What this calculator does

It sums up each segment’s envelope heat loss (walls, roof, windows, doors) and air-exchange loss (infiltration + ventilation), then subtracts any internal heat credits (people, lights, equipment sensible). You get total heating capacity in kW and kBTU/h, plus a warm-air airflow estimate from your chosen supply temperature rise. Optional costing converts capacity to budget and energy bills for India, US, or UK.

Standards presets (ASHRAE / ISHRAE / CIBSE) “Values derived from <Standard>, clause <xx> (user must verify)”

How it works (plain English)

Show formulas & symbols

Envelope: Q_{env} = (U_w A_w + U_r A_r + U_g A_g + U_d A_d) · (T_{in}-T_{out})

Air exchange: Q_{air} = ρ c_p (V̇_{inf} + V̇_{vent}) · (T_{in}-T_{out}), where V̇_{inf} = ACH · Volume / 3600 and V̇_{vent} = L/s_{pp} · N / 1000.

Credits: Q_{cred} = N·q_{S,pp} + LPD·A + (Equip_W · SF).

How to use the calculator

1. Choose the unit system (SI or Imperial).
2. Select a standard (ASHRAE / ISHRAE / CIBSE) to prefill typical U-values and set-points. A note appears: “Values derived from <Standard>, clause <xx> (user must verify).”
3. Add segments (one per room/zone), give each a name.
4. Enter geometry & envelope (areas, U-values), design ΔT, people, lighting, equipment, infiltration, and ventilation.
5. Optional costing: choose country (India / US / UK), set equipment cost per kW, tariffs, markup, and usage hours.
6. Press Calculate. Outputs update below (KPIs, table, and cost summary).
7. Use Reverse mode to size warm-air flow or radiator length from a target kW.
8. Click Generate Report for a printable HTML report (KPIs, tables, assumptions, metadata).

Worked example — Two-room apartment (SI units)

Standard: ASHRAE (indicative) • Indoor 21 °C • Outdoor 5 °C → ΔT = 16 K • Supply air 45 °C (ΔT_air=24 K). ρ=1.2 kg/m³, c_p=1005 J/kg·K.

Segment A — Living room

• Geometry: Area 24 m², Height 3.0 m → Volume 72 m³
• Envelope areas: Wall 32 m², Roof 24 m², Window 5 m², Door 2 m²
• U-values (W/m²·K): U_wall=0.45, U_roof=0.25, U_win=2.6, U_door=2.0
• People & internals: 3 persons @ 70 W sensible each; LPD 6 W/m²; Equipment 500 W (SF 0.8)
• Air: Infiltration 0.6 ACH; Ventilation 6 L/s·person

Item	Calculation	Load
Envelope	(0.45·32 + 0.25·24 + 2.6·5 + 2.0·2)·16	0.5984 kW
Air exchange	1.2·1005·(0.6·72/3600 + 6·3/1000)·16	0.5789 kW
Credits (subtract)	3·70 + 6·24 + 500·0.8	0.7540 kW
Total	0.5984 + 0.5789 − 0.7540	0.4233 kW
Airflow (warm-air)	Q/(ρ cp ΔTair)	0.01462 m³/s (≈ 31 CFM)

Segment B — Bedroom

• Geometry: Area 14 m², Height 3.0 m → Volume 42 m³
• Envelope areas: Wall 20 m², Roof 14 m², Window 2 m², Door 1.8 m²
• People & internals: 2 persons @ 60 W sensible; LPD 4 W/m²; Equipment 3 W/m² (SF 0.8)
• Air: Infiltration 0.5 ACH; Ventilation 5 L/s·person

Item	Calculation	Load
Envelope	(0.45·20 + 0.25·14 + 2.6·2 + 2.0·1.8)·16	0.3408 kW
Air exchange	1.2·1005·(0.5·42/3600 + 5·2/1000)·16	0.3055 kW
Credits (subtract)	2·60 + 4·14 + (3·14)·0.8	0.2096 kW
Total	0.3408 + 0.3055 − 0.2096	0.4367 kW
Airflow (warm-air)	Q/(ρ cp ΔTair)	0.01509 m³/s (≈ 32 CFM)

Grand totals

Costing (same load, different countries)

Assume electric heat pump with COP = 3.0; usage 6 h/day, 30 days/month. Equipment markup +10%.

Country	Equipment rate	Equip. cost	Tariff	Input (kW)	Energy ₹/$/£ /hr	₹/$/£ /month
India	₹7,000/kW	₹6,622 (0.86×7000×1.10)	₹7.00/kWh	0.287	₹2.01	₹361
US	$200/kW	$189.20	$0.1747/kWh	0.287	$0.050	$9.01
UK	£350/kW	£331.10	£0.2573/kWh	0.287	£0.074	£13.28

Gas/LPG alternative (for comparison)

With a boiler/furnace at η = 0.90, fuel input = 0.86/0.90 = 0.956 kW (HHV). Using editable placeholders: India ₹5/kWh-eq → ₹4.78/hr (≈ ₹860/month at 180 h). US $0.06/kWh-eq → $0.057/hr (≈ $10.32/month). UK £0.12/kWh-eq → £0.115/hr (≈ £20.65/month). Actual tariffs vary—update in the calculator.

Pro tips for reliable heating numbers

• Check ΔT carefully. Big changes here swing results most. Use your climate’s winter design point.
• U-values matter. If you know construction (cavity wall, insulated roof, double glazing), set realistic U’s.
• Ventilation drives losses. ACH and L/s-per-person add up; reduce infiltration via sealing where practical.
• Credits are optional. For conservative sizing, switch them off (no subtraction).
• Keep a paper trail. Use the built-in Report to capture assumptions for sign-off and future tweaks.

Related reading

Methodology & standards note

The calculator uses steady-state envelope conduction and sensible air-exchange losses. Internal sensible gains can be credited. Warm-air airflow is derived from the supply temperature rise. When you select a standard (ASHRAE / ISHRAE / CIBSE), the widget populates indicative U-values, set-points, and starter ventilation and displays: “Values derived from <Standard>, clause <xx> (user must verify).”

Make readers stay longer

• Add a 20-second GIF: add two rooms → tweak ΔT → generate report.
• Insert a mini quiz: “Which input increases heating load most—U-values, ACH, or ΔT?” (Reveal with <details>.)
• Show a before/after: zero credits vs credited internals—let the KPI cards animate.
• Offer a downloadable report from the calculator for sharing with clients.