The heat loss calculation in HeatAlgo: seven steps from documents to result

The heat loss module calculates the building's design heat loss room by room to EN 12831-1:2017, with the thermal resistance of envelopes to EN ISO 6946 and heat exchange with the ground to EN ISO 13370. The wizard leads through seven steps - you can fill them in with Algo or by yourself - and the result is computed live as soon as you enter the area and the rooms. This article describes each step, the compliance status and what a report unlocks. What a heat loss calculation is in physical terms and what depends on it is explained in a separate guide: what a heat loss calculation is.

Before you start: data from the form and from Algo

If the client filled in the form, on the step 1 start screen you see the line "Ready to bring in - Client form" with the figures from the form and the "Bring it into this module" button; the building data and the room list fill in by themselves. You can also work with the Algo assistant - you start from step 1, described below. Both sources are described in data flows between modules.

Step 1: Project

The first step is the start screen: a picture of the calculation and the "Start with Algo", "I'll fill it in myself" and "Guide" buttons. There are no fields to fill in and no bottom bar - at the top there is only the arrow back to the project, and the buttons lead on:

• "Start with Algo" opens the assistant beside the wizard (on a narrow screen on its own page, with this project) with the question of what it needs. Algo answers with a list: the building's location, the heated area, the storeys and the rooms with their areas and heights (from the floor plans), the wall, roof and floor constructions - layers or U values (from the sections and the technical description), the windows and doors with sizes and U values (from the joinery schedule), what lies on the other side of each element, the ventilation type and, with heat recovery, its efficiency (from the building services description), and the n50 result of a pressure test, if there was one. You type the answers in the conversation or attach documents to the message, and Algo enters the values into the steps; you check each one on its step before you read the result. How Algo reads a whole building design is described in the section below.
• "I'll fill it in myself" goes on to step 2.
• "Guide" opens this article in a new tab.

You can ask Algo the same question on any step - the answer is the same. The step list, the module settings and the warnings are on the bottom bar from step 2.

You see step 1 while the project is still empty. A project that already holds any data - typed in, entered by Algo or taken from the client form - opens straight on step 2, and you go back to step 1 from the step list. After "Clear data" the start screen comes back.

Algo AI reads the building design

Instead of copying the building design by hand, you can give it to Algo to read:

1. On the step 1 start screen press "Start with Algo". Algo asks for the documents.
2. Attach them in the conversation: the floor plans, the sections, the technical description and the joinery schedule. Algo reads PDF and photos (JPG, PNG, WEBP) - up to 5 files and 20 MB at once.
3. Algo starts the reading by itself as soon as you attach the files. While it runs, the wizard shows the overlay "I'm reading the building design" with "It usually takes 1-3 minutes." and a "Stop" button.

Algo enters the rooms (names and areas), the building elements (layers or the U value) and the windows and doors - placed in the walls from the marks on the plan and the joinery schedule. In "Propose" mode the values read wait in Algo's changes panel and you save them yourself. In "Automatic" mode they are saved when the reading ends. "Undo" reverses the write.

Algo never overwrites your value with one the documents do not state. When two documents disagree, Algo asks which value to take. Values that break hard rules (e.g. a U-value, λ or layer thickness out of range) are not written into the form.

After the reading Algo lists what is left to complete, in groups: windows, room floor areas, the U values of building elements, room temperatures and other data. Each group has "show", which takes you to the right step and field. The values it assumed without a document Algo gives in one line: "I assumed: ... You can change this."

What it costs: a reading uses the same monthly Algo limit as the chat and does not use a report. The limit grows once you buy a report and is largest with a subscription; when it is used up, Algo says so before the start. One reading also fills heat pump sizing and heat recovery ventilation at no extra cost. One account can run up to 10 readings per 24 hours. Administrative pages (decisions, applications, pages about a person) are deleted as soon as they are recognised and never go on to the reading. We delete the files 24 hours after the reading starts, and its result after 30 days. More about the assistant in Algo, the AI assistant.

