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INDEPENDENT REVIEW · EXISTING SYSTEMS
Independent Heat Loss Report for a Poorly Performing Heat Pump
Independent room-by-room heat loss report for an existing heat pump.
- Last updated: September 2026
- Heat Loss Hub · Independent Review Services
If your heat pump is already installed but you are questioning the figures used to design it, an independent heat loss calculation can help establish whether the original numbers actually stack up.
Perhaps the house struggles to reach temperature. Maybe certain rooms remain cold. The heat pump may appear much larger or smaller than expected. You may need unexpectedly high flow temperatures to keep the property comfortable. Or another engineer may have looked at the system and questioned the original heat-loss calculation.
Heat Loss Hub can independently recalculate the property room by room, compare our findings with the original calculation where available, and explain what any material differences mean for the heat pump and radiators already installed.
The aim is not simply to produce another kW figure.
KEY TAKEAWAY
What does the property actually require — and does that support the system that was designed and installed?
GET STARTED
Concerned the heat loss used to design your installed heat pump was wrong?
Not sure whether the heat-loss calculation is actually the cause of the problem? Our Independent Heat Pump Performance Review may be the better starting point.
Why Recheck the Heat Loss After a Heat Pump Has Been Installed?
A room-by-room heat loss calculation sits at the foundation of a heat pump design. It helps establish how much heat the whole property requires, how much heat individual rooms require, what size heat pump may be suitable, what output the radiators or other emitters need to provide, and what heating water temperatures the system may need.
If the original calculation contains significant errors, those errors can affect decisions made further down the design process.
That does not mean every poorly performing heat pump has a bad heat-loss calculation. But if the calculation itself is in doubt, it makes sense to establish the building requirement before trying to decide whether the heat pump, radiators or controls are wrong.
Typical reasons homeowners ask us to recheck the heat loss include the house not reaching the expected temperature in cold weather, one or more rooms remaining consistently cold, the heat pump appearing significantly oversized or undersized, the system needing much higher flow temperatures than expected, the radiator schedule not appearing to match the rooms, another installer or engineer questioning the original figures, the original report containing property details that appear incorrect, construction or insulation recorded wrongly, important parts of the property appearing to have been missed, or an independent report being needed before discussing the system further with the original installer.
KEY TAKEAWAY
Before deciding that equipment needs replacing, it is useful to establish the most basic question: how much heat does the property actually lose?
What Does an Independent Heat Loss Report Do?
This service is designed for properties where the heat pump is already installed and there is a specific concern about the heat-loss calculation used during the original design.
We independently recalculate the property room by room. Where you can provide the original calculation, we can also compare the two.
The process is essentially: original calculation → independent recalculation → installed heat pump and emitters.
That allows us to establish whether the overall property heat loss is broadly consistent, whether individual room results differ materially, which assumptions are causing the differences, whether those differences could affect heat-pump sizing, whether radiator sizing should be reconsidered, and whether the heat loss could reasonably explain the problem you are experiencing.
This is different from a full Independent Heat Pump Performance Review, which can investigate the wider system including hydraulics, commissioning, controls, operation and available performance evidence.
KEY TAKEAWAY
This service has a much narrower question: was the heat loss used to design the installed system reasonable?
We Don’t Just Produce Another kW Number
A second heat-loss calculation is only useful if you understand what it means.
Suppose your original report says: whole-house heat loss: 10.4kW. And our independent calculation produces: whole-house heat loss: 7.2kW.
The useful question isn’t simply: which number is right? We need to understand why they differ.
Perhaps the original calculation treated insulated cavity walls as uninsulated, used different room dimensions, assumed a higher ventilation loss, recorded glazing incorrectly, missed insulation, used a different external design temperature, or made another conservative assumption.
The opposite can happen too. An original calculation may underestimate the property because part of the building fabric was recorded too favourably or because a room or heat-loss area was missed.
The value of an independent review is in understanding the difference and its consequence.
Step 1 — Recalculate the Property Room by Room
We first establish our own independent assessment of the building. Depending on the property and information available, this can include room dimensions, ceiling heights, external wall areas, wall construction, floors, roof and ceiling construction, insulation, windows, external doors, adjoining heated spaces, adjoining unheated spaces, ventilation and infiltration, internal room temperatures, and the relevant external design temperature.
