What formula does the greenhouse heating calculator use?

It uses the standard two-component steady-state method. Fabric (conduction) heat loss is Q_fab = U × A × ΔT, where U is the cladding U-value in W/(m²·K), A is total surface area in m² and ΔT is the temperature difference in Kelvin. Infiltration loss is Q_inf = 0.333 × N × V × ΔT, where N is air changes per hour and V is volume in m³ — the constant 0.333 is the volumetric heat capacity of air (1200 J/(m³·K)) divided by 3600 s/hr. Total load is the sum of both components; the recommended heater output multiplies that by a safety factor.

What U-value should I use for my greenhouse?

Single-layer glass or polyethylene film has a U-value around 5.8 W/(m²·K) — the highest (worst) insulation. Double-glazed units drop to around 2.9. Twin-wall 6 mm polycarbonate is typically 3.5 and triple-wall 10 mm about 2.1. Adding a bubble-wrap inner liner to single glass brings it to roughly 2.3. If you have manufacturer data, use that figure instead.

What air change rate (ACH) should I enter?

For a modern, well-sealed greenhouse with good door seals use 0.5 ACH. A typical older glass greenhouse is around 1.0 ACH. A draughty or poorly sealed structure can be 1.5–2.0. ACH has a big impact on total heat load — improving draught-proofing is often the quickest win before buying a bigger heater.

Why is there a safety factor?

The steady-state formula assumes the coldest possible night persists continuously, which it rarely does. Even so, a 1.25 safety factor (25% extra capacity) is standard practice for horticultural heating — it covers unusually cold snaps, temporary heat loss when doors open, and heater output degradation over time. For tropical or tender crops where a few degrees of frost kills the plants, some growers use 1.5.

How do I convert the result to kilowatts for buying a heater?

The calculator already gives kW directly. To convert to BTU/hr multiply kW by 3,412. Gas heaters in the UK are rated in kW; electric fan heaters and tube heaters are also rated in kW. Oil-fired or LPG cabinet heaters are often labelled in BTU/hr — use the BTU/hr figure shown in the results.

Does the calculator include heat loss through the floor?

No. Ground heat loss through a soil or concrete floor is typically small compared with walls, roof and infiltration in a vented greenhouse, and is conventionally omitted in standard horticultural heat-loss calculations. If your greenhouse has an unusually cold concrete slab or underfloor pipes, add roughly 5–10% to the total as a manual allowance.

Greenhouse Heating Calculator

Sizing a greenhouse heater correctly saves money on fuel, prevents plant losses on cold nights, and ensures your heating system can actually keep up on the coldest days of the year. This calculator follows the standard two-component steady-state heat-loss method used in horticultural engineering — the same approach recommended by CIBSE and ASHRAE for glazed structures — and gives you the result in kilowatts and BTU/hr within seconds.

How it works

Heat escapes a greenhouse by two routes: conduction through the glazing and structure (fabric loss), and infiltration — cold outside air leaking in through gaps, door seals and vents.

Fabric heat loss

The conduction formula is:

Q_fab = U × A × ΔT

where U is the overall heat-transfer coefficient of the cladding in W/(m²·K), A is the total surface area of walls and roof in m², and ΔT is the difference between the desired inside temperature and the design outside temperature in Kelvin (or equivalently °C). A single layer of horticultural glass has U ≈ 5.8 W/(m²·K); twin-wall polycarbonate is around 3.5 and triple-wall around 2.1.

Infiltration heat loss

The infiltration formula is:

Q_inf = 0.333 × N × V × ΔT

where N is the number of air changes per hour, V is the internal volume in m³, and the constant 0.333 comes from the volumetric heat capacity of air (1200 J/(m³·K)) divided by 3600 seconds per hour. A typical well-maintained greenhouse has N ≈ 1.0 ACH; a draughty structure can reach 2.0.

Recommended heater size

Total load is Q_total = Q_fab + Q_inf. The recommended heater output adds a safety factor (default 1.25, i.e. 25%) to cover especially cold nights, door openings and heater output degradation.

Worked example

A 6 m × 3 m rectangular greenhouse with 2.0 m eave height and 2.5 m ridge:

Roof area: two panels, each √(1.5² + 0.5²) × 6 ≈ 9.5 m² → 19 m² total
Wall area: two end gables + two side walls ≈ 22 m²
Total surface area: ≈ 41 m² | Volume: ≈ 37 m³
Cladding: single glass, U = 5.8 | ΔT: 15°C inside − (−5°C) outside = 20 K
Fabric loss: 5.8 × 41 × 20 ≈ 4,756 W (4.76 kW)
Infiltration (1 ACH): 0.333 × 1 × 37 × 20 ≈ 246 W (0.25 kW)
Total: ≈ 5.0 kW | With 1.25 safety factor → 6.3 kW heater

Switching to twin-wall 6 mm polycarbonate (U = 3.5) drops fabric loss to 2.87 kW and the recommended heater to about 3.9 kW — a saving of over a third on running costs.

Formula note

The factor 0.333 in the infiltration equation is derived from the volumetric specific heat of air: ρ × c_p ≈ 1.2 kg/m³ × 1005 J/(kg·K) ≈ 1206 J/(m³·K) ÷ 3600 s/hr ≈ 0.335 W·h/(m³·K), which is conventionally rounded to 0.333. This gives infiltration loss directly in watts when volume is in m³, air change rate in h⁻¹ and temperature difference in K.

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