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HOMEOWNER GUIDE · HEAT PUMP SIZING
Has My Heat Pump Been Undersized?
Signs, causes, and how heat-pump capacity is properly checked.
- Last updated: September 2026
- Heat Loss Hub · Homeowner Advice
If your heat pump runs for long periods but the house still struggles to stay warm, it is reasonable to question whether the installed unit is too small.
Perhaps the problem only appears during colder weather. Maybe the backup heater is operating more often than expected. The thermostat target may never quite be reached. Or another engineer may have suggested that the heat pump does not have enough capacity for the property.
Those are all valid reasons to investigate. But a heat pump is not proved undersized simply because it runs continuously or because the house feels cold.
The real question is: can the installed heat pump provide the property’s credible design heat requirement at the outdoor and water temperatures it was supposed to operate at? And just as importantly: can the heating system actually deliver that available heat into the rooms?
KEY TAKEAWAY
If you want the installed model independently checked, see our Independent Heat Pump Sizing Review.
CHECK YOUR SYSTEM
Want the installed capacity independently verified against your property?
Quick Answer — How Do I Know If My Heat Pump Is Undersized?
Possible warning signs include the whole house struggling to maintain temperature during cold weather, the compressor running continuously while indoor temperatures continue to fall, supplementary electric heating operating frequently, the property being comfortable in mild weather but increasingly struggling as outdoor temperature drops, or the installed heat pump’s actual output appearing lower than the credible building heat load.
But none of those signs proves undersizing on its own.
A proper assessment needs to consider: credible building heat loss → design outdoor temperature → required flow temperature → exact heat-pump output → actual system behaviour.
KEY TAKEAWAY
There is also another important question: is the heat pump failing to produce enough heat, or is enough heat being produced but not reaching the rooms? Those are different problems.
Homeowner Assessment Pathway — Where Should I Start?
You do not need to diagnose the heat pump yourself. But a few simple questions can help you work out which part of the system needs investigating first.
Step 1 — Is It One Room or Most of the House?
One or two rooms are cold: Start with the individual rooms. Check room heat loss, radiator or underfloor-heating output, balancing, local water flow, thermostatic valves, and room controls. If most of the property is comfortable, the heat pump itself is less likely to be the first thing to blame.
Most or all of the house is cold: Move to Step 2.
Step 2 — Does the Problem Mainly Appear During Cold Weather?
No, the house struggles even in mild weather: This may point towards a wider system problem such as controls, flow temperature, emitters, hydraulics, commissioning, or equipment operation. A broader Independent Heat Pump Performance Review may be the better starting point.
Yes, the house is normally comfortable but struggles as outdoor temperatures fall: Move to Step 3.
Step 3 — Is the Heat Pump Running Continuously While the House Gets Colder?
No, the heat pump keeps stopping even though the house is cold: Before assuming undersizing, investigate why it is stopping. Possible causes include thermostat operation, zoning, schedules, control settings, domestic-hot-water priority, faults, or other commissioning issues. That is more likely to need a Performance Review than a sizing-only assessment.
Yes, it runs continuously but still cannot maintain temperature: Capacity becomes a much stronger line of investigation. Move to Step 4.
Step 4 — Is the Original Heat Loss Calculation Reliable?
I have a detailed room-by-room calculation and it appears credible: Move to Step 5.
I don’t have one, or I think the original figures may be wrong: The building demand needs establishing before the heat pump size can be checked properly. Start with an Independent Heat Loss Report.
Step 5 — Does the Installed Heat Pump Have Enough Output at the Actual Design Conditions?
This is where the technical sizing assessment begins. We need to compare credible building heat loss against the exact heat pump’s available output at the property’s design outdoor temperature and required flow temperature.
Output appears sufficient: The problem may lie elsewhere in the system. Investigate radiator output, system flow, pipework, hydraulics, controls, commissioning, and actual equipment operation. An Independent Heat Pump Performance Review may be the most appropriate route.
Output appears insufficient or the model selection is unclear: An Independent Heat Pump Sizing Review can establish whether the installed heat pump has enough capacity for the property.
A Simple Way to Think About It
KEY TAKEAWAY
One cold room? Check the room and emitter first.
