Home heat and airtightness

Start assessing heat demand with a building section, not with the boiler or floor area. Mark the rooms that will be heated continuously, the unheated garage, basement, attic and seasonal veranda. The boundary surrounding the heated volume must be clear and continuous: sometimes it follows the attic floor, sometimes the roof slope. Sharing a house address does not mean that every part of the building has the same temperature.

List walls, roof, floors, windows and doors separately, recording the environment on the other side of each element. Obtain the net wall area by subtracting window and door openings, which will be calculated separately, from the gross area measured using an agreed convention. Use the same dimensioning and thermal-bridge accounting system throughout the assessment so that junctions are neither omitted nor counted twice.

A partition facing a cooler garage is not equivalent to an external wall. It needs a justified adjacent-zone temperature or a suitable calculation model, not an arbitrary common factor. Mark an unknown layer or temperature as unconfirmed rather than replacing it with zero. This envelope schedule provides a verifiable starting point for renovation scenarios and for selecting the heating output of individual rooms.

Thermal insulation slows heat transfer through a construction, whereas the airtight layer limits uncontrolled air movement through its gaps. A thick layer of mineral wool therefore does not, by itself, demonstrate airtightness. Conversely, sealing a thin wall does not necessarily give it good thermal resistance. In a renovation assessment, distinguish these tasks in the quotation and drawings, but combine them into one coordinated solution.

The air barrier selected in the drawing must remain continuous at wall, ceiling, floor and window junctions. Pipe, cable and duct penetrations and later fixings need particular attention. Select tape or sealant for the substrate, joint movement, exposure conditions and the manufacturer's system. Merely covering a joint does not demonstrate durable adhesion: before finishes conceal it, retain photographs of accessible junctions and an inspection of the completed work.

Air control and vapour control also have different purposes, although a properly designed layer can perform both functions. An external wind barrier is not an automatic substitute for internal airtightness. Before work starts, agree which contractor completes each junction and how the next trade will avoid damaging it. Coordinate air sealing with functioning ventilation, not with taping over fresh-air routes.

A fan-assisted airtightness test measures building air leakage under an artificially created pressure difference. The reference pressure for n50 is 50 Pa and its unit is h⁻¹. It is not the air-change rate occurring naturally in ordinary weather and does not replace a ventilation design. Results from two houses cannot be meaningfully compared without checking which part of the building and which volume were included in the test.

Look in the report for the method, measured volume, envelope area, building preparation, closed or temporarily sealed openings and measurement conditions. Where Q50 is the leakage flow in m³/h, n50 = Q50 / V, with volume V in m³. For the area-based indicator we use q50 = Q50 / A here, with area A in m² and q50 in m³/(h·m²); always check the symbols and reference area used in the particular report.

An intermediate test is useful when the airtight layer is continuous but its junctions remain accessible for repairs. The final document must represent the agreed building condition, not just an attractive lowest number. Leave leakage investigation and appliance preparation to a specialist, particularly where combustion appliances are present. This explanation does not establish Lithuanian energy-class limits or guarantee certification.

A thermal camera records infrared radiation emitted and reflected by a surface; it does not see directly through a wall. A cold patch in an indoor image may indicate a thermal bridge, uneven insulation, air movement or another cause. A vivid colour alone identifies neither the origin of a defect nor the amount of heat lost. Even an image of the same wall changes when its temperature scale is adjusted.

Assessment requires a suitable and sufficiently stable indoor-to-outdoor temperature difference. Wind, rain, earlier sunshine, heating operation, furniture, viewing angle and surface emissivity affect the result. Glass or shiny metal may reflect the sky, a person or another heat source. Ask for measurement conditions, a normal photograph and an explained conclusion, not just coloured images.

Professional airtightness testing and thermography are sometimes combined to confirm an air leak; a cold stripe alone does not prove one. A construction's U-value cannot reliably be determined from a single image: that requires an appropriate separate measurement method and data. A useful report clearly distinguishes the observed sign, its likely cause and the confirmation still required. That allows a specific junction to be repaired instead of guessing from the most striking image.

Before air sealing, establish how air enters living rooms, travels towards extract points and leaves the building. Mark the air supply for a stove, fireplace or other combustion appliance separately. Gaps in the construction are not a reliable ventilation solution, but reducing them can change the existing pressure balance. Replacing windows must therefore be considered together with air exchange rather than as an isolated purchase.

