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Heat Pump Not Heating the House Properly: Is the Design Wrong?
How to separate design, installation, commissioning, control and equipment problems.
- Last updated: September 2026
- Heat Loss Hub · Homeowner Advice
If your heat pump is installed and running but the house still does not heat properly, it is easy to assume the heat pump itself is faulty.
Sometimes it is. But the heat pump is only one part of the heating system. Poor comfort can also result from an incorrect heat-loss calculation, the wrong heat pump being selected, undersized radiators or underfloor heating, an unrealistic design flow temperature, insufficient water flow, pipework or hydraulic problems, poor balancing, incorrect controls, commissioning settings, or the installed system not matching the original design.
The useful question is therefore not simply “is my heat pump broken?” It is: which part of the heating system is failing to do what the design required?
A cold house does not tell you whether the problem started with the design, happened during installation, arose during commissioning or is the result of a later fault. This guide explains how to start separating those possibilities.
KEY TAKEAWAY
If the cause is unclear and you want the complete system independently assessed, see our Independent Heat Pump Performance Review.
GET STARTED
Not sure which part of your heating system is at fault?
Why Is My Heat Pump Running but the House Still Cold?
There are five broad areas to investigate.
1. Design
Was the right system specified in the first place? This includes building heat loss, heat-pump sizing, radiator and UFH sizing, design flow temperature, pipework, hydraulics, zoning, and controls.
2. Installation
Was the design actually installed? A technically sound design can still underperform if different equipment, pipework or emitters were fitted.
3. Commissioning
Was the completed system correctly set up? This includes water flow, balancing, pump settings, weather compensation, and control configuration.
4. Controls and Operation
Is the system being allowed to operate as intended? Heating schedules, thermostats, zoning, TRVs and domestic-hot-water priority can all change how the system behaves.
5. Equipment Condition
Has something stopped working properly? This becomes particularly relevant when a system that previously performed well suddenly deteriorates.
The heat pump itself can therefore be operating while the heating system around it still fails to deliver enough heat into the property.
First Question — Has the Heat Pump Ever Heated the House Properly?
This is one of the most useful starting points.
It Has Never Worked Properly
If the system has struggled from the day it was installed, investigate the original design, equipment selection, radiator and UFH sizing, pipework, hydraulic arrangement, commissioning, and control setup.
That does not prove the design is wrong. But a design, installation or commissioning issue becomes more plausible than a new component failure.
It Used to Work Properly
If the house was comfortably heated for one or more winters and performance has suddenly changed, ask: what changed?
Possible areas include altered controller settings, a failed pump, valve problems, blocked filters or strainers, air in the system, pressure changes, thermostat changes, service work, sensor faults, equipment faults, or alterations to the property.
A system that previously demonstrated that it could heat the house requires a different investigation from one that has never achieved the intended comfort.
How Much of the House Is Affected?
The pattern of cold rooms can provide useful clues.
One Room Is Cold
Start locally. Possible causes include radiator too small, inadequate UFH output, insufficient water flow, balancing, TRV or valve issue, local control, or incorrect room heat loss.
One cold bedroom does not strongly suggest that the whole heat pump is too small.
Several Rooms on One Floor or Zone Are Cold
Think about something those rooms share — for example one pipework branch, a zone valve, UFH manifold, circulation pump, zoning controls, or balancing.
Most or All of the House Is Cold
Now whole-system issues become more relevant. These include design flow temperature, heat-pump capacity, whole-system water flow, original heat loss, controls, commissioning, and equipment operation.
The pattern matters.
When Does the Problem Happen?
Try to identify the conditions in which the problem appears. Is the house cold even when it is mild outside? Only during colder weather? Only after an overnight setback? While the hot-water cylinder is reheating? On one particular heating zone? All the time? Only since something was changed?
A system that cannot heat the house at +10°C outside may have a very different problem from one that performs well until outdoor temperature approaches the property’s winter design condition.
Record when the problem occurs before deciding what is causing it.
Could the Original Heat Pump Design Be Wrong?
Yes, but that is only one possibility.
A heat-pump system is a chain of design decisions. Building heat loss determines required heat output, which influences heat-pump selection, while individual room heat losses determine emitter requirements, which influence design flow temperature, and the complete system then needs suitable pipework, flow and controls.
A problem anywhere in that chain can affect comfort.
Possible design issues include building heat loss underestimated, heat pump too small, radiators undersized, UFH output inadequate, unrealistic design flow temperature, inadequate pipe sizing, unsuitable zoning, or hydraulic design that prevents reliable heat delivery.