Whole house cold only in winter? Check heat-pump capacity.
Heat pump keeps stopping while the house is cold? Check controls, commissioning and operation.
Heat pump runs continuously but cannot maintain temperature? Capacity becomes more suspicious.
Don’t trust the original heat loss? Recalculate the building first.
Heat loss is sound but you question the installed model? Independent Heat Pump Sizing Review.
Don’t know which part of the system is responsible? Independent Heat Pump Performance Review.
What Does an Undersized Heat Pump Actually Mean?
A genuinely undersized heat pump does not have sufficient available heating capacity to meet the building’s design heat demand under the conditions it was selected to operate at.
For example: building design heat loss: 8kW, but at the relevant outdoor temperature and required heating-water temperature, installed heat pump output: 6.5kW. That gives us a genuine capacity concern.
It does not automatically follow that every cold house has an undersized heat pump. Other problems can produce very similar symptoms, including insufficient radiator output, poor system flow, restrictive pipework, incorrect balancing, weather compensation set too low, control problems, hydraulic issues, equipment faults, or an incorrect original heat-loss calculation.
The symptoms tell us where to investigate. They do not give us the diagnosis.
The Most Important Question — Is the Heat Pump Producing Enough Heat, or Is the System Failing to Deliver It?
This is one of the most important distinctions when investigating an apparently undersized heat pump.
Imagine the house requires 8kW. There are two very different ways the property could fail to get enough heat.
Scenario 1 — Genuine Heat-Pump Capacity Shortfall
At the required operating conditions, the heat pump can only provide 6.5kW. The heat source itself may not have enough capacity.
Scenario 2 — Enough Heat Is Available, but It Is Not Reaching the Rooms
The heat pump can provide 8.5kW. But the heating system can only transfer a smaller amount because of insufficient water flow, undersized emitters, hydraulic resistance, balancing, control restrictions, or another distribution problem.
The homeowner experiences the same thing: the house is cold. But fitting a larger heat pump in the second situation may not solve the actual problem.
That is why heat-pump capacity and heat-delivery capacity need to be considered separately.
Common Signs That Make Homeowners Suspect Undersizing
The House Struggles in Cold Weather
This is one of the strongest reasons to investigate heat-pump capacity. If the property is comfortable in mild weather but cannot maintain temperature as outdoor conditions approach the design winter temperature, the relationship between building heat loss and available heat-pump output should be checked.
The Heat Pump Runs Almost Constantly
This is not automatically a problem. Heat pumps are designed to operate for long periods. In cold weather, continuous running can be completely normal.
The more meaningful distinction is: heat pump running continuously while maintaining a comfortable indoor temperature, versus heat pump running continuously while the indoor temperature continues to fall. The first may be exactly what we want. The second deserves investigation.
The Thermostat Target Is Never Reached
This can suggest insufficient available heat, but it can also result from emitters that are too small, low flow temperature, balancing problems, thermostat location, zoning, hydraulic issues, or control settings. Again, the whole system needs context.
The Backup Heater Keeps Operating
Frequent supplementary electric heating may be a reason to investigate capacity. But backup heater operation does not automatically prove the heat pump is too small. We first need to understand why the heater is operating and whether that operation forms part of the intended system design.
Another Engineer Says the Heat Pump Is Too Small
That is a sensible reason to seek a second opinion. But the conclusion should still be supported by a credible heat-loss calculation, the exact installed model, the required operating temperatures, and manufacturer performance data.
Does Running Continuously Mean My Heat Pump Is Too Small?
No.
Long runtimes are a normal feature of heat-pump heating. A traditional boiler often operates in relatively short, high-output bursts. An inverter heat pump can instead run for much longer while adjusting output to match the building demand.
During cold weather, a correctly sized heat pump may run almost continuously. That can be desirable. The important question is: is it maintaining the required room temperatures while it runs?
KEY TAKEAWAY
If yes, continuous running alone is not evidence of undersizing. If the heat pump is already working hard and the property is still losing temperature, capacity becomes a stronger line of investigation.
One Cold Room or the Whole House?
This distinction can tell us a lot.
