Fast Heat Loss — Guide

This tool gives a quick whole-house heat loss estimate — enough to gauge the size of heat generator a property needs, in a couple of minutes on your phone. This guide explains how it works, how to read the numbers, and how to spot a building's construction.

Using the tool

Enter five things: postcode (sets your design temperatures), building type, storeys, floor area, and age. The moment you pick an age, the tool fills the wall, floor, loft and glazing with the construction typical for that era — shown in orange. Change anything you know is different.

If you change a wall or floor to something unusual for the age, a note appears under that section:

• Amber ⚠ note — unusual for the era, likely a mistake (e.g. a solid wall on a 2010 house). Check before continuing.
• Grey ℹ note — a genuine judgement call where practice was mixed (e.g. cavity fill in the early 80s, or floor insulation in the late 90s). Default to the safer choice unless you have evidence — a borescope, drill-hole patches, the EPC or the plans.

The two results

The tool shows two figures:

Heat loss — heat generator (the main number)

The peak heat the building loses on the coldest design day for your area (inside 21°C, outside your local design temperature). This is what you size the heat generator against — boiler, heat pump or hybrid.

Typical load

The heat loss on an average day (inside 21°C, outside the yearly-average "reference" temperature — far milder than the design day). The generator must be able to modulate down to roughly this in heating mode, or it short-cycles: firing, overshooting and switching off again and again. This matters for combi boilers and heat pumps alike.

How the HTC method works

The tool uses the Heat Transfer Coefficient (HTC) — the watts a building loses for every 1°C difference between inside and outside. Heat loss is then simply:

The HTC itself is built up from:

• Fabric loss — each element's U-value × its area (walls, roof, glazing, floor). The U-values come from RdSAP 10.
• Thermal bridging — a standard allowance for heat escaping through junctions.
• Ventilation — heat carried out by air changes, scaled by the building's age (older = draughtier).

Tap the HTC line in the tool to see the full breakdown — each element's U-value and the watts it accounts for.

Houses with extensions

An extension is often built to a different (usually higher) standard than the original house — a 1930s house with a 2015 extension is really two fabric ages in one building. This whole-house tool works from a single age and fabric, so handle an extension one of two ways:

• Small extension — if it's a modest proportion of the house, enter the whole property at the original age. The result reads slightly high (the extension is better than the original fabric), which errs on the safe side for sizing.
• Substantial extension — run it as its own quick estimate at its own age and fabric, then add the two figures together. Enter the original house (its floor area and age) and the extension (its floor area and age) separately, and sum the heat losses.

Either way, a full room-by-room calculation handles mixed-age fabric properly, room by room — another reason it's the right tool for designing a new system rather than just gauging generator size.

Reading a building's walls

The biggest single input is the wall type. You can often read it straight off the brickwork:

Common brick bonds. A cavity wall (stretcher bond) shows only long brick faces — the cross-sections show an open cavity and a filled one side by side. Solid walls (Flemish or English bond) show a regular pattern of short brick ends (headers), because the bricks tie the wall's full thickness together.

• All long faces (stretcher bond) → cavity wall. The two leaves aren't brick-tied, so no headers show.
• A pattern with short ends showing (headers, in Flemish or English bond) → solid wall.
• Wall thickness at a door or window reveal — solid ≈ 230 mm; cavity ≈ 250 mm or more.
• Age — pre-1920s is almost always solid; cavity became common through the 1920s–30s.

Evidence of retrofit blown cavity insulation: the regular grid of filled drill-holes in the mortar is what you'll actually see on site once the work is done.

Has the cavity been filled?

• Look for drill-hole patches in the mortar — a regular grid of filled holes is clear evidence of retrofit cavity fill.
• Check the EPC, building records, or a CIGA guarantee (issued for cavity fill).
• Ask the client — often the quickest route.
• A borescope through a drilled hole confirms it if you're equipped.

Around 20 million UK homes built since 1920 have cavity walls; roughly two-thirds are now insulated, so filling is common — but far from universal. If you're not sure, choose unfilled: it's the safer assumption (higher heat loss) and errs toward not undersizing.

Method & sources

Fabric U-values come from RdSAP 10 — the standard age-and-construction defaults, so no intrusive survey is needed. Design and reference temperatures come from CIBSE 2025 Weather Data (Table 2-3), matched to your postcode across 28 UK zones. The default is the 99th percentile, which CIBSE recommends for most UK homes; the 99.6th percentile is reserved for exposed locations — though CIBSE notes the final choice is a judgement to agree between designer, client and occupier at the design stage. A toggle beneath the postcode field switches between the two. Each figure is drawn from its own authoritative source, as is standard for heat loss work.

Under Approved Document L (2026), a whole-dwelling calculation — this tool — is the appropriate method when only the heat generator is being replaced. The replacement must be sized from the calculated heat loss, not from the output of the old appliance. Fitting a new system that includes emitters needs a room-by-room calculation instead, because each room's loss sizes its own radiator or underfloor circuit.

This tool gives a whole-house estimate for heat generator replacement. Use a full room-by-room heat loss calculation when emitters are being sized.

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