But before blaming the design, check whether the completed installation actually matches it.
Was the Original Design Actually Installed?
This is frequently overlooked.
Suppose the design specified living-room heat loss: 1,450W, and radiator selected: capable of approximately 1,600W at the design temperatures. But the radiator actually fitted is a smaller unit providing only 1,100W under the same conditions.
The original calculation could be correct. The heat pump could be correctly sized. The problem would be that what was installed does not match what was designed.
The same issue can occur with heat-pump model, radiator sizes, UFH circuit design, pumps, valves, pipework, buffer or volumiser, and controls.
If you have the original design documents, compare them with the physical installation.
Was the System Properly Commissioned?
A good design still needs to be correctly commissioned.
Commissioning is the process of setting up and checking the completed heating system so that it operates as intended. Relevant areas include system water flow, radiator balancing, UFH balancing, pump settings, weather compensation, target flow temperatures, thermostats, and zone controls.
A design may say that a particular circuit requires a certain water flow. If commissioning never achieves that flow, the system may not transfer the intended amount of heat.
KEY TAKEAWAY
A useful way to think about it: the design tells us what the system needs to achieve. Commissioning should demonstrate that the completed installation can actually achieve it.
Could Controls Be Preventing the House From Heating Properly?
Yes.
A technically capable system can perform poorly if the controls restrict it. Possible examples include weather compensation set too low, thermostat ending demand too early, thermostat located in an unusually warm area, short heating schedules, deep overnight setbacks, excessive zoning, TRVs closing most emitters, third-party controls conflicting with manufacturer controls, or domestic-hot-water priority interrupting heating.
This does not mean poor comfort should automatically be blamed on the homeowner. A properly designed and commissioned installation should include controls that suit the intended operating strategy and should be explained during handover.
Could There Be an Equipment or Maintenance Problem?
Particularly if the system worked correctly before, yes.
Possible areas include circulation pump, motorised valve, sensor, blocked filter or strainer, air in the heating system, system pressure, heat-pump fault, controller fault, or valve not opening correctly.
These are generally engineer-level investigations. The important point is that a new performance problem does not automatically mean the original system design was wrong.
Can the Heat Pump Produce Enough Heat?
This is one side of the investigation.
The question is not simply “is this an 8kW heat pump?” It is: how much heat can this exact model provide at the relevant outdoor temperature and heating-water temperature?
Heat-pump output can change with operating conditions. If the whole property loses temperature during cold weather while the heat pump is already operating continuously or close to its available output, heat-pump capacity deserves investigation.
See: Has My Heat Pump Been Undersized? for a more detailed explanation of how insufficient capacity is checked.
Can the Heating System Deliver Enough Heat?
This is the other half of the problem.
A heat pump may be able to produce enough heat while the system fails to move or release it effectively.
Imagine the property requires 8kW and the heat pump is capable of providing 8.5kW. That sounds fine. But if inadequate water flow, restrictive pipework or undersized emitters mean the house can only absorb 6kW, the homeowner can still experience a cold property.
Fitting a larger heat pump would not automatically solve that problem.
That is why we separate heat generation from heat distribution.
The Radiators Are Warm but the House Is Cold — What Does That Mean?
A warm radiator is not proof that it is large enough.
The important question is: can the radiator provide enough heat to replace the room’s heat loss at the water temperature being used?
For example: room heat loss 1,200W, but at the actual flow and return temperatures, radiator output 800W. The radiator may feel warm. But the room can still gradually cool.
The relevant relationship is: room heat loss → emitter size → water temperature.
If only one or two rooms are affected, emitter sizing and local flow should be high on the investigation list.
My Radiators Do Not Feel Very Hot — Is That a Problem?
Not necessarily.
Heat pumps are usually intended to operate at lower water temperatures than traditional boilers. Weather compensation may deliberately lower the flow temperature during milder weather. So radiators may feel warm, lukewarm, or much cooler than you remember from a gas boiler.
If the room is maintaining its required temperature, that is not necessarily a problem.
The question is not “does the radiator feel hot?” It is “is the room receiving enough heat to remain at its target temperature?”
Does the House Work at the Design Flow Temperature?
This is one of the most useful design checks.
Suppose the system was designed for 45°C flow temperature at the winter design condition. But in practice the house will only remain comfortable after the controller is increased to 55°C.
That discrepancy deserves investigation. Possible explanations include room heat loss underestimated, radiators too small, insufficient water flow through emitters, installed emitters differ from design, or another system-design issue.