One or Two Rooms Are Cold
If most of the house is comfortable but one bedroom or living room remains cold, the first investigation should usually be local. Possible causes include radiator too small, underfloor-heating output too low, poor balancing, restricted flow, air in the circuit, incorrect room heat loss, or local control issues.
That does not strongly suggest that the entire heat pump is undersized.
Most or All Rooms Are Losing Temperature
If the whole property gradually becomes colder during winter conditions while the heat pump is already running continuously, whole-system capacity becomes much more relevant.
That is when we should compare whole-house heat loss against available heat-pump output, while also checking that the distribution system is capable of transferring that heat.
How Is Undersizing Properly Checked?
A proper capacity check works through the evidence in sequence.
1. Establish a credible heat loss — how much heat does the property actually require?
2. Establish the design outdoor temperature — what winter condition was the system supposed to cope with?
3. Establish the required heating water temperature — at what flow temperature does the heating system need to operate?
4. Identify the exact heat pump — not simply an “8kW model”, but the exact make and model installed.
5. Check actual manufacturer output — how much heat can that particular unit provide at the relevant outdoor and water temperatures?
6. Compare the two — does available output adequately cover the credible building heat demand?
7. Check whether the system can deliver that heat — are flow, emitters, pipework and controls allowing the available heat to reach the rooms?
That gives a much stronger answer than looking at the model badge alone.
Start With a Credible Heat Loss
Any heat-pump sizing conclusion depends on the building heat load being correct.
Suppose the original calculation says 6.5kW and the installer selects a heat pump accordingly. But an independent calculation later shows 8.3kW. The heat-pump selection may have been reasonable against the information provided at the time. The deeper problem may have been that the original building load was underestimated.
That matters when deciding what went wrong.
If you specifically question the heat-loss calculation, see: Was My Heat Pump Heat Loss Calculation Wrong?
Check the Exact Heat Pump — Not Just Its Nominal kW
A heat pump described as an “8kW model” does not necessarily provide exactly 8kW under every operating condition.
Available output can change with outdoor air temperature, heating water temperature, exact product, compressor operating conditions, and manufacturer controls.
That is particularly important when investigating undersizing. A heat pump may appear to match the house on paper: house heat loss 8kW, heat pump 8kW. But if the exact model can only provide materially less than 8kW at the required winter outdoor temperature and flow temperature, the comparison changes.
The installed model’s actual performance data is therefore much more useful than the number in its name.
Why Flow Temperature Can Change the Answer
Suppose a heat pump was selected on the basis that the heating system would operate at 45°C. But after installation the house actually needs 55°C to remain comfortable.
That matters for two reasons. First, heat-pump output can vary with water temperature. Second, operating efficiency generally changes too.
So the sizing review needs to ask: at what flow temperature was the heat pump selected? And what flow temperature does the installed heating system genuinely require?
If those are substantially different, what looks like an undersized heat pump may partly be an emitter-design problem.
Could the Radiators Be the Real Problem?
Yes.
The heat pump may have enough capacity for the house while individual rooms still struggle because the emitters cannot transfer enough heat at the intended water temperature.
For example: bedroom heat loss 900W, but at the actual operating temperature, installed radiator output 600W. The room may remain cold even though the heat pump itself has spare capacity.
Increasing the heat-pump size does not automatically solve that mismatch. The useful relationship is: room heat loss → emitter output → required flow temperature.
That is why radiator sizing and heat-pump sizing need to be considered separately.
Can Pipework or System Flow Make a Heat Pump Look Undersized?
Yes.
The heat pump needs to move heat through water. If sufficient heat is being produced but the system cannot circulate the required amount effectively, the property may still receive less heat than it needs.
Possible areas include pipe diameter, hydraulic resistance, pump operation, balancing, bypass arrangements, valves, filters, and system configuration.
Small pipework does not automatically mean the system is wrong. What matters is whether the required flow can be achieved within sensible operating conditions.
The correct diagnosis is based on what the system is actually doing rather than the pipe diameter alone.
Can Controls Make a Heat Pump Look Too Small?
Yes.