Each heated room is assessed individually. This gives us:
Room-by-Room Heat Loss
The amount of heat each room is calculated to require at design conditions.
Whole-Property Heat Loss
The overall design heat demand of the building.
Both matter. The whole-property figure helps us consider heat-pump capacity. The room-by-room figures help us assess the radiators, underfloor heating or other emitters.
Step 2 — Check the Original Assumptions
If you have the original detailed heat-loss report, we can compare its inputs with the property information we establish independently. This can be one of the most useful parts of the assessment.
We may compare room dimensions, wall areas, wall construction, assumed U-values, insulation levels, floor construction, roof construction, window dimensions, glazing type, door areas, ventilation assumptions, internal temperatures, external design temperature, extensions, conservatories, rooms over garages, rooms adjoining unheated spaces, and any other assumption materially affecting the result.
Sometimes the reason for the difference is immediately obvious. For example: original report says solid uninsulated wall, but property evidence shows an insulated cavity wall. Or: original report says no loft insulation, but property evidence shows 300mm insulation. Or: original report uses a bedroom design temperature of 21°C while our independent calculation uses a different room design condition. Or: original report treats an extension as original construction, but property evidence shows it was built to significantly different standards.
KEY TAKEAWAY
The report should reflect the actual property as closely as the available evidence allows.
Why Can Two Heat Loss Calculations Give Different Answers?
Two calculations for the same property will not always produce identical results. That does not automatically mean one designer has made a mistake.
Existing homes often contain uncertainty. The exact construction of a wall may not be visible. Insulation may have been added later. Parts of the building may have been constructed at different times. Air leakage may not have been measured. Original drawings may no longer represent the finished property.
Two competent designers may therefore make slightly different assumptions. Differences can arise from dimensions, construction interpretation, U-values, insulation assumptions, window and door areas, ventilation rates, airtightness assumptions, room temperatures, outdoor design conditions, thermal bridging, extensions, adjoining spaces, and the evidence available at the time.
KEY TAKEAWAY
The important distinction is between a reasonable difference caused by professional judgement and a material difference caused by incorrect or unsupported assumptions.
That is why we do not treat every numerical difference as proof that the original calculation was wrong.
Is the Difference Large Enough to Matter?
This is one of the most important questions.
A difference between two heat-loss calculations does not necessarily change the system design.
If one calculation produces 7.4kW and another produces 7.6kW, that difference may have little practical consequence. If one calculation produces 6.5kW and another produces 10.5kW, the implications can be much more significant.
Likewise, the total property heat loss could be reasonably close while one individual room is substantially different. That may have little effect on the heat-pump size but a major effect on whether a particular radiator is large enough.
We therefore look at material differences, not simply whether the numbers match exactly. The questions are: does the difference change the likely heat-pump requirement? Does it change the emitter requirement in individual rooms? Does it affect the proposed design flow temperature? Could it reasonably explain the performance concern?
What Does the Whole-House Heat Loss Mean for the Heat Pump?
The whole-property heat loss gives us an important benchmark.
But heat-pump sizing is not as simple as saying: house heat loss = 8kW, therefore any heat pump labelled 8kW is correct.
The actual output of a heat pump can vary with outdoor temperature, heating water temperature, model, operating conditions, and manufacturer performance characteristics.
So the logical comparison is: building design heat loss against actual heat-pump output at the required design conditions — not simply the model name.
A heat-loss report tells us what the building requires. The heat pump’s technical performance data tells us whether the installed unit is capable of meeting that requirement under the relevant conditions.
Can the Report Tell Me Whether My Heat Pump Is Undersized?
It can provide one of the most important pieces of evidence.
If the independent heat loss is significantly higher than the load used to select the heat pump, the installed capacity should be reassessed.
However, we do not determine undersizing purely from the nominal kW printed on the heat pump. We also need to consider the unit’s actual output at the required outdoor and water temperatures.
For example: a heat pump marketed as an 8kW model does not necessarily deliver exactly 8kW under every operating condition.
The independent heat-loss result therefore establishes the demand side of the comparison.
Can the Report Tell Me Whether My Heat Pump Is Oversized?
It can also help investigate suspected oversizing.
An inflated heat-loss calculation could lead to a larger heat pump being selected than the property actually requires. That can matter, particularly during milder weather when the heating demand is much lower than the winter design load.