Simply increasing flow temperature may improve comfort. But it does not explain why the system cannot achieve the original design at the temperature it was supposed to use.
Could the Original Heat-Loss Calculation Be Wrong?
Yes.
If the original room-by-room heat losses were incorrect, several later design choices may also be affected. An underestimated room heat loss can lead to radiator too small or insufficient UFH output. An underestimated whole-building heat loss can contribute to insufficient heat-pump capacity.
The opposite can also happen. An exaggerated heat loss can lead to unnecessarily large equipment and different performance problems.
If you specifically question the original calculation, see: Was My Heat Pump Heat Loss Calculation Wrong?
Could the Heat Pump Be Too Small?
Possibly.
Heat-pump capacity becomes more relevant when most or all of the house is affected, the problem becomes worse as outdoor temperature falls, the heat pump is already running continuously, room temperatures continue to drop, and other obvious heat-delivery problems have been considered.
The comparison should be between credible building heat loss and actual output from the exact installed heat pump at the relevant conditions.
See: Has My Heat Pump Been Undersized? for the detailed capacity checks.
Could the Radiators or Underfloor Heating Be Too Small?
Yes.
Individual rooms require enough emitter output to match their heat loss. A heat pump can therefore be perfectly adequate for the whole house, but particular rooms may still fail because their emitters are too small.
Potential signs include one or two persistent cold rooms, rooms that only become comfortable at much higher flow temperature, radiator schedule differing from the original design, or large room heat loss compared with emitter output.
Could Pipework or Water Flow Be the Problem?
Yes.
The system needs sufficient water flow to transfer heat from the heat pump to the heating circuits. Possible restrictions include unsuitable pipe sizing, excessive hydraulic resistance, valves, blocked filters, poor pump selection or settings, badly balanced circuits, air, or hydraulic separation issues.
Small-bore pipework does not automatically mean the system is wrong. Likewise, there is no single flow rate or temperature difference that proves every system is operating correctly.
The actual requirements should be assessed against the design and the particular equipment.
Could Poor Balancing Cause Cold Rooms?
Yes.
If the heating system is poorly balanced, some circuits can receive too much water while others receive too little. Possible patterns include upstairs very warm while downstairs is cold, radiators nearest the heat pump heating strongly while distant radiators struggle, one branch of the system consistently underperforming, or several UFH loops behaving differently.
This points towards distribution rather than automatically towards insufficient total heat-pump capacity.
Could Zoning Be Making the Problem Worse?
Potentially.
Heat pumps generally work best when they have a stable heating load and adequate water flow. Multiple thermostats, zone valves, TRVs and UFH actuators can substantially change how much of the system is open at any one time.
This can affect water flow, heat distribution, thermostat behaviour, cycling, and room comfort.
That does not mean zoning is always wrong. It means the zoning strategy needs to work with the heat pump and hydraulic design.
Could Weather Compensation Be Set Too Low?
Yes.
Weather compensation adjusts heating-water temperature according to outdoor temperature. In broad terms, milder outside means lower flow temperature and colder outside means higher flow temperature.
If the heating curve is set too low, the system may not increase the water temperature sufficiently as the weather becomes colder. The property can then drift below target temperature even though the equipment itself has sufficient capacity.
There is no universal correct heating curve. It depends on building heat loss, emitter sizing, target room temperatures, and system design.
Could the Thermostat Be Ending the Heating Demand Too Early?
Yes.
Consider a thermostat located in a warm hallway, near a kitchen, close to another heat source, or in a room that heats more easily than the rest of the house.
If that area reaches target temperature first, it may stop or restrict the heating demand while colder rooms still need heat. The exact effect depends on the control strategy.
A thermostat reading 21°C therefore does not necessarily mean every room in the property is at 21°C.
Could Domestic Hot Water Be Interrupting the Heating?
Yes.
Many heat pumps temporarily prioritise reheating the hot-water cylinder. During this period, some or all of the heat pump’s output is diverted away from space heating.
Usually this is not a problem. But during colder weather, prolonged or frequent cylinder reheating can become more noticeable.
If the property regularly cools at a particular time, check whether that coincides with domestic-hot-water operation. This may indicate a control or system-strategy issue rather than insufficient heat-pump capacity.
Could Defrost Be Making the System Look Worse?
Air source heat pumps need to defrost their outdoor heat exchanger during cold and damp conditions.
During defrost you may notice temporary reduction in space-heating output, changing flow temperature, fan stopping, or water or steam around the outdoor unit. That is normal operation.