The heat pump may have sufficient capacity but be prevented from using it properly. Examples include weather compensation set too low, a thermostat ending the heating demand too early, heating schedules restricting runtime, zones closing, control conflicts, or incorrect commissioning parameters.
If the heat pump is being asked to produce less heat than the building requires, a cold house does not prove a capacity problem. That is why controller settings and actual operating state matter.
What About Backup or Booster Heating?
Many heat-pump systems include supplementary electric heating. Its presence does not mean the heat pump was necessarily designed incorrectly.
The heater may have several possible roles depending on the equipment and design. These can include supplementary space heating, domestic-hot-water operation, emergency operation, frost protection, or other manufacturer-controlled functions.
So if you notice the backup heater running, the first question should be: why has it activated?
If a system designed for the heat pump to meet the full space-heating requirement relies heavily on direct electric heating during normal design conditions, that deserves investigation. But the heater operating at some point does not itself prove undersizing.
Full Heat-Pump System or Hybrid System?
Before deciding a heat pump is too small, we also need to understand the original system strategy.
Some systems are intended for the heat pump to provide the complete space-heating requirement. Others are deliberately designed as hybrid or bivalent systems where another heat source forms part of the heating strategy. That might include a boiler, another heat pump, or another designed heat source.
A heat pump in a genuine hybrid system should not automatically be judged as though it were supposed to carry the entire peak load by itself.
The first question is: what was this system actually designed to do?
What Happens During Defrost?
Air source heat pumps periodically need to remove ice from the outdoor heat exchanger during cold and damp conditions.
During defrost, normal heating operation is temporarily interrupted or altered. You may notice flow temperature dropping, heating output reducing, changes in fan/compressor behaviour, or a brief reduction in room heat input. That is normal.
Defrost itself does not prove that the heat pump is too small. The relevant question is whether the complete system can maintain the required indoor conditions through normal winter operation, including repeated defrost cycles.
The House Is Cold Only in Very Cold Weather — Is That Proof of Undersizing?
Not automatically. We need to know how cold it actually was.
A heating system is designed against a defined outdoor design condition. An unusually severe event below that design basis is different from a system failing under the conditions it was specifically designed to meet.
Useful information includes actual outdoor temperature, design outdoor temperature, indoor target temperatures, flow temperature, heat-pump operating state, backup-heater operation, and whether the system was in normal steady operation.
The question is not simply “was it freezing outside?” It is: how did the system perform relative to the conditions it was designed around?
Maintaining Temperature Is Different From Recovering From a Setback
This is a common source of confusion when moving from a boiler to a heat pump.
Imagine the house is normally maintained at 21°C but the heating is turned down overnight to 16°C. The homeowner then expects the heat pump to recover those 5°C rapidly in the morning. That can require substantially more heat than simply maintaining 21°C.
A heat-pump system may be correctly sized for the building’s steady heat loss while still taking longer than a powerful boiler to recover from a deep setback.
So there are two separate questions: can the system maintain the design temperature? And how quickly can it recover a cold building? Slow recovery alone does not prove undersizing.
Starting Conditions Matter
Avoid judging heat-pump capacity from one unusual event.
For example, the building may have been unheated for several days, recovering after a power cut, left with windows or doors open, heavily set back overnight, or newly occupied after being cold.
Heating the building fabric back to normal temperature requires additional energy. The more useful capacity test is how the system performs once normal steady-state conditions have been established.
Can Domestic Hot Water Affect Space Heating?
Yes.
Many heat-pump systems temporarily prioritise the hot-water cylinder. During cylinder reheating, some or all of the heat pump’s operating time may be diverted away from space heating.
In normal circumstances that may have little noticeable effect. But during very cold weather, frequent or prolonged hot-water reheating can reduce the time available for heating the property.
So if the house repeatedly loses temperature at a particular time, it can be useful to ask: was the heat pump heating the house, or was it reheating the cylinder? This does not necessarily mean the heat pump is undersized. It may be a system-strategy issue.
How Do I Know Whether the Heat Pump Is Actually Working at Maximum Output?
A heat pump running continuously is not necessarily running at full capacity.