But we need to be careful. Frequent cycling does not automatically prove the heat pump is oversized. Cycling can also be influenced by minimum heat-pump output, system volume, zoning, thermostats, hydraulic design, control settings, and actual building demand.
KEY TAKEAWAY
A heat-loss recalculation can therefore show whether the original sizing basis appears excessive, but it does not by itself diagnose every cause of cycling.
What Do the Room-by-Room Figures Mean for the Radiators?
The whole-house figure is only part of the picture. Individual rooms need individual emitter capacity.
For example: if a bedroom loses 900W under design conditions, the installed radiator needs to be capable of delivering approximately that requirement at the system’s intended operating temperature.
If the radiator only provides 600W at that temperature, the room may struggle to reach its design condition. One way to compensate would be to increase the water temperature. But increasing the water temperature affects the operating conditions of the entire heating system.
That is why room-by-room heat loss is so important. The useful chain is: room heat loss → installed emitter output → required operating temperature.
KEY TAKEAWAY
An independent heat-loss report can therefore help establish whether radiator sizing should be investigated further.
Could the Original Heat Loss Be Why My Flow Temperature Is So High?
Potentially.
Imagine the original calculation underestimates the heat requirement of several rooms. The radiators may then have been sized against loads that were too small. If those radiators cannot provide enough output at the intended design temperature, the system may need hotter water to compensate.
But the opposite situation can occur too. An exaggerated heat-loss calculation can lead to emitters being larger than necessary.
The important point is that the heat loss and emitter schedule should make sense together.
High flow temperatures can also result from other factors, including weather-compensation settings, control strategy, balancing, hydraulic issues, commissioning, and homeowner settings.
A heat-loss recalculation therefore helps test one part of the design rather than automatically proving the cause.
Airtightness and Ventilation Can Make a Significant Difference
Not all heat is lost through walls, roofs, windows and floors. Heat is also lost as warm indoor air is replaced by colder outdoor air.
The amount allowed for ventilation and infiltration can therefore materially affect a heat-loss calculation.
In existing homes, actual airtightness is often uncertain unless the property has been tested. Designers may need to use assumptions. Those assumptions can sometimes be conservative.
If two heat-loss calculations differ substantially, it can therefore be useful to separate fabric heat loss from ventilation and infiltration heat loss to understand where the difference originates.
KEY TAKEAWAY
We do not invent an airtightness figure simply to achieve a preferred result. Where information is uncertain, the assumption should be stated clearly.
What If I Don’t Have the Original Heat Loss Calculation?
We can still independently calculate the property. You do not need the original report for us to establish our own room-by-room result.
However, there is an important limitation. If all you have is a final figure such as “property heat loss: 9.8kW”, we may be able to tell you whether our independent result is substantially different. But we may not be able to establish why the original designer reached 9.8kW.
For a detailed comparison, we ideally need their room-by-room calculation and assumptions.
There are therefore two slightly different services within the assessment:
Independent Recalculation
We establish the heat loss of the property independently.
Original Calculation Comparison
Where the original detailed report is available, we compare its assumptions and results against ours.
The more original information you can provide, the more forensic the comparison can be.
What Information Should You Send Us?
If available, useful information includes:
Original Heat Loss Information
Room-by-room heat-loss calculation, whole-house heat-loss report, radiator schedule, heat-pump selection information, original system design, commissioning documents.
Property Information
Floor plans, architectural drawings, EPC, extension plans, insulation certificates, building-control documents, window specifications, photographs of construction where available.
Installation Information
Heat-pump make and model, radiator sizes, underfloor-heating information, original quotation, system schematic, installation photographs.
Even if you only have part of this information, we can establish what can reasonably be assessed.
How Do We Verify the Property?
A retrospective report is only useful if the new calculation itself is based on credible information.
Depending on the agreed survey method, information may come from measured room dimensions, plans, photographs, video, property documentation, EPC information, known building age, insulation records, visible construction evidence, and homeowner information.
KEY TAKEAWAY
Where something cannot be confirmed, it should be treated as an assumption rather than presented as fact. That distinction matters when the purpose of the report is to challenge or verify an existing calculation.
Can the Assessment Be Carried Out Remotely?
Often, yes.