The important question is not whether defrost occurs. It is whether the complete system can still maintain satisfactory indoor conditions through normal winter operation.
Does Hydraulic Separation Change the Temperatures You Are Looking At?
It can.
Some installations contain buffer tanks, low-loss headers, plate heat exchangers, mixed circuits, or secondary pumps.
In these systems, heat-pump leaving-water temperature may not be the same as the temperature reaching the radiators or UFH.
So if the heat-pump controller displays 45°C but the emitter circuit is receiving substantially less, that may be relevant.
The correct measurement points depend on the hydraulic arrangement.
Are External Pipes Losing Useful Heat?
Potentially.
Some heat pumps have heating-water pipework running outside between the heat pump and the property. That pipework should be appropriately insulated and protected.
Poor insulation can allow useful heat to be lost before it reaches the building.
A small length of exposed pipe is unlikely to explain every comfort problem. But badly insulated external primary pipework is still worth identifying during a whole-system investigation.
Compare the Original Design With What Was Installed
If you have the design documents, carry out a simple comparison.
| Design information | Check against the installed system |
|---|---|
| Heat-pump make and model | Is the exact specified model installed? |
| Radiator schedule | Are the stated sizes/types actually fitted? |
| UFH design | Do circuits and heated areas match? |
| Design flow temperature | Is this still the basis of operation? |
| Pipework | Were significant routes/sizes changed? |
| Pump arrangement | Does it match the schematic? |
| Buffer/volumiser | Was one added or removed? |
| Controls | Were the intended controls installed? |
| Zoning | Does the actual zoning match the design? |
A discrepancy does not automatically prove that it is causing the problem. But it tells you where to investigate.
Check the Commissioning Information
If available, look for the commissioning record. Useful information may include heating flow rate, configured flow temperature, weather-compensation settings, system balancing, pump configuration, control settings, and commissioning date.
If the design depends on a particular flow rate but there is no evidence the system ever achieved it, that is relevant.
Likewise, if the designed heating curve bears no resemblance to the current controller setup, investigate why it changed.
Record an Evidence Snapshot Before Changing Everything
One of the most useful things a homeowner can do is record how the system behaves while the problem is happening. For example:
Outdoor temperature: 1°C
Living-room target: 21°C
Living-room actual: 18.5°C
Heating flow target: 45°C
Actual flow: 44°C
Return: 38°C
Heat pump: Running continuously
Hot water: Off
Heating zones: Open
Also save controller screenshots, room temperatures, outdoor temperature, error messages, and energy-consumption information where available.
One individual number rarely gives the answer. The combination is much more useful.
Do Not Change Five Settings at Once
It is tempting to start changing the weather curve, thermostats, heating schedule, TRVs, pump settings, and flow temperature all at once.
If everything is changed at once, you may improve the system but have no idea which change mattered.
Where practical, record a baseline first. Then make one appropriate change at a time and record the result.
Anything involving installer-level settings, electrical components, refrigerant system, pumps, valves, or hydraulic changes should be handled by an appropriately competent engineer.
Several Small Problems Can Add Up
Not every poor-performing heat-pump system has one spectacular design mistake.
Imagine heating curve slightly too low, one living-room radiator slightly marginal, downstairs circuit poorly balanced, deep overnight setback, and thermostat in the warmest part of the house.
Each problem may look relatively minor. Together they can create a cold and frustrating house.
A good investigation therefore looks at how the complete system behaves rather than stopping after finding the first imperfect setting.
Has the Property Changed Since the Heat Pump Was Installed?
The original system may have been designed for a different property or different heating requirement.
Changes might include extension, loft conversion, newly heated rooms, radiator replacements, removal of insulation, different ventilation, increased target room temperatures, or major changes to zoning.
This creates an important distinction: was the original design wrong? versus: is the original design no longer appropriate for the property today? They are not necessarily the same.
Is This a Comfort Problem or an Efficiency Problem?
It helps to separate the two.
The House Cannot Get Warm
First investigate heat delivery, heat-pump capacity, emitters, water flow, controls, and commissioning. Comfort is the immediate problem.
The House Is Warm but Electricity Use Seems Excessive
The system is clearly capable of heating the property. The investigation may instead focus more heavily on flow temperature, controls, cycling, domestic hot water, backup heating, and efficiency.
In that situation an Independent Heat Pump Performance Review can look at the wider operating picture.