Step 2: Building

At the top of the step you enter two figures for the whole project:

• the heated area, that is the sum of the heated rooms' areas; if the sum of rooms from step 5 exceeds it, you get a warning,
• the design outdoor temperature for the building's location - from the building design or a local station's data; Algo can set it from the national table for the nearest of eight cities (Warsaw, Kraków, Gdańsk, Wrocław, Poznań, Szczecin, Łódź, Katowice).

When the project has a wall to a neighbouring building (semi-detached or terraced), an "Annual mean outdoor temperature" field appears beside them too. The other side of such a wall is taken at this temperature, never below 5 °C (EN 12831-1:2017, § 6.3.2.5, Table 7), and without it the calculation does not run.

Below them, the data shared by the whole building:

• the dimension basis - internal (between walls, the most common for detailed designs), external (outlines) or measured; measure envelopes consistently on one basis,
• thermal bridges - the construction class adds a blanket ΔU_TB: very good 0.02, standard 0.05, retrofitted 0.10, poor 0.15 W/(m²·K),
• airtightness - class I-IV suggests a typical value, and if you have a Blower Door result, enter n50; above 3 1/h the app suggests switching to the detailed method with per-room flows.

Step 3: Ventilation

You choose the type: natural, mechanical extract, or mechanical with heat recovery. With heat recovery the recovery efficiency field appears (85% is typical for a good unit) and the "Insert default airflows" button, which fills supply and extract in the rooms with typical values; fields already filled are not overwritten. The air change rate defaults to 0.5 1/h. If the project has a designed ventilation system in the ventilation module, the heat loss calculation checks consistency: the ventilation type, the efficiency and the room flows must agree.

Step 4: Envelope library

Here you save the project's constructions so you do not enter them in every room from scratch. The step has five groups as rows: Walls, Roofs, Floors, Ceilings, and Windows and doors. Beside each group's name are two counts - a folder icon for the constructions in this project, a book icon for the entries in "My library" - with the first names under it. The plus on a row creates a new construction of that group, and the pencil (or the whole row) opens its list, with search. The list's first row is Algo, which answers questions about the elements and layers in this project.

A new construction is made in one dialog, in three steps:

• "Kind" - for example an external wall, a wall to an unheated space or a wall to another heated room; Roofs have one kind, so they skip this step,
• "Starting point" - "Describe it to Algo" (the dialog closes and Algo asks for the element's description from the design, a photo of the section or its layers, computes the U and, when you ask, saves the construction in "My library"), "Ready-made construction" (a typical layer build-up from the catalogue, e.g. 25 cm ceramic block + 20 cm graphite EPS: search by name, material or maker, and narrow the list by building period, construction, insulation and whether it meets WT 2021; on a phone the filters sit behind the "Filters" button), "My library" (a copy of one of your entries), "Enter a U value" (from the building design or an audit, no layers) or "From scratch" (an empty list of layers),
• "Layers and name" - the element's section and a table of layers from the inside: material, thickness in cm and λ, with moving and removing layers; the "Non-homogeneous" switch adds the λ of the structural part for walls with studs. The U value is computed as you type, to EN ISO 6946. The "Save in My library" switch saves the construction to your account as well.

A window or a door is one step: its kind, width, height and U. "Choose from ready-made windows and doors" lists typical windows of each period and the WT 2021 limits - the one you pick sets the kind, the U and the name, and you type the size yourself. The "Use a ready-made construction" button beside the step's title opens the same dialog straight on the ready-made constructions. Ready-made constructions are proposals - check thicknesses and λ against the building documentation, because the result matches reality only when the layers reflect the real cross-section.

"My library" holds the constructions, windows and doors saved on your account, available in every project. You open it from the menu on the left ("My library") - it has the same groups, lists and dialog as this step. An account without an active subscription keeps 5 entries in it in total; an active subscription lifts the limit, and when it lapses the saved entries stay. Algo reads your library and, when you ask, adds, changes or deletes entries, each with an Undo.

