Building & Method Settings
Typical UK winter design around -1 °C to -3 °C.
Adds headroom for warm-up and uncertainties.
Infiltration at design: 0.4–1.0 typical; older homes higher.
If no MVHR, leave at 0.
Common default where details are reasonably good.
For display in notes only.
Rooms & Envelope Inputs
| Room | Length (m) | Width (m) | Height (m) | Setpoint (°C) | Wall area (m²) | Wall U (W/m²·K) | Window area (m²) | Window U | Door area (m²) | Door U | Roof/Ceiling area (m²) | Roof U | Floor area (m²) | Floor U | Actions |
|---|
Tip: If room shares elements internally, reduce corresponding exposed areas.
Results
0.00 kW
Total design heat loss (with margin)
0 W/K
Fabric + thermal bridge heat loss coefficient
0 W/K
Ventilation / infiltration heat loss coefficient
ΔT = 0 K
Overall design temperature difference
| Room | Area (m²) | Volume (m³) | ΔT (K) | H_fabric (W/K) | H_tb (W/K) | H_vent (W/K) | Design loss (W) | With margin (W) |
|---|---|---|---|---|---|---|---|---|
| Totals | 0 | 0 | — | 0 | 0 | 0 | 0 | 0 |
Note: Use ΔT50 radiator charts or manufacturer ratings to select emitters.
Example Data
Click “Add Room” twice and copy the example figures.
| Room | L | W | H | Setpoint | Wall A | Wall U | Win A | Win U | Door A | Door U | Roof A | Roof U | Floor A | Floor U |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Lounge | 4.8 | 3.8 | 2.4 | 21 | 32 | 0.30 | 6 | 1.40 | 1.9 | 1.80 | 18.2 | 0.18 | 18.2 | 0.22 |
| Bedroom | 3.6 | 3.2 | 2.4 | 19 | 24 | 0.30 | 3 | 1.40 | 0 | 1.80 | 11.5 | 0.18 | 11.5 | 0.22 |
Formula Used
Fabric loss (per room): H_fabric = Σ(U_i × A_i)
Thermal bridge (approx.): H_tb = y × A_exposed where A_exposed = A_walls + A_windows + A_doors + A_roof + A_floor
Ventilation/infiltration: H_vent = 0.33 × n_eff × V, where n_eff = ACH × (1 − η_MVHR)
Design heat loss: Q = (H_fabric + H_tb + H_vent) × ΔT, with ΔT = T_set − T_out
With margin: Q_margin = Q × (1 + margin%)
0.33 ≈ ρ_air × c_p / 3600 with ρ≈1.2 kg/m³, c_p≈1000 J/(kg·K).
How to Use
- Set the external design temperature appropriate for your UK locality.
- Enter ACH and any MVHR efficiency, else leave defaults.
- Click “Add Room”, fill geometry, areas, and U-values for each surface.
- Use setpoint temperatures per room; living spaces often 20–21 °C.
- Press “Calculate” to see per-room and total heat losses.
- Adjust the safety margin to match commissioning intentions.
- Export results to CSV or PDF for records or quotations.
FAQs
Recent UK regs target about 0.18–0.25 for roofs, 0.22 floors, 0.26–0.30 walls, 1.2–1.6 modern windows. Use your as-built specification if known.
-1 °C is typical nationally; colder northern/continental exposures may use -3 °C or below. Local guidance or CIBSE design data may refine this value.
Party walls or internal partitions typically do not contribute to external heat loss. Exclude their areas or assign negligible U-values when shared with similarly heated spaces.
This tool uses a y-value approximation. Detailed assessments use ψ-values per junction and lengths. If you have them, sum ψ×L and add to H_fabric instead.
Yes. Effective ACH is scaled by (1 − efficiency). For example, 80% recovery at 0.6 ACH behaves like 0.12 ACH for heat loss purposes, assuming balanced flows.
10–20% is common for radiator selection. Larger margins increase responsiveness but may reduce condensing efficiency for modulating systems if emitters are oversized.