An inverter compressor can operate at different outputs. Depending on the manufacturer and controller, useful information may include compressor frequency, compressor demand, electrical input, target flow temperature, actual flow temperature, return temperature, operating mode, and backup-heater state.
If the heat pump is running continuously at relatively low output, that tells us something very different from a unit already operating at or near its available maximum while the building continues to cool.
The exact information available varies by manufacturer.
Could High Electricity Bills Mean My Heat Pump Is Undersized?
Possibly, but not automatically.
A genuine capacity shortfall can become expensive if direct electric backup heating is regularly covering the missing heat.
But high electricity consumption can also result from high flow temperatures, domestic-hot-water use, immersion-heater settings, poor controls, high indoor temperatures, weather, property heat demand, or electricity tariff.
If high bills are your main concern and you do not know the cause, an Independent Heat Pump Performance Review may be more appropriate than assuming undersizing.
Could the Original Heat Loss Be Too Low?
Yes. This is one of the most important things to establish.
Suppose the original designer calculated 6.5kW and selected a heat pump accordingly. But important heat losses were omitted or underestimated and the property actually requires 8.3kW.
The heat pump may now appear too small. But the root cause may be the original heat-loss calculation rather than the equipment-selection process that followed it.
If you suspect this, an Independent Heat Loss Report can recalculate the property room by room and compare the findings with the original calculation where available.
Has the Property Changed Since the Heat Pump Was Installed?
A heat pump can also become insufficient for a property that has changed.
Examples include building an extension, converting the loft, increasing the heated floor area, heating previously unused rooms, increasing ventilation, removing insulation, or regularly maintaining higher indoor temperatures.
KEY TAKEAWAY
This creates an important distinction: was the heat pump too small when it was installed, versus is it too small for the property as it exists and is used today? Those are not always the same question.
Can an Undersized Heat Pump Be Fixed Without Replacing It?
Sometimes the apparent capacity problem lies elsewhere. Depending on the cause, useful remedial work could include:
Improving Emitter Capacity
Larger or additional radiators may allow the property to receive enough heat at a lower water temperature.
Correcting System Flow
Pipework, pumps, balancing or hydraulic issues may be preventing available heat reaching the rooms.
Optimising Controls
Weather compensation, thermostat settings and heating schedules may need correcting.
Correcting Commissioning
The equipment may not be operating as originally intended.
Reducing Building Heat Loss
Insulation, draught reduction or other fabric improvements may reduce the heating requirement.
Reviewing the Supplementary or Hybrid Strategy
Where another heat source is deliberately part of the design, the operating strategy may need review.
KEY TAKEAWAY
No control adjustment or radiator change creates additional compressor capacity that genuinely does not exist. If the installed heat pump itself has a material capacity shortfall, that needs to be addressed as a capacity problem.
When Might a Larger Heat Pump or Different System Need to Be Considered?
Only after the evidence supports that conclusion.
Before replacing equipment, establish credible building heat loss, appropriate outdoor design temperature, actual required flow temperature, exact installed heat-pump output, emitter capability, system flow, controls, and intended supplementary-heating strategy.
If there is a genuine material shortfall, possible solutions may then include a different heat pump, changes to the overall system design, reducing building heat loss, an intentionally designed hybrid solution, or, for suitable larger-load applications, more than one heat pump.
The correct remedy depends on the scale and cause of the shortfall.
What Evidence Should I Gather?
If you suspect your heat pump is undersized, useful information includes:
Original Design Information
Room-by-room heat-loss calculation, whole-property heat load, radiator schedule, original proposal, design flow temperature, system schematic.
Heat-Pump Information
Manufacturer, exact model, controller, installation date.
Cold-Weather Information
Outdoor temperature, target indoor temperatures, actual indoor temperatures, flow temperature, return temperature, heat-pump runtime, compressor information where available, backup-heater operation.
System Information
Radiator or UFH details, heating zones, pumps, buffer/volumiser where fitted, any known pipework constraints.
Operating Information
Weather-compensation settings, thermostats, heating schedule, domestic-hot-water schedule, controller/app screenshots.
This makes it much easier to distinguish lack of capacity from a wider system problem.
What Does the Evidence Usually Lead To?
There are several possible outcomes.
