the system can replace it.

Reasons why actual heat loss may exceed the calculated figure include:

• Insulation that was assumed in the design was not installed, or was installed poorly and provides less thermal performance than specified

• Draughts through windows, external doors, loft hatches, and service penetrations that were not included in the original calculation

• A heat loss calculation that used generic or assumed U-values for wall and roof construction rather than measured values from the actual property

• Extensions, conservatories, or alterations carried out after the original installation that increase the total heated floor area without a corresponding increase in heat pump or emitter capacity

• A property in an unusually exposed location, at altitude, or subject to prevailing winds that increases the effective heat loss above what regional tables suggest

A useful distinguishing sign is that the system heats the property adequately in mild autumn and spring weather but consistently falls short during cold snaps, because the heat pump’s output matches design conditions but not the actual peak demand. Our article on heat loss in a house and what it means for heat pump performance explains what a proper heat loss assessment should cover and why it matters.

The Weather Compensation Is Not Set Correctly

Weather compensation is the control mechanism that automatically adjusts the flow temperature sent to the radiators based on how cold it is outside. When outdoor temperature drops, flow temperature increases to compensate for the greater heat demand. When conditions are mild, it reduces, so energy is not wasted overheating the system.

This is the primary control strategy for maintaining consistent indoor temperatures across a range of outdoor conditions. Without it properly configured, either the system runs at a fixed high temperature that wastes energy in mild weather, or a fixed low temperature that falls short in cold weather. It is perhaps the single most impactful setting on a heat pump system.

When the weather compensation curve is set incorrectly, the resulting symptoms are often misattributed to other causes:

• The property is comfortable in autumn and spring, but rooms become cold during cold snaps because the curve does not rise steeply enough at low outdoor temperatures

• Running costs are higher than expected even when the house feels comfortable, because the curve is set too aggressively and the flow temperature is unnecessarily high in mild conditions

• The heat pump short cycles because the curve combined with a room thermostat creates a stop-start pattern rather than steady long runs

• If switched off entirely, the system runs at a fixed flow temperature that may be adequate in cold weather but is grossly inefficient in mild conditions, or adequate in mild weather but insufficient when temperatures fall below 2 or 3 degrees

Weather compensation curves can be adjusted through the heat pump controller. Our guide on how to set weather compensation on a heat pump explains the settings and how to read the system’s response. A real example of what happens when this setting is missing entirely is illustrated in our case study on a Berkshire property where weather compensation had been disabled at installation handover, leaving the homeowner paying high electricity bills throughout winter with no fault code to indicate anything was wrong.

Could The Heat Pump Be Undersized?

When a heat pump runs throughout the day but the house remains cold, the instinctive conclusion for many homeowners and some engineers is that the heat pump is too small. This is sometimes correct, but it is also one of the most frequently wrong conclusions reached in the UK heat pump sector.

Many homeowners are advised to upgrade to a larger heat pump before simpler and less expensive causes have been properly investigated and ruled out. The problem is that a larger heat pump installed on a poorly configured system will not necessarily improve performance. If the real issue is radiator sizing, system balancing, or weather compensation settings, those problems will persist on a new larger heat pump.

In the worst cases, a larger heat pump on an underpowered emitter circuit short cycles more aggressively than the smaller unit it replaced. Our case study on a couple in Yorkshire who were told they needed a bigger heat pump when the real problem was how the system was designed illustrates how quickly this conclusion is reached and how often it is incorrect.

Before concluding undersizing, the following should all be checked and confirmed as correct:

• An accurate room-by-room heat loss calculation that reflects the property as it actually exists, not a desktop estimate based on floor area

• Flow temperatures that are appropriate for the emitter sizes and sufficient to meet the heat demand during cold weather

• Radiator sizing that has been checked at heat pump flow temperatures, not just carried over from a previous boiler installation

• System balancing that distributes flow evenly to all radiators throughout the property

• Weather compensation settings that adjust flow temperature appropriately across the full range of outdoor conditions

• Circulation issues including dirty filters, closed valves, or low system pressure that may be limiting effective output

Only when all of those elements have been verified and the shortfall continues should the heat pump’s kilowatt output be questioned. Our guide on the signs that a heat pump may genuinely be undersized helps distinguish between genuine undersizing and poor configuration.

Could It Be A Design Issue?

Some persistent underperformance is not explained by a fault, a settings error, or a component limitation. It reflects the original system design being wrong from the beginning, usually because the design process was inadequate before installation started.

Identifying design problems requires reviewing the original heat loss calculation, the commissioning records, the radiator schedule, and the controller settings together as a complete picture. Our guide on common commissioning mistakes with air source heat pump installations covers the design and setup decisions that most consistently lead to a system that has never performed as expected.

• A heat pump sized from floor area rules of thumb rather than a room-by-room heat loss calculation, leading to systematic undersupply in cold weather

• Radiators retained from the previous boiler installation without recalculating output at heat pump flow temperatures across every room

• Hot water cylinder or coil sizing that does not match the heat pump’s output capacity, creating competition between heating and hot water demand

• System layout with components such as buffer tanks that dilute flow temperature and slow system response without providing a corresponding benefit

• Control strategies configured at commissioning with default settings that were never adjusted to suit the property’s actual heat loss profile

The consistent indicator of a design problem rather than a developing fault is that the system has never performed correctly. Problems were present from the first winter, not something that appeared after the system had previously been working well.

If you are planning a heat pump installation and want an independent assessment of whether the proposed design is suitable before any work begins, our Pre-Installation Design Review reviews the heat loss methodology, heat pump sizing, emitter selection, flow temperature strategy, and overall system design.

Design problems are significantly easier and less expensive to identify before installation than to correct after the system is in place.

Need Help Diagnosing A Heat Pump Heating Problem?

If your heat pump is running but your home still does not feel warm, our Fix My Heat Pump service can identify the specific cause.

We review your system details, photographs, controller settings, flow temperatures, and performance evidence to identify what is actually limiting the heating output and what needs to change.

Whether the cause is a settings configuration, a distribution fault, an emitter shortfall, or an original design problem, knowing the precise cause is the only effective starting point for resolving it without wasting money on the wrong solution.