Taping over an air inlet, plugging a ventilation duct or arbitrarily closing a chimney is not an energy-saving measure. A powerful cooker hood or extract fan may affect the draught of an open-flued combustion appliance. A suitably qualified specialist must assess safe combined operation, the required combustion air and flue-gas removal using appliance instructions and applicable requirements. Do not create a makeshift depressurisation test by switching on all fans while a stove is burning.

Carbon monoxide CO cannot reliably be detected by smell, so a suitable alarm and its maintenance provide important additional protection but do not replace correct installation. If an alarm sounds or poisoning is suspected, move into fresh air and call emergency assistance. After air sealing, check ventilation flows, air-transfer paths and actual indoor humidity. An airtight house needs a controlled air supply, not no air supply at all.

A material's thermal conductivity λ is measured in W/(m·K): a lower value means lower conductivity under the same assessment conditions. The resistance of a homogeneous layer is R = d / λ, with thickness d entered in metres and the result in m²·K/W. For example, a 0.20 m layer with λ = 0.040 W/(m·K) gives R = 5.00 m²·K/W. This describes that layer, not the whole wall or house.

For a simple flat opaque element, the layer resistances and the internal and external surface resistances Rsi and Rse are considered. Only with suitable calculation assumptions can U ≈ 1 / Rtotal be used, with U expressed in W/(m²·K). A higher R and lower U indicate better resistance to heat transfer, but this simple arrangement cannot be transferred directly to windows, ground heat transfer or complex junctions.

Timber framing, metal fixings, non-homogeneous layers and air spaces require an appropriate assessment; an air space's resistance is not its thickness divided by an arbitrarily selected mineral-wool λ. Calculations need a design value appropriate to service conditions, not merely a declared number in an advertisement. If the construction is unknown, provide a justified range and identify missing data instead of presenting a supposedly precise result.

A statement about additional insulation thickness does not describe a complete facade solution. You need to know the existing wall materials, their condition, causes of dampness, the substrate's suitability for fixings and the compatibility of the whole system. Address water ingress or structural damage first. Concealing a damp wall beneath a new layer does not resolve the moisture problem, even if the chosen material has good declared thermal conductivity.

Design comparisons should consider not just a product's λ but also the continuity of the installed layer. Fixings, brackets, reveals, the plinth, balcony connections and roof edges may create local heat-flow paths. The design must show how new insulation connects with windows, foundations and the roof. Before ordering, ask for drawings of these junctions and a defined scope of work rather than a general facade illustration.

Select the material and system by assessing fire safety, loads, rain exposure, drying potential and the manufacturer's installation conditions. Declared λ is not automatically the design value at every moisture level or service condition. There is no universal ranking of the best material and no thickness suitable for every house. Compare quotations by whole-element performance, junction quality and clearly confirmed assumptions.

If the attic remains unheated, the boundary of the heated house usually follows the top-floor ceiling. If the roof space becomes habitable accommodation, the thermal and airtight boundary must move coherently into the roof, knee walls and their connections. These options cannot be merged into one order for insulation thickness. First decide which spaces will be heated now and whether there is a clear sequence for a future conversion.

In a cold attic, check ceiling junctions, hatch seals, hatch insulation and service penetrations. Designed ventilation gaps and air inlet and outlet routes must not be accidentally blocked by insulation. Building airtightness and roof-space ventilation are not opposing tasks: warm, humid indoor air must not flow uncontrolled into the cold roof construction. Also assess protection for pipes and equipment remaining in the cold zone.

A habitable roof space needs a specific design for roof layers, vapour control, rainwater and drying. There is no universally suitable membrane sequence or ventilation-gap dimension: the covering, underlay, timber and local conditions matter. Before concealing junctions, check safety clearances and loads associated with chimneys, lights and electrical installations. A complicated or damaged roof needs assessment by a specialist, not just an insulation supplier.

Ground is not outdoor air, so ground-floor heat loss cannot be assessed simply by assigning the winter outdoor temperature to its entire area. The heat path depends on building geometry, exposed perimeter, insulation position, foundation depth and soil properties. Seasonal ground-temperature changes also differ from rapid air cooling. Consequently, results for identical floor layers in small and large buildings are not necessarily proportional to floor area alone.

For the initial data, collect a floor plan, the boundary between heated and unheated zones, exposed perimeter length, floor level and a foundation section. Distinguish a slab on ground, a floor over a ventilated void and a heated or unheated basement: these are different cases. If edge insulation is unknown, do not record it as present. State the assumption and check documents or investigate the construction safely through an agreed inspection.

