Why Installer Heat Loss Estimates Get Heat Pump Sizing Wrong

Why This Question Matters Before You Install Anything

Almost every heat pump problem we are asked to look at traces back to one of two things: how the system is being controlled, or how it was sized in the first place. Sizing is decided by a single number: your property's heat loss. Get that number right and the rest of the design, the flow temperature, the radiators, the controls, has a fighting chance of being right too. Get it wrong and no amount of adjusting settings afterwards will fully fix it.

The problem is that "doing a heat loss calculation" and "doing a heat loss calculation properly" are two very different things. Most homeowners assume that if their installer has produced a number, that number is accurate. In our experience reviewing systems across the UK, that assumption is often the root cause of the underperformance we are called in to diagnose.

This article explains how installer heat loss estimates are usually produced, the specific places they tend to go wrong, and what a proper room-by-room heat loss survey checks that a quick estimate does not.

How Most Installer Heat Loss Estimates Are Actually Produced

To be clear, most installers are not being careless on purpose. Heat loss software is quick, the sales process moves fast, and a rough figure is often treated as good enough to get a quote out. The typical process looks like this:

• A room-by-room or whole-house calculation is run using standard software

• Default construction values are applied for wall, floor and roof type based on the property's age band

• Room dimensions are taken from a floor plan, an estimate, or a quick walk-round rather than measured directly

• Insulation levels are assumed from the build era rather than confirmed on site

• The result is a single kW figure used to select the heat pump size

This can work reasonably well for a straightforward, unmodified property. It works far less well for the kind of homes we see problems in most often: older properties, homes with extensions, part-renovated houses, mixed insulation levels between rooms, or anything that does not match the software's default assumptions.

If you want the mechanics of how a heat loss calculation works in more detail, our guide on what a heat loss calculation is and why it matters before installation and our related piece on how heat loss calculations are worked out and why they decide your heat pump size both cover the calculation itself. This article focuses specifically on where the numbers going into that calculation come from, and why an assumed figure and a measured figure can produce very different results.

The Specific Ways Installer Estimates Go Wrong

1. Default U-values instead of actual construction

Every wall, floor, roof and window has a U-value, a measure of how much heat passes through it. Software defaults assign a U-value based on the property's age band. A 1930s solid-wall semi and a 1930s semi that had cavity wall insulation retrofitted in 2005 will often be given the same default figure, even though their actual heat loss through the walls can be very different. Unless someone has physically confirmed what is actually in the walls, floor and loft, the calculation is working from an assumption, not a fact.

2. Rooms treated as uniform when they are not

A whole-house figure hides a lot of detail. Two rooms of the same size can have very different heat loss if one has a large single-glazed bay window and an external corner, and the other is an internal room with one small window. A heat pump can be correctly sized for the house overall and still leave individual rooms struggling, because the radiators in those rooms were never sized against their actual heat loss.

3. Air permeability and draughts are rarely measured

Software estimates typically apply an assumed air change rate based on property type and age. Actual air leakage, through old sash windows, unsealed floorboards, loft hatches, extractor fans, or extensions where the junction with the original building was never properly sealed, is very rarely tested on site. Draughts and ventilation losses are a real, sometimes significant, component of total heat loss that a desk-based estimate cannot capture.

4. Extensions and part-renovated areas are frequently miscounted

If a property has had an extension, a loft conversion, or partial insulation upgrades, the calculation needs to reflect that mixed construction accurately. It is common to see extensions built to modern building regulations sitting next to original solid-wall sections with no additional insulation, and for that distinction to be lost in a single whole-house estimate.

5. Existing radiators are assumed to be adequate without being checked

A heat loss figure only tells you what the property needs. It does not tell you whether your existing radiators can deliver that heat at heat pump temperatures. Radiators sized for a boiler running at 70 to 80°C can produce roughly 40% of their rated output at the lower flow temperatures a heat pump uses. Our article on whether heat pumps need bigger radiators explains this in more detail. If radiator output was never checked room by room against the actual heat loss for that room, a technically correct whole-house heat pump size can still leave you with cold rooms.

What Happens When the Heat Loss Figure Is Wrong

An inaccurate heat loss figure does not usually show up as an obvious fault. It shows up as ongoing, low-grade performance problems that get blamed on the heat pump itself:

• Undersizing leads to a system that runs constantly without ever quite reaching temperature, because it is trying to meet a heat demand it was never designed to cover. We see this pattern regularly, including in cases like a heat pump that was constantly turning on and off because of a zoning design issue that increased running costs, where the root cause traced back to how the system had been designed around the property rather than a fault with the unit.

• Oversizing leads to short cycling, poor efficiency, and a system that struggles to modulate down to the low, steady output a mild day actually needs.

• Wrong radiator sizing downstream of a wrong heat loss figure leads to rooms that never quite feel warm, even though the heat pump itself is working correctly, a pattern we cover in our guide on why some rooms stay cold with a heat pump.

• Higher running costs than expected, because an incorrectly sized system is often forced to run at a higher flow temperature than necessary to compensate. Our guide on heat pumps using too much electricity and our related article on why a heat pump might be using so much electricity both cover flow temperature in detail, and flow temperature strategy is directly shaped by the heat loss figure used at design stage.

The pattern we see again and again is homeowners contacting us months, sometimes years, after installation, convinced the heat pump itself is faulty, when the actual root cause was a heat loss figure that was never accurate to begin with. Our article on heat pump problems after installation and who is actually responsible covers this territory in more depth, including how to work out whether a problem originated at design stage or afterwards.

