Greenhouse Heating Worksheet
Two numbers make or break this: your glazing's U-value and your design low. Get both from a real source.
BFCBrilliance · bfcbrilliance.com/tools/greenhouse-heating-calculator
The arithmetic
- Heat loss
- area x U-value x temperature gap
- Surface area
- 4 walls + roof (with roof allowance)
- Loss per degree
- area x U-value
- Heater size
- loss x (1 + margin)
- 1 kW
- 3412.14 BTU/hr
- Halve the gap
- halve the heat needed
- ⚠ U-value
- from YOUR glazing manufacturer
- ⚠ Design low
- from your met service, not a record
- ⚠ Not included
- leakage, wind, ground loss
This greenhouse
- Greenhouse
- Length
- Width
- Height to eaves
- Roof allowance
- SURFACE AREA
- Glazing type
- U-value
- Source of U-value
- Inside target
- Design low
- TEMPERATURE GAP
- GLAZING LOSS
- Margin
- HEATER SIZE
- kW
What a lower target would save
| Inside target | Gap | Loss BTU/hr | kW | Worth it? |
|---|---|---|---|---|
Before buying a heater
- U-value taken from the glazing manufacturer's data
- Design low from a met service, not a record low or an average
- Roof allowance set for a pitched or hooped roof
- Margin allowed for leakage, wind and ground loss
- Door and vent gaps sealed before sizing - cheaper than capacity
- Thermal screen considered and modelled through the U-value
- Lower inside target considered - the cheapest saving available
- Rounded UP to the next available heater size
- Electrical circuit checked against the kW figure
- Ventilation planned separately - overheating is a real risk
Seal it before you size it
Air leakage through door gaps, vents, glazing laps and the base joint can be a large share of total loss on an older structure, and it varies far more between greenhouses than glazing does. Sealing is nearly always cheaper than the heating capacity it saves, and it should be done before the heater is chosen rather than after.
A lower target is the cheapest saving
Loss is proportional to the temperature difference. Holding 45°F instead of 50°F against a 20°F night takes the gap from 30 to 25 - a sixth off the requirement, for no capital cost. Ask what the plants actually need before assuming the round number.