A design assessment must use a suitable ground heat-transfer model and coordinated accounting for edge thermal bridges so that losses are not counted twice. Foundation insulation must also address moisture, waterproofing, loads and safe working beside existing foundations. A simple wall-layer resistance formula does not solve these issues. For a quotation and heating output, clearly distinguish verified construction from a preliminary scenario.

A thermal bridge occurs where heat flow differs from the adjacent regular part of an element: at corners, floor edges, balcony or window junctions and supports. A good wall U-value does not eliminate a poorly resolved connection. Such places can increase heat loss and lower the internal surface temperature, so comfort and moisture risk must be assessed together rather than by insulation thickness alone.

A linear bridge is described by ψ, measured in W/(m·K), and its length L in metres; a point bridge by χ, measured in W/K. Under appropriate assumptions, ψ × L and χ contribute to the heat-loss coefficient H in W/K, with the temperature difference ΔT additionally applied to calculate heat flow. Values depend on the specific geometry and chosen dimensioning convention, so a catalogue detail is suitable only after checking that it matches the house.

Dew point depends on air temperature and humidity; a U-value alone does not show where or when moisture will accumulate. Mould risk may develop before water droplets become visible. Assess the coldest surfaces, ventilation, rain, construction drying and operating conditions. Internal insulation in particular needs separate assessment because it changes the existing wall's temperature and drying potential. A questionable junction needs appropriate thermal and moisture analysis, not a promise based on one diagram.

A window is more than its insulating glass unit. Ug describes heat transfer through the glazing, Uf through the frame, and Uw through the complete window. These coefficients are expressed in W/(m²K): under comparable conditions, a lower value means less heat transfer. If a quotation highlights only Ug, it does not yet establish the thermal performance of the complete window you are buying. Ask the supplier to identify the component represented by every stated value.

The simplified relationship is Uw = (Ag × Ug + Af × Uf + lg × ψg) / (Ag + Af). Ag and Af are the glazing and frame areas, lg is the glazing edge length, and ψg is the linear thermal transmittance at the glazing-to-frame junction. Consequently, windows with the same profile can have different Uw values when their size, subdivisions or opening arrangement differ.

Compare the final assemblies proposed for your actual openings, rather than just a catalogue example of a reference window. Request the dimensions, glazing specification, frame and calculation or declaration basis. The thermal bridge where the installed frame meets the wall is assessed separately; Uw alone does not describe the installation joint, air leakage or the heat loss of the entire room. Replacing windows must also preserve the intended fresh-air supply.

A profile name identifies a system, but is not a thermal guarantee for the complete window. For SOFTLINE 70 AD, VEKA's official pages list a system depth of 70 mm, glazing thickness up to 42 mm and Uf 1.3 W/(m²K). The corresponding figures for SOFTLINE 76 MD are 76 mm, up to 48 mm and Uf 1.1 W/(m²K); for SOFTLINE 82 MD they are 82 mm, up to 54 mm and Uf 1.0 W/(m²K).

These are manufacturer system reference figures checked on 2026-10-27, not a promise for every manufactured window. The particular profile combination, reinforcement and construction affect the actual value. Permitted glazing thickness does not itself establish Ug, solar factor or acoustic performance. The MD systems use a centre seal; more chambers or a deeper profile alone do not prove that a particular complete window is better in every respect.

Request the exact system variant, glazing specification and Uw for your dimensions and opening arrangement. Check fixing, thresholds, reveals and the installation scope too: good components do not replace correct installation. VEKA is used here as a technical comparison example, without claiming dealership or representative status. Compare other manufacturers through complete-product documentation as well, rather than profile branding or an advertised figure for the best-performing variant alone.

Choose glazing through several performance measures, not simply the description 'triple glazed'. Ug describes heat transfer caused by a temperature difference, while solar factor g describes the fraction of solar energy entering through the glass. Light transmittance is a separate measure again. Two glass units with similar Ug can transmit different amounts of solar heat and daylight, so a single figure cannot answer every comfort question.

For large sunny windows, a lower g can help limit summer solar gains, but also reduces the potential benefit of sunshine during the heating season. The decision depends on orientation, shading, glass area and how the room is used. Different sides of a house do not necessarily need identical glazing. Colour or reflectivity in a photograph is insufficient for selection: request documented Ug, g and light transmittance for the actual glass unit.