Step 5: Rooms

Most of the work is here. "Add room" creates a card with a name, a type (living room, bedroom, kitchen, bathroom 24°C, corridor 18°C and others, office and service rooms too), area, height, design temperature and storey (basement, ground floor, upper floors). "Choose typical values" sets the temperature and ventilation category with one click. A room can be duplicated, and "Add attic" runs the wizard that generates slopes, knee walls, gables, floor and collar tie ceiling from the building width, roof angle, knee wall and length.

In every room the "Envelope elements" section shows its elements as rows: the kind's icon, the name, and under it a gray line with the area, the U value or the number of layers, and the windows. The pencil opens an element for editing, and the "⋯" menu beside it has "Remove" (with an Undo) and "Duplicate to…", which copies the element with its windows into this room or another one - the line under it says "Copied to: …" with "Undo". The "Add an element" plus in the section header and the pencil on an element open the same dialog, on one screen in three parts:

• "Construction" - a list of constructions only, right in the dialog: the current one first and ticked, then "In this project" and "My library", each with its U-value; a "Search constructions" field above the list when there is something to choose from. When editing, only the chosen construction shows, with its layers, name and U - "Change" opens the list, and a construction made of layers has "Edit layers" beside it. The "New construction" button by the heading opens the same dialog as step 4 - from layers, from ready ones, from a U-value or with Algo. An element with a given U-value has its "U value" field in the same part,
• "In this room" - the name, the gross area, what lies on the other side and the net area,
• "Windows & doors" - the rows of windows and doors, and under them "Add a window or door" and - when another element in the project already has windows - "Copy from another element", which adds copies of its windows and doors to this element. "Save" finishes. Picking copies the construction into the room - the library stays as it was, and the copy can be changed in this room alone.

A room that borders nothing colder is marked with "This room has no elements to calculate" - only its ventilation is then calculated, and a note goes into the result and the report. You pick an element's type from ten: "External wall", "Wall to unheated space (garage, pantry)", "Wall to another heated space", "Roof / flat roof", "Floor on ground", "Floor above unheated space (cellar/garage)", "Floor over outdoor air (passage, overhang)", "Floor over a heated space", "Ceiling under unheated space (attic)" and "Ceiling under a heated space". For each you enter the gross area (with openings - windows and doors from the table subtract themselves, and losses are computed from the net area) and the U-value: directly or from layers copied from the library. Windows and doors cannot be larger than the element they sit in - if they are, the calculation stops. Non-homogeneous walls (timber frame with studs) have their own mode with a structural fraction.

The details the standard asks about:

• floor on the ground - perimeter P and depth z; without a measured perimeter the app assumes 4·√A and lowers the compliance status; the groundwater level affects the f_GW correction,
• envelopes to unheated spaces - you pick what is on the other side (a room with 1-3 external walls, a basement, an insulated or uninsulated attic, a circulation space, a suspended floor void) and the b_u factor follows; you can also give an exact temperature. A new "Floor above unheated space (cellar/garage)" starts with "Basement - with external doors/windows (b_u ≈ 0.8)" set, per EN 12831-1, Table B.2; elements already saved stay as they were,
• envelopes to heated spaces - on the other side you pick from the "Heated" group: "Another heated room" (a temperature; an empty field is the same as this room, so no loss, and the partition counts for this room only, not in the building's load), "A neighbour's flat in this building" (temperature required) or, for a wall, "Neighbouring building (semi-detached, terraced)" (the annual mean outdoor temperature from step 2, never below 5 °C; with the first such wall the dialog asks for it right away), and those two count in the building's load too; when the other side is warmer, the partition is a gain, and elements in this group carry no thermal bridge supplement,
• floor over outdoor air (passage, overhang) - calculated like an external element, at the design outdoor temperature and with no path through the ground,
• windows and doors - a table with width, height, U and quantity,
• the ventilation category - a habitable room or a wet room with a window (n_min 0.5) or an internal secondary room (0.0); with heat recovery also supply and extract,
• high rooms (from 4 m) - a heat emission system must be selected.

Step 6: Adjustments

For a typical detached house you can skip this step. It holds internal heat gains (ignored by default; non-zero values are an explicit assumption to agree with the client, with a per-room override), intermittent heating with the heating-up power φ_hu (from the auxiliary table or entered by hand, with the setback period, the air change during setback, the building's thermal mass and the heating-up time) and the heat source sizing correction - a factor that helps size the source and deliberately does not change the rooms' loads.

