Is this an MCS-compliant heat loss calculation?

No. MCS requires a room-by-room calculation with U-values for every element (walls, roof, windows, floor, doors) and documented ventilation rates. This tool uses typical envelope factors and fabric U-values by age band — accurate enough for a quoting conversation, not a design.

What design temperature difference should I use?

24 °C is the UK default — internal 21 °C and external -3 °C. Scotland and the north use 26-27 °C. Coastal south can go as low as 22 °C. Check the CIBSE design external temperature for the nearest weather station if you want precision.

Why do older properties lose so much more heat?

Solid walls (pre-1919) have an effective U-value of around 2.1 W/m²K versus 0.2 for a modern build — more than 10× the loss through the same wall area. Ventilation is also higher — draughty sash windows, suspended timber floors, and open chimneys mean more air changes per hour.

How does property type affect the result?

Envelope area matters. A detached house has four exposed walls, a roof, and a floor — about 2.5× the floor area in exposed surface. A mid-floor flat has two exposed walls and no roof or floor exposure — 1.2× floor area. Same floor space, half the heat loss.

Why is the heat pump kW lower than the boiler kW?

Boilers are intentionally oversized by 10-15% so they can recover a cold house quickly. Heat pumps are sized tight to the calculated heat loss — oversizing causes short-cycling, which wrecks efficiency and lifespan. A properly designed heat pump runs close to flat out on the coldest day and modulates down the rest of the year.

Does this account for hot water demand?

No. This is the heating load only. For a combi boiler, DHW demand often drives the selection (24 kW for one bathroom, 30+ for two). For a heat pump, hot water is a separate cylinder with its own calculation based on occupancy.

How accurate is this compared to a full heat loss?

Expect ±20-25% on a typical property. The biggest sources of error are unusual floor-to-wall ratios (long thin extensions, double-height spaces), windows well above average size, and rooms with significantly different insulation levels. For those properties, commission a proper survey.

What about ventilation in newer airtight homes?

Post-2011 builds often have MVHR (mechanical ventilation with heat recovery), which recovers 80-90% of the heat in extract air. The 0.4 ACH figure used here assumes MVHR or similar — if a property has no controlled ventilation and an extractor fan on high, ventilation losses will be significantly higher.

Free calculator for engineers

MCS Heat Loss Calculator (simplified)

Whole-house heat loss in under a minute. A simplified model for quoting conversations — enter the property, read off the kW, then commission a proper heat loss before you fit anything.

Heated floor area (m²)

Ceiling height (m)

Property type

Age band

Insulation quality

Design temperature difference (°C)

UK default is 24 °C (21 °C indoor, -3 °C outdoor). Scotland and the north use 26-27.

Whole-house heat loss

6.42kW

Fabric losses: 5.28 kW

Ventilation losses: 1.14 kW

Heated volume: 240 m³

Boiler

8 kW

+15% margin

Heat pump

7 kW

Tight sizing

Simplified whole-house estimate for quoting conversations. For commissioning, do a room-by-room MCS calculation.

How it works

1
Enter property basics
Floor area, ceiling height, property type and age band.
2
Pick insulation level
From "no upgrades" to "full retrofit" — this scales the fabric U-values.
3
Read off kW
Whole-house heat loss in kW, split between fabric and ventilation, plus boiler and heat pump sizing.

What this model is doing

Heat loss breaks into two streams: fabric (conduction through walls, roof, floor, windows) and ventilation (warm air being replaced by cold). This calculator uses typical envelope areas by property type, U-values by age band, and air change rates that scale with how airtight the building is.

Fabric loss is envelope area × U-value × Δt. A 100 m² semi has about 200 m² of exposed envelope. A 1960s semi with partial insulation has an effective U-value around 1.5-1.8 W/m²K. At Δt 24, that's roughly 8 kW through the fabric alone.

Ventilation loss is air changes per hour × volume × 0.33 × Δt. The 0.33 is the specific heat capacity of air in Wh per m³ per kelvin — the amount of energy needed to warm a cubic metre of air by one degree. Multiply by volume, ACH, and design Δt and you've got your vent load in watts.

When to trust this and when not to

Trust it for: quoting conversations with homeowners, sanity-checking a quick boiler replacement, pre-survey heat pump screening, comparing two retrofit scenarios.

Don't trust it for: MCS certification, heat pump design for a property with unusual geometry, commercial calculations, or anywhere the number needs to be within 10%. For those jobs, do a full room-by-room calculation with measured U-values.

Biggest sources of error

Window area. The envelope factor assumes around 15% of floor area as glazing. A glass-fronted extension or a conservatory blows that up and the calculation will underestimate.

Hybrid construction. A 1950s semi with a 2010s extension has two different U-values side by side. Averaging them loses information — the extension might be fine but the original building needs radiators doubling in size.

Party wall assumptions. The model treats party walls as non-losing. In practice a terrace with a vacant neighbouring house loses heat through that wall at close to external U-values. Worth flagging on properties you know are empty next door.

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Frequently asked questions

Heat pump suitability →

Quick screening before you commission a full heat loss.

Radiator BTU calculator →

Room-level sizing once you know the whole-house load.