A warm-edge spacer forms part of the insulating glass unit's perimeter. Its effect is evaluated in the particular glazing-to-frame junction through ψg, rather than by promising an identical improvement for every window. It does not replace appropriate indoor humidity management and ventilation. Ultimately compare whole-window Uw, the need for safety glazing and planned exterior shading. This avoids confusing winter insulation with protection against summer sun.

A good window can perform poorly if its connection to the wall is neglected. The installation detail must separately address load transfer, thermal insulation, airtightness and drainage. Installation foam may form part of the thermal infill, but saying 'filled with foam' does not establish correct fixing or every other function. The solution must suit the wall material, the condition of the opening and the instructions for the particular window.

The internal joint must connect to the building's air-control layer, while the external protection must manage wind and rain and suit the construction's moisture behaviour. Adhesion of tapes, membranes or sealants depends on substrate preparation and compatibility. Supports, the sill connection and the drainage route are particularly important at the bottom. Product drainage openings must not be covered, nor should the intended drainage be replaced by decorative finishing alone.

Before covering the joints, photograph fixings and layer connections, and identify responsibility for them in the contract. Airtightness testing or leak detection helps verify the result, but a warm-looking thermal image is not complete acceptance evidence. When replacing old windows, assess damp or damaged reveals first. This describes the functions that need resolving, not a universal sequence of installation layers suitable for every building.

Start with an identical opening schedule: dimensions, opening modes, subdivisions, colour and intended use. A fixed window quotation is not directly equivalent to an opening window of the same size. For each item, request the exact system, glazing specification and basis for Uw. If Uw applies only to a reference size, this must be clearly identified rather than presented as a property of every window in your order.

Alongside heat transfer, compare solar factor, light transmittance, required safety glazing and substantiated air, water and wind performance. Not every opening needs the same solution: height, exposure and room use matter. For a noisy location, the phrase 'triple glazing' is insufficient. Ask for documentation for the intended assembly, not a better result obtained for another size or a different glass specification.

Compare the work separately: removal, waste, opening repairs, supports, sealing, sills and finishing. Agree responsibility for measurements, delivery conditions, acceptance, adjustment and maintenance information. Check how ventilation will operate after replacement; do not order the sealing of vents intended for it. A like-for-like schedule helps explain price differences, but does not make a profile name or Ug figure sufficient to declare a universal winner.

When choosing an external door, consider Ud for the complete door product, not just the advertised insulation of the panel. Leaf, frame, glazing and their junctions all contribute. Frame Uf or a panel U-value cannot automatically be presented as the Ud of the whole door. The quotation must identify the dimensions, glazing and construction represented by the value. An added sidelight or another leaf can change the characteristics of the complete assembly.

The threshold is more than its visible profile. Its support and connections to floor insulation, wall insulation and water-control layers must be planned before installation. A convenient low passage still requires assessment of rain, snow and external surface levels. Product drainage must not be sealed arbitrarily. Agree the levels of internal flooring, the terrace or path, and structural support beneath the whole door assembly in advance.

The sealing line only works when the door is installed, adjusted and used correctly. Investigate a persistent draught instead of concealing it with random adhesive seals. Acceptance checks include closing, gaskets, hardware and the connection to the building's air barrier. Whole-door heat transfer, air leakage and installation thermal bridges are different assessments; a good Ud alone does not confirm that the actual entrance detail has been resolved.

A large patio opening brings daylight, but also changes heat losses, solar gains and loads. Fixed sections and moving leaves have different frame proportions, so a profile name alone is insufficient. Request thermal data for the complete proposed assembly, clearly identified dimensions, glazing specification and opening arrangement. Plan shading for a sunny façade from the outset, rather than treating it as an extra only when the room begins to overheat.

VEKA VEKAMOTION 82, intended for lift-and-slide doors, is one example described in official system information. The manufacturer also presents a separate MAX design variant; similar names do not make them identical. A system description does not confirm the dimensions, watertightness or wind resistance of your actual door. Check suitability through product documentation and the building's real exposure conditions, rather than an attractive general visualisation.

A heavily glazed assembly needs designed support and adequate rigidity around the opening. Threshold, floor insulation, terrace and drainage must be coordinated at the bottom; convenient access does not remove these requirements. Before ordering, discuss moving-leaf weight, operation, maintenance access and safety glazing. At acceptance, check operation and specified clearances, and request the operating instructions. Arbitrarily changing supports or sealing drainage can damage an otherwise correctly selected system.