A remote assessment may work well where reliable plans are available, dimensions can be verified, construction is relatively straightforward, good photographs and video can be supplied, insulation information is available, and the property owner can provide the required survey information.
For many homes, a structured remote survey can provide enough information for a detailed room-by-room assessment.
When Might a Site Survey Be Better?
A physical survey may be more appropriate where construction is uncertain, dimensions conflict, the property has several extensions, different parts of the building were constructed at different times, rooms are complex in shape, roof spaces or floors need closer assessment, or a disputed assumption could materially change the result.
The aim is not to insist on a site visit unnecessarily. It is to use a survey method that gives sufficient confidence in the calculation.
Is Thermal Imaging the Same as a Heat Loss Calculation?
No.
Thermal imaging and heat-loss calculations answer different questions.
A thermal camera can help identify temperature differences across building surfaces. It may help investigate missing insulation, cold bridges, air leakage, or unexpected thermal anomalies.
But a thermal image does not provide a complete room-by-room design heat load. A heat-loss calculation considers the dimensions, building fabric, temperatures, ventilation and other design conditions required to establish how much heat each room needs.
Thermal imaging can sometimes provide useful supporting evidence. It does not replace the heat-loss calculation.
What Do I Receive?
The objective is to provide a report that explains the result rather than simply issuing a number.
Depending on the agreed scope and information available, your Independent Heat Loss Report can include:
Property and Survey Basis
The information used to assess the property.
Calculation Methodology
The basis on which the room-by-room loads have been established.
Key Assumptions
Important areas where construction or other information could not be directly verified.
Room-by-Room Heat Loss
The calculated requirement for each heated space.
Whole-Property Heat Loss
The overall design heat load.
Original Calculation Comparison
Where the original report is available, comparison of important assumptions and results.
Material Discrepancies
Differences that are significant enough to potentially affect system-design decisions.
Heat-Pump Sizing Implications
What the revised building load means when considering the installed heat pump.
Radiator and Emitter Implications
Where room loads indicate that installed emitter sizing should be reconsidered.
Limitations
Anything that cannot reliably be determined from the evidence available.
Recommended Next Steps
What should be checked or investigated next.
How Do We Present the Findings?
A technical report is more useful when the conclusions are clear.
Rather than labelling every difference simply right or wrong, findings may be described as:
Broadly Consistent
Our independent result is sufficiently close to the original calculation that the difference is unlikely to materially alter the design conclusion.
Material Discrepancy Identified
A difference exists that could materially affect heat-pump sizing, radiator sizing or other system-design decisions.
Assumption Cannot Be Verified
The outcome depends on property information that cannot currently be confirmed.
Further Investigation Required
Additional information, measurement or a wider system review is needed before a reliable conclusion can be reached.
This avoids turning normal calculation tolerances into problems while still identifying differences that genuinely matter.
What If the Original Heat Loss Was Broadly Correct?
Then we will say so.
The purpose of an independent report is not to find a mistake simply because a homeowner is unhappy with the performance of the system.
If our calculation broadly supports the original design load, that is useful information. It means the cause of the poor performance may lie elsewhere.
Possible areas could include radiator or emitter design, hydraulic arrangement, pipe sizing, system flow, commissioning, controls, weather compensation, hot-water settings, or the heat pump itself.
In that situation, an Independent Heat Pump Performance Review may be the more appropriate next step.
What If We Find a Material Heat Loss Error?
The next step depends on what the error affects.
For example, it may be necessary to reassess the installed heat pump’s capacity, reassess individual radiator sizes, review the intended design flow temperature, reconsider original performance assumptions, investigate whether other parts of the design were based on the same incorrect figures, or develop a remedial design.
The report can help establish where further work should be focused. It does not automatically mean the entire heating system needs replacing.
Can the Report Help With an Installer Complaint?
Potentially.
An independent room-by-room heat-loss calculation can provide technical evidence for discussions with the original installer or another relevant organisation.
Where the original report is available, the comparison may help explain where the figures differ, which assumptions are responsible, whether those differences are material, and what effect they may have had on system design.
However, the purpose of the assessment is to establish the technical position. It is not written on the assumption that the installer must be wrong.
A standard Independent Heat Loss Report is also not automatically an expert-witness report, a legal opinion, proof of negligence, or a determination of contractual liability.