Homeowner Diagnostic Pathway
Use the symptoms to decide where to start.
| What you are seeing | First area to investigate | Useful Heat Loss Hub route |
|---|---|---|
| One room is cold | Emitter, local flow, balancing | Radiators & Heat Emitters |
| Several rooms on one floor or zone are cold | Zoning, balancing, branch flow | Pipework & Hydraulics |
| Whole house is cold only during colder weather | Capacity and design conditions | Has My Heat Pump Been Undersized? |
| Whole house is cold but the heat pump repeatedly stops | Controls and commissioning | Flow Temperature & Controls |
| House only becomes comfortable at much higher flow temperature than designed | Heat loss, emitters, flow and design | Independent Performance Review |
| Original heat-loss figures look questionable | Original design heat load | Independent Heat Loss Report |
| System used to work properly but has suddenly deteriorated | Fault, maintenance or changed setting | Engineer / Performance Review |
| You cannot identify the cause | Complete system | Independent Heat Pump Performance Review |
Which Heat Loss Hub Service Fits My Problem?
I Do Not Know What Is Causing the Problem
Choose an Independent Heat Pump Performance Review. This is the broader investigation for an installed heat-pump system where the underlying cause is not yet clear.
I Specifically Think the Heat Pump Is the Wrong Size
Choose an Independent Heat Pump Sizing Review. This compares the credible building requirement with the actual performance of the installed model.
I Think the Original Heat-Loss Calculation Was Wrong
Choose an Independent Heat Loss Report. This provides a fresh room-by-room calculation and can compare it with the original figures where available.
Frequently Asked Questions
Possible causes include insufficient heat-pump output, undersized radiators, low water flow, poor balancing, incorrect flow temperature, control problems, commissioning, or an equipment fault. The pattern of affected rooms and the conditions in which the problem occurs can help narrow the investigation.
Compare the intended design with actual performance. Useful information includes heat loss, heat-pump model, radiator schedule, design flow temperature, pipework, and commissioning data. A system that cannot achieve its documented design requirements deserves further investigation.
Yes. The heat pump may produce enough heat while insufficient water flow, undersized emitters or control problems prevent that heat reaching the rooms.
The radiators may not be producing enough heat at their current water temperature. A warm radiator is not necessarily a correctly sized radiator. Its output needs to be compared with the room heat loss.
Not necessarily. Heat pumps often operate radiators at lower temperatures than boilers, especially during mild weather. If the room remains comfortable, a cooler radiator may be completely normal.
Yes. If radiator output is lower than the room heat loss at the design water temperature, the room may not maintain its required temperature.
Potentially. Pipework needs to allow the required water flow without excessive resistance. But pipe diameter alone does not prove whether a system is correctly or incorrectly designed.
Yes. Poor balancing can send too much water through some circuits and too little through others. This can leave certain rooms or floors cold even when total heat-pump capacity is adequate.
Yes. If the heating curve does not raise flow temperature sufficiently as outdoor temperature falls, the property may gradually become colder.
Potentially. If the thermostat is in a room or area that reaches temperature sooner than the rest of the house, it may reduce heating demand while other rooms remain cold.
Many systems give domestic hot water priority. While the cylinder is reheating, space heating may temporarily stop or reduce. Whether this causes a noticeable comfort problem depends on the system and operating conditions.
Defrost temporarily reduces normal space-heating operation and is a normal part of an air source heat pump’s operation. Occasional temperature changes during defrost do not automatically indicate a design problem.
That makes the original design, installation and commissioning worth investigating. Check heat loss, heat-pump sizing, emitters, design flow temperature, water flow, controls, and whether the installed system matches the original design.
A new problem is more suggestive of changed settings, flow restriction, pump or valve problems, controls, maintenance issues, or equipment failure. That does not rule out a longstanding design weakness, but the recent change is important evidence.
An Independent Design Review is primarily for checking a proposed system before installation. If your heat pump is already installed and the property is not heating properly, an Independent Heat Pump Performance Review is normally the more appropriate starting point.
Heat Pump Running but the House Still Not Heating Properly?
Do not assume the heat pump itself is automatically faulty.
The cause may lie in the original design, what was actually installed, how the system was commissioned, how the controls are operating, or the condition of the equipment today.
The most useful investigation compares what the system was supposed to do with what the completed system is actually doing.
If the cause is unclear, Heat Loss Hub can independently review the available design information, the installed system and its current operation to help identify where the problem is most likely to lie.
GET AN INDEPENDENT REVIEW
Not sure where the fault lies? Get the complete system independently assessed.