Start choosing a roof window with the roof construction and the instructions for the specific product. Roofing type, pitch, installation height and safe access all matter. A permitted pitch range for one model family cannot automatically be applied to another. A pitched-roof window and a flat-roof rooflight are not the same product; check the exact code, glazing variant, installation kit and stated limits of use.

When assessing thermal performance, establish what the declared Uw includes and the conditions under which it was determined. Installation inclination affects heat transfer, so not every catalogue figure can be compared directly. Do not relabel a flat-roof product's Urc or a value based on a different reference area as Uw. A low Ug alone does not describe the complete roof window, its junctions and its installation detail.

Balance daylight with solar gains, views, cleaning access and exterior overheating protection. A high-level window may need remote operation, with power supply, servicing and response to rain planned according to the selected system's instructions. A window is not a substitute for designed ventilation. Entrust changes to the roof construction and work at height to qualified specialists; these are selection questions, not instructions for safe do-it-yourself installation.

A roof window interrupts the roof layers, so insulation packed around the frame is not enough. The window model, roofing, compatible flashing kit and installation position must work together. Both water running over the roof covering and water that may reach the layer beneath it need a planned route around the opening. A façade-window detail cannot simply be rotated and assumed suitable for a roof.

The detail must maintain insulation continuity, connect to the roof's air and vapour-control layers, and provide exterior protection. The VELUX instruction catalogue separately lists BDX insulation, BFX underfelt protection and BBX vapour-barrier products. These perform different functions and are not arbitrarily interchangeable. Whether a particular window needs an additional drainage component, and how it connects, must be established from the exact assembly and instructions.

Before the detail is covered, request photographs and verify that the intended layer connections were actually made. Internal reveals must also suit the manufacturer's solution without interrupting insulation. Surface condensation may arise from the junction, air exchange or indoor humidity, not only the glass. Altering roof structures and working at height are not do-it-yourself experiments: agree installation, safety and responsibilities with a competent contractor in advance.

A well-insulated house can still overheat in summer. Heat enters through sunlit glazing, while occupants and equipment add heat indoors. Window area, orientation, glazing g and surrounding shade must be considered together. Low wall U or good window Uw does not provide solar protection. A large west-facing opening may require a different solution from a south-facing window beneath a suitably designed roof overhang.

External blinds, awnings or other purpose-designed shading can intercept solar energy before it crosses the glass. Indoor curtains help with light and privacy, but should not be treated as equivalent to exterior protection. Selection must suit wind exposure, fixings and the particular window; plan operation and maintenance too. A general advertising percentage for a product cannot promise the same room-temperature reduction in every house.

Use shading before the room heats up. Ventilation can assist cooling when the outdoor air is cooler, but noise, security, rain and possible air pollution must be considered. A heat-recovery ventilator's summer bypass is not an air conditioner and does not guarantee sufficient cooling capacity. For extensive glazing or sensitive rooms, request an assessment of the whole summer operating strategy rather than automatically increasing heat-pump size using a winter heat-loss calculator.

Start with a list of the elements enclosing the heated space, rather than the total floor area. Each external wall, roof, window and door needs an area A, a thermal transmittance U and the temperature difference ΔT across it. Deduct windows and doors from the wall area if their losses are included separately. This prevents counting the same opening twice. U is measured in W/(m²K), A in m², and U × A gives a heat transfer coefficient in W/K.

An educational example: an 80 m² wall with U = 0.18 W/(m²K), with +21 °C indoors and −22 °C outdoors, loses 0.18 × 80 × 43 = 619.2 W, or 0.6192 kW. These temperatures are chosen for the example, not supplied as the design conditions for any Lithuanian location. A wall adjoining an unheated garage must be assessed using the adjoining zone's conditions, not automatically the outdoor temperature.

The sum for external elements is not yet the home's complete heating load. Thermal bridges, air exchange and ground heat transfer need separate treatment. Do not apply the outdoor temperature difference directly to a ground floor: enter its loss from a separate appropriate calculation. This section's calculator helps explain the assumptions; it does not replace a design heat-load calculation. Unknown inputs must remain explicitly identified, rather than becoming zero losses.