No Clear Evidence of Undersizing
The installed heat pump appears capable of meeting the credible building demand. The performance problem should be investigated elsewhere.
Heat-Pump Capacity Needs Verification
The symptoms are consistent with insufficient capacity, but the building load and exact model output still need to be checked properly. An Independent Heat Pump Sizing Review can assess the installed model against the property.
The Building Heat Loss Is Questionable
If the original building load cannot be trusted, sizing cannot be verified reliably. An Independent Heat Loss Report can independently recalculate the property.
Heat-Pump Capacity Appears Adequate but the Heat Is Not Reaching the Rooms
This points towards areas such as emitters, flow, hydraulics, balancing, controls, or commissioning. An Independent Heat Pump Performance Review may then be the better route.
Want to Know Whether Your Heat Pump Was Actually Sized Correctly?
Heat Loss Hub can independently review the sizing of an existing heat-pump installation. The assessment can compare the property’s credible design heat load, the exact installed heat pump, its available output at the relevant outdoor temperature, the required heating-water temperature, and the original design information.
The aim is to establish whether the installed unit genuinely has enough capacity for the property rather than diagnosing undersizing from long runtimes or cold rooms alone.
CHECK YOUR SYSTEM
Want the installed capacity independently verified against your property?
Frequently Asked Questions
The strongest warning sign is usually a whole property that cannot maintain the required temperature during relevant cold-weather conditions while the heat pump is already operating continuously or close to its available capacity. Long runtime alone is not enough.
No. Continuous operation can be normal for an inverter heat pump, particularly during cold weather. It becomes more concerning if the heat pump is running continuously while indoor temperature continues to fall.
Possible causes include insufficient heat-pump capacity, undersized emitters, inadequate system flow, incorrect weather compensation, hydraulic restrictions, controls, or an incorrect heat-loss calculation. The cause needs to be established rather than assumed.
Usually not by itself. If the rest of the house is comfortable, investigate the individual room’s heat loss, radiator or UFH output, balancing and local flow first.
Yes. If the radiators cannot deliver enough heat at the intended water temperature, the property can remain cold even when the heat pump itself has sufficient capacity.
Potentially. The required water flow needs to be achieved through the heating system. However, small pipework does not automatically mean the installation is inadequate. The actual hydraulic performance needs to be considered.
Not necessarily. Backup heating can operate for several reasons depending on the equipment and system design. The reason for activation needs to be established first.
That does not automatically prove undersizing. The relevant comparison is with the property’s actual design outdoor temperature and the heat pump’s output under those conditions.
No. Defrost is normal for an air source heat pump. The important question is whether the complete system can maintain the required indoor temperatures through normal winter operation including defrost cycles.
Heat pumps are generally better suited to steady heating than large rapid temperature recoveries. A system that maintains the house comfortably can still take several hours to recover after a deep overnight setback.
Yes. Steady-state building heat loss and rapid warm-up load are not the same thing. Slow recovery alone does not prove undersizing.
Yes. If the heated area or building heat loss has increased significantly since the heat pump was installed, the original unit may no longer have sufficient capacity for the current property.
It depends on what is actually causing the apparent shortfall. If the problem is emitters, flow, controls or commissioning, remedial work may resolve it without replacing the heat pump. If the heat pump itself genuinely lacks sufficient capacity, a wider redesign may be required.
No. A larger heat pump should only be considered after confirming the building heat loss, installed model output, design conditions, emitters and system operation.
An Independent Heat Pump Sizing Review can compare a credible building design load with the exact installed heat pump’s available output under the relevant operating conditions. That gives a much stronger answer than relying on symptoms alone.
Think Your Heat Pump May Be Too Small?
A cold house and a heat pump that runs all day can make undersizing seem obvious. But they do not prove it.
The evidence comes from establishing what the building actually requires, what the installed heat pump can genuinely produce, what water temperature the system needs, and whether the heating system can transfer that heat into the rooms.
If you want the installed sizing independently checked, Heat Loss Hub can assess the evidence and explain whether insufficient heat-pump capacity is actually the problem.
CHECK YOUR SYSTEM
Check whether your heat pump was sized correctly.