Step 7: Result

The result shows the building's design heat loss, the heating output index in W/m², the heated area and the split into transmission, ventilation and thermal bridges. The "Room breakdown" carousel switches rooms, and clicking a card opens the details: envelopes with layers and the U computation, windows and doors, load components, ground corrections, ventilation. When the sum of the rooms differs from the building's load, one line under the carousel reads "Sum of rooms X W · Building Y W · difference Z W" with the reason. The first is the wind: each room is counted with the wind on its own side, the building with the wind once. The second is partitions to other heated rooms - they go into their rooms' loads (EN 12831-1:2017, § 6.3.1.3, Formula (3)) but not into the building's (§ 6.3.1.2, Formula (2)). You size the heat source on the building's load, and radiators and loops on the room loads. A room that gains more heat from warmer neighbours than it loses can come out at 0 W or below - its card then says "No heating needed". The "Calculation method" tab describes the assumed parameters. You download the PDF with a square button on the bottom bar (described below) - see the article on reports.

Until there is a result, step 7 shows a sample result under frosted glass, and in the middle the step's heading and what the result still needs: rows with a "Fill in" or "Fix" button that takes you to the right step and field, and a "Complete with Algo" button. When the calculation fails on our side, you see "The result could not be calculated" - your data is saved - with "Try again" and "Ask Algo what happened". When the calculation stops at a value you entered, the list names that value, and "Fix with Algo" asks the assistant about it.

Issues, blockers and the compliance status

The icon with the "N to fix" count on the wizard's bottom bar, beside the step number, collects everything that needs completing; with nothing to fix it is not shown. Items labelled "Blocks the calculation" (e.g. no rooms, a room without envelopes, heated area exceeded, missing flows with mechanical ventilation, the annual mean outdoor temperature missing for a wall to a neighbouring building, windows and doors larger than the element they sit in) stop the calculation; the "Fix" button takes you to the field. The remaining remarks do not block the result but lower its status.

The status on the result has two values: "Result meets EN 12831-1:2017" or "Approximate result - does not meet strict EN 12831-1 compliance". You see the reasons in the "About this result" dialog: the status is lowered by an approximate perimeter of the floor on the ground or by heating-up power from the table without confirming the conditions of Annex F.2. Missing air flows, the old temperature correction factor, the annual mean outdoor temperature missing for a wall to a neighbouring building, and windows and doors larger than the element they sit in do not lower the status but block the calculation - the same dialog lists them under "To fix". A project with air transfer devices between zones (ATD) or designed flows between rooms is outside the module's scope - the app says so plainly.

What a free account shows

The calculations run in full. On screen and in the PDF you see the building's demand, the index and one fully calculated room; the other room cards show "Value available once the project is unlocked", and the method details (source sizing, bridges, n50, ventilation) wait for the unlock. One report unlocks the whole project - see what is free and what a report unlocks. You can also buy a report for the heat loss calculation alone, for that project.

Module settings and clearing data

From step 2 on, the wizard's bottom bar has three parts: on the left a square "Back" button, in the middle a glass island, on the right "Next" (on a phone, the arrow alone). The island holds, in order: the module settings gear, the step number (e.g. "4/7" - clicking it opens the list of every step) and the "N to fix" count. On step 7 two square icon buttons take the place of "Next": report notes and the PDF download; the tooltip gives their names ("Report notes", "Download PDF"). Both are grey until there is a result. Algo opens from the button in the top right corner (on a phone, Algo's mark alone).

The gear opens the "Module settings" panel with the module's name under the title. The "Data sources" section has a "Client form" row with a "Bring in" button and an "Algo AI" row with a "Fill in" button, which opens Algo with the same question as "Start with Algo" on step 1. The "Module data" section has a "Clear data" row with a red "Clear" button, which wipes the whole heat loss form in this project after confirmation. The heat loss result feeds pump sizing, underfloor heating and ventilation - after changes in the heat loss calculation those modules offer an update.