What Heatworx measures, assumes and lets you edit
Every heat loss calculation is built from inputs. Some are measured directly. Some are chosen by the person doing the survey. Some are looked up from tables. Some are assumptions that should be reviewed before the result is trusted.
Most heat loss tools hide these distinctions. The number comes out of a box, and you have no way to tell which parts were measured and which were guessed. Heatworx takes a different approach: every input is visible, and almost every input is editable.
This page explains where each input comes from and what you can do about it.
Some inputs are measured from the room scan
When you scan a room with Heatworx or enter dimensions manually, the app captures the physical geometry of the space: wall lengths, floor area, ceiling height and room volume.
These are the most concrete inputs in the calculation. They come from direct measurement rather than assumption. Even so, they are not perfect — a phone-based room scan has tolerances, and manual entry depends on where and how you measured.
Heatworx lets you review and edit the geometry after capture, so if a wall length or ceiling height looks wrong, you can correct it.
Some are selected by the user
Certain inputs require a deliberate choice from the person doing the survey. These include the room type (living room, bedroom, kitchen, bathroom), the construction type for walls and floors, the glazing type for windows and doors, and the emitter type installed in the room.
These selections drive downstream calculations. The room type sets the default internal design temperature. The construction type determines the U-value lookup. The emitter type determines the heat output available in the room.
Where you are unsure, Heatworx will use a reasonable default — but defaults are starting points, not answers.
Some are inferred from property age, construction and evidence
When a construction type is selected or assumed from the property age, Heatworx looks up a default U-value from recognised tabulated data for that construction era and type. These tables cover common UK wall, floor, roof and glazing constructions across different building periods.
Identifying the actual construction of a wall is often the hardest part of a domestic heating survey. You can sometimes tell from the wall thickness, the sound when you knock it, what is visible in the loft space, or whether cavity wall insulation marks are present. But in many older homes, the exact build-up is genuinely uncertain.
Airtightness is another inferred input. In most existing homes, air permeability has not been measured. Heatworx uses survey evidence — such as draughty windows, suspended timber floors, open flues and the age of the building — to estimate airtightness rather than requiring a blower door test.
Some are defaults that should be reviewed
Several inputs are set to sensible defaults but deserve conscious review:
- Internal design temperatures vary by room type. Living rooms default to 21 degrees C, bedrooms and kitchens to 18 degrees C (for pre-2006 buildings), and bathrooms to 22 degrees C. These follow recognised UK guidance, but your household may use different temperatures.
- Outside design temperature is set from the property location, using data from one of 28 UK weather zones with altitude correction. This represents a cold design day, not an average winter day.
- Airtightness assumptions are based on survey evidence, but if the evidence is limited, the estimate may be conservative or optimistic. An incorrect airtightness estimate can lead to significant oversizing or undersizing of the heating system.
These defaults are not hidden. They are shown in the survey and can be changed.
Some are calculated from other inputs
A few values are derived automatically and cannot be directly overridden:
- Surface areas are calculated from the room geometry — wall area comes from wall length multiplied by ceiling height, minus any window and door openings.
- Ventilation heat loss is calculated from the room volume, the assumed air change rate and the temperature difference.
- Heating margin — the comparison between the room's design heat loss and the installed emitter output — is calculated by Heatworx from the other inputs.
You influence these values by changing the inputs they depend on, not by editing the calculated result directly.
Input provenance table
| Input | Where it comes from | Can the user edit it? |
|---|---|---|
| Room dimensions | Room scan / manual edit | Yes |
| Wall, floor and ceiling areas | Room model | Yes |
| Windows and doors | Scan / user edit | Yes |
| Construction type | User selection / default | Yes |
| U-value | Lookup / calculated from construction | Yes |
| Internal room temperature | Room type default | Yes |
| Outside design temperature | Location / default | Yes |
| Draught / airtightness evidence | Survey selections | Yes |
| Radiator output | User input / selected emitter | Yes |
| Heating margin | Calculated by Heatworx | No |
Why transparent assumptions are better than hidden ones
A heat loss result is only as reliable as its inputs. If you cannot see what was assumed, you cannot judge whether the result is reasonable for your building.
Many tools present a single number with no way to inspect the assumptions behind it. If the U-value was wrong, or the airtightness was guessed badly, or the room temperature was set to a value nobody in the household actually uses, the result will be wrong — and you will have no way to tell.
Heatworx takes the opposite approach. Every assumption is shown. Every editable input can be changed. The calculation updates immediately, so you can see how sensitive the result is to each assumption. An editable estimate you can interrogate is more useful than a black-box number you have to accept on faith.
How this appears in Heatworx
In the app, each room's survey shows the inputs grouped by category: geometry, surfaces, construction, ventilation evidence, temperatures and emitter. Inputs that came from a scan or default are labelled, and you can tap any editable value to change it.
When you change an input — say, switching a wall from solid brick to cavity with insulation — the room heat loss recalculates immediately. This lets you explore how different assumptions affect the result, which is especially useful when the actual construction is uncertain.
The room model page explains how Heatworx turns a scan into surfaces. The heat loss calculation page explains how those surfaces and inputs become a heat loss figure.
Limitations and assumptions
No survey tool captures everything. Heatworx does not measure U-values directly, does not conduct a blower door test, and does not model dynamic thermal behaviour or thermal mass. The calculation is a steady-state design estimate for a cold design day.
Construction identification — particularly for older buildings with mixed or modified fabric — remains inherently uncertain. The best survey evidence narrows the range of plausible assumptions, but rarely eliminates uncertainty entirely.
Where inputs are uncertain, the most useful thing a tool can do is make the assumptions visible and let you test what difference they make.
Heatworx helps produce and review a structured heat loss estimate. It does not replace professional judgement, manufacturer requirements, regulatory obligations, or a full heating system design where one is required.
Frequently asked questions
Can I edit the assumptions Heatworx uses?
Yes. Almost every input in Heatworx is editable. You can change construction types, U-values, room temperatures, airtightness evidence and more. The only values you cannot directly edit are those calculated automatically from your other inputs, such as the heating margin.
How does Heatworx choose default U-values?
Default U-values are looked up from the construction type you select or that is assumed from the property age and build. These lookups are informed by recognised UK domestic heating design guidance, including tabulated values for common construction types across different building eras.
What if I don't know the construction of my walls?
That is normal. In many homes, the exact wall build-up is not visible. Heatworx will use a default based on the property age and type, but you should review it. Knocking the wall, measuring its thickness, or checking what is visible in the loft can help narrow it down.
Which inputs have the biggest effect on heat loss?
Wall construction and U-values typically have the largest effect, followed by glazing area, airtightness assumptions and internal design temperature. A small change in U-value applied across a large external wall area can shift the room heat loss significantly.