What a Proper Room-by-Room Heat Loss Survey Actually Checks

A proper survey is not just a more careful version of the same software exercise. It is a physical, room-by-room assessment carried out on site, and it typically includes:

• Measured room dimensions, not estimates taken from a floor plan

• Confirmed construction type for each external wall, floor and roof section, checked rather than assumed from the property's age

• Actual insulation levels, including any retrofit insulation that would not show up in a default age-based assumption

• Window and door specification, including glazing type, frame condition and approximate age, room by room

• Orientation and exposure, since a north-facing room and a south-facing room of identical construction do not lose heat at the same rate in practice

• Ventilation and air permeability factors, including extractor fans, trickle vents, and any obvious draught sources

• Existing radiator sizes and types, checked against the heat loss calculated for that specific room, not just the property as a whole

The output is a room-by-room heat loss figure your installer can use to size the heat pump correctly, specify the right flow temperature, and confirm which radiators need upgrading and which do not. That is the specific gap our Heat Loss Survey service is built to close: a survey carried out by an experienced heat pump engineer, with accurate room-by-room calculations and practical advice on what your system actually needs, independent of any installer's sales process.

How an Accurate Heat Loss Figure Feeds Into the Rest of the System

Heat loss is not just an input for choosing the size of the outdoor unit. It is the foundation for several other design decisions:

• Flow temperature strategy. Our guide on what flow temperature a heat pump should run at explains why the right flow temperature depends on an accurate understanding of how much heat each room actually needs to lose.

• Weather compensation curve settings. A weather compensation curve that is not matched to the property's real heat loss will either run too hot in mild weather or fail to keep up in cold weather. Our article on what weather compensation actually does covers this relationship in detail.

• Overall system sizing. If you have not yet chosen a heat pump size, our guide on what size heat pump you need walks through how sizing decisions should be made once accurate heat loss figures are available.

Get the heat loss figure right and these downstream settings have something accurate to be based on. Get it wrong and every setting built on top of it inherits the same error.

Signs You May Be Working With an Estimated, Rather Than Measured, Heat Loss Figure

• Your installer produced a heat pump size quickly, without visiting every room or measuring anything on site

• You were not asked about insulation upgrades, retrofit work, or extensions when the figure was produced

• The heat loss figure was described as a whole-house number, with no room-by-room breakdown

• Your existing radiators were never assessed against the proposed flow temperature

• Since installation, specific rooms have consistently underperformed compared to others, despite the heat pump itself showing no faults

If several of these apply to your situation, whether you are still planning your installation or already living with the results of one, it is worth getting the heat loss figure independently checked before assuming the heat pump itself is at fault.

Heat Loss Survey or Pre-Installation Design Review: Which Do You Need?

These two services are closely related but serve slightly different purposes. Our Heat Loss Survey service focuses specifically on producing accurate, room-by-room heat loss figures, ideal if you need reliable numbers to give to an installer, or if you want to check figures you have already been given. Our Pre-Installation Design Review takes a wider view, assessing your proposed system design as a whole, including flow temperature strategy, radiator sizing and heat loss, ideal if you already have a quote or design and want an independent check before committing to the work.

If your heat pump is already installed and you suspect a sizing issue based on how it is currently performing, such as rooms that never reach temperature or a system that runs constantly, our Fix My Heat Pump service can help establish whether the issue is a settings problem or something that traces back to the original sizing.

What This Means in Practice

None of this is a criticism of installers as a rule. Many are working within tight timeframes and commercial pressure to produce a quote quickly, and default software assumptions exist for a reason. The issue is simply that a fast estimate and an accurate, room-by-room measured figure are not the same thing, and the difference between them can be the difference between a heat pump that performs exactly as expected and one that leaves you chasing settings changes for years afterwards.

If you are planning an installation, getting an independent, room-by-room heat loss survey done before your installer finalises their design gives them the accurate numbers they need to size the system correctly the first time. If you already have a heat pump and suspect the sizing was never right, an independent survey can confirm whether that is genuinely the case, and what it would take to correct it.

Frequently Asked Questions

Does every installer get heat loss calculations wrong?
No. Some installers carry out thorough, room-by-room assessments as standard. The issue is that there is no way to tell from a single kW figure alone whether it was measured on site or generated from default assumptions, which is why an independent check is useful either way.

Can a heat loss survey be done without disrupting my home?
Yes. A proper survey involves an experienced engineer assessing each room, checking construction, glazing, insulation and existing radiators. It does not require any building work or system downtime.

I already have a heat pump installed. Is it too late for a heat loss survey to help?
No. A survey carried out after installation can confirm whether the original sizing was correct and identify exactly which rooms or radiators need attention, which is often more useful than guessing at settings changes without knowing the underlying numbers.

Will a heat loss survey tell me if my radiators need upgrading?
Yes. A proper survey checks existing radiator output against the heat loss calculated for each room, not just the property as a whole, so you will know specifically which radiators are adequate and which are not.

Need Accurate Heat Loss Figures for Your Property?

Whether you are planning a new installation or trying to understand why an existing system is underperforming, our Heat Loss Survey service provides accurate, room-by-room calculations carried out by an experienced heat pump engineer, along with practical advice on insulation, radiators and system design based on what your property actually needs, not a default assumption.

Learn About Heat Loss Surveys