If a separate specialist report is required for legal proceedings, that would need to be discussed independently.
What Doesn’t an Independent Heat Loss Report Prove?
A heat-loss calculation is an important piece of the system-design puzzle. It is not the entire puzzle.
A fresh calculation does not by itself prove the heat pump is operating inefficiently, the actual seasonal COP is poor, the pipework is incorrectly sized, the hydraulic design is wrong, the system was badly commissioned, the controls are inappropriate, the refrigerant circuit has a fault, or the installer is legally responsible for the performance problem.
It tells us what the building requires. That information can then be used to assess whether other parts of the system make sense.
Which Heat Loss Hub Service Do I Need?
I Am Planning a Heat Pump
If the heat pump has not yet been installed and you simply want to know the room-by-room heat loss of your property:
Explore: Choose our Independent Heat Loss Calculation service
My Heat Pump Is Installed and I Specifically Question the Original Heat Loss
If you want the property independently recalculated and compared with the figures used for the installation:
Choose an Independent Heat Loss Report for an Existing Heat Pump — this service.
My Heat Pump Is Installed but I Don’t Know What Is Wrong
If the issue could involve heat loss, radiators, controls, hydraulics, commissioning, electricity consumption or the equipment itself:
Explore: Choose an Independent Heat Pump Performance Review
That wider assessment is designed to investigate the complete system rather than beginning with the assumption that the heat-loss calculation is the problem.
Frequently Asked Questions
Yes. We can independently recalculate the property room by room. If you provide the original detailed report, we can also compare its assumptions and results with ours.
Yes, where the original calculation contains sufficient detail. We can compare areas such as room loads, construction assumptions, dimensions, insulation, ventilation, temperatures, and whole-house heat loss. If you only have the original final kW figure, the level of comparison will be more limited.
We can still carry out an independent room-by-room calculation. What we may not be able to establish is exactly why the original designer reached a different result.
The independent heat loss establishes what the building requires. We can then compare that requirement with available performance information for the installed heat pump at the relevant operating conditions.
The recalculation can show whether the original building load appears to have been substantially overstated. However, heat-pump oversizing and cycling also depend on factors such as minimum modulation, system volume and control strategy.
Yes, where suitable information is available. The room-by-room heat loss can be compared against installed radiator output at the relevant system temperatures.
Potentially. If room heat losses were underestimated, emitters may have been sized too small to meet the actual requirement at the intended water temperature. But high flow temperatures can also have other causes.
It may contribute. For example, an incorrect calculation could influence heat-pump selection, emitter sizing or operating temperature. However, high electricity consumption can also be caused by many other factors. If electricity use is your main concern and you do not specifically suspect the heat-loss calculation, an Independent Performance Review may be more appropriate.
Not always. Many properties can be assessed remotely where sufficient plans, measurements, photographs, video and construction information are available. Where the property is complex or important information cannot be verified remotely, a site survey may be recommended.
No. Thermal imaging may provide useful evidence about parts of the building fabric, but it does not replace a room-by-room heat-loss calculation.
You can use your report when discussing technical concerns with your installer or another relevant organisation. The standard report is an independent technical assessment, not automatically an expert-witness report for litigation.
No. The Independent Heat Loss Report focuses specifically on the building heat loss and its implications for heat-pump and emitter sizing. If you also need hydraulics, commissioning, controls, electricity consumption and wider operating behaviour investigated, our Independent Heat Pump Performance Review is the more appropriate service.
Then we will tell you. That is still a useful result because it suggests the cause of the problem should be investigated elsewhere rather than repeatedly recalculating the building.
Concerned Your Original Heat Loss Calculation May Be Wrong?
A heat pump design starts with the building. If the figures used at the beginning are in doubt, the most useful next step may be to establish them independently.
Heat Loss Hub can recalculate the property room by room, compare the result with the original report where available, identify material differences and the assumptions behind them, and explain what those differences mean for your installed heat pump and radiators.
The purpose is not to produce the lowest or highest possible heat-loss figure. It is to establish a technically defensible one.
GET STARTED
Concerned the heat loss used to design your installed heat pump was wrong?
Explore: Independent Heat Pump Performance Review
Explore: Room-by-Room Heat Loss Calculations
Explore: Independent Heat Pump Design Reviews
Explore: Independent Heat Pump Design