Cold air entering a home needs heating even when the walls are well insulated. A simplified sensible-heat expression is Φ = 0.33 × q × ΔT: q is in m³/h, the temperature difference is in K, and the result is in W. If the natural air-change rate n and heated volume V are known, the flow would be q = n × V. However, a test result n50, measured at a pressure of 50 Pa, cannot simply be entered as the normal operating air-change rate n.

For balanced mechanical ventilation, the educational formula becomes Φ = 0.33 × q × (1 − η/100) × ΔT. Here η means a justified effective heat recovery under the actual operating conditions, not an uncritically copied maximum advertising figure. For example, q = 120 m³/h, η = 75 % and ΔT = 43 K give 425.7 W. This covers only heating the incoming air, not the fans' electricity consumption or the unit's entire energy balance.

Additional leakage not already included elsewhere is assessed separately, without reducing it by the heat recovery percentage. Do not count the same flow as both ventilation and infiltration. Actual outcomes depend on flow balance, wind, filters, defrost and controls. Do not reduce the necessary fresh-air supply to save energy: check the system and assumptions first. If a flow is unknown, treat the calculator's partial sum as partial, not as the whole home's heating load.

Equal floor areas do not mean equal heat losses. A corner room has more external surfaces than a room in the middle of the building; an upper storey may face roof losses, while the ground floor is affected by the ground or a cold basement. Window sizes, ceiling height, desired indoor temperature and neighbouring zones also matter. Orientation helps assess sunlight and summer overheating, but incidental solar gains should not become an unjustified promise to cover the heating load in cold conditions.

Dividing the home's total kW by its floor area produces an overall W/m² indicator. It is useful for comparing assumptions, but is not an exact recipe for every room's radiator or underfloor circuit. Each room needs its own loss calculation, ventilation conditions and defined thermal boundary. Heat transfer between rooms at different temperatures matters when sizing their emitters, but internal transfers must not simply be added to the whole-home total as new losses to outdoors.

Check an emitter's output at the intended supply-water, return-water and room temperatures. An old radiator catalogue's kW figure may not apply to a lower-temperature heat-pump system. Floor finishes and permissible surface temperatures also matter for underfloor heating. This section's whole-home calculator does not design individual rooms: reliable distribution needs a room plan, dimensions and a separate designer's assessment, rather than only the final overall sum.

kW describes power under particular conditions; kWh describes energy transferred over time. The load at design cold conditions does not tell you how many such hours occur in a year. Multiplying it by every hour of the year therefore does not produce the actual annual heating demand. An annual assessment needs local weather, indoor temperatures, operating patterns, solar and internal gains. Domestic hot water and the system's energy consumption are also not the same as heat transfer through an external wall.

The educational expression Q = H × HDD × 24 / 1000 gives gross transmission energy in kWh when H is the transfer coefficient for the stated scope in W/K and HDD is heating degree days in K·days. HDD data need a known location, period and explicit base. Do not mix data with different bases: for example, Eurostat uses an 18 °C reference for days with a mean outdoor temperature no higher than 15 °C. This is a statistical method, not this home's controller setting.

This calculator's annual expression covers only the entered external planar elements and known thermal bridges. It excludes ground heat transfer, ventilation, gains, controls and domestic hot water, so it is not the whole home's annual demand or bill. An unknown bridge value or missing HDD does not generate an annual result. Do not turn it into electricity consumption simply by dividing by an advertised SCOP: a consistent assessment of the complete system and its use is needed.

If you plan both insulation improvements and a heating replacement, calculate at least two clearly defined cases: the home now and the home after specific works. Record which walls, roof sections, openings and airtightness details will actually change. A vague promise to insulate later is not a reliable sizing basis. Coordinate the sequence with the building's condition, moisture and ventilation: lower losses must not be achieved by blocking necessary air supply. The inputs for each case should be traceable.

Compare heat pumps using the manufacturer's heating output at the intended outdoor and supply-water temperatures, not just the kW figure in the model name. Also check the minimum modulating output in milder weather: an oversized unit may cycle frequently after renovation. Assess radiators or underfloor heating alongside flows, controls and the selected water temperatures. Backup heating and defrost conditions should be explicit, not concealed within one headline capacity figure.

Assess domestic hot water separately according to use, storage and operating schedule; it cannot simply be added as a continuously operating extra space-heating load. This section's result supports discussion and checking assumptions, not automatic equipment ordering. A room plan, details of the current system and a realistic list of renovation works are useful with an enquiry. Select the final model only after refining the calculation and confirming compatibility with the system.