01
Formula
BTU/h = exposed area × U × temperature difference.
Exposed area = 2 × (length + width) × height + length × width. The floor is not counted as exposed.
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Greenhouse heating calculator for heat loss
This greenhouse heating calculator estimates steady heat loss through the walls and a flat roof. Heat loss is exposed area times a U-factor times how many degrees warmer you hold the house than the outside. It is not an infiltration or a solar-gain model.
Heat loss
45,408BTU/h
Exposed area1,032 sq ft
U-factor1.10
RuleArea × U × temperature difference
Planning estimate from the numbers you enter. Coverage, density, waste and price are assumptions, not a supplier quote or an engineering specification.
How it works
BTU per hour equals area times U times the temperature difference.
Exposed area is the four walls plus the roof footprint: 2 times (length plus width) times height, plus length times width. A 30 by 12 foot house with 8 foot walls has 1,032 square feet of that surface. At a U-factor of 1.1 and a 40 degree difference, heat loss is 45,408 BTU per hour. A single layer of poly is often planned near a U of 1.1. A double layer is lower. Type the U you were given. Wind, cracks, and the heater's own efficiency are extra. This greenhouse heating calculator sizes the loss, not the fuel bill.
02
Example
30 by 12 feet, 8 foot walls, U of 1.1, 40 degrees.
The surface is 1,032 square feet. The greenhouse heating calculator shows 45,408 BTU per hour.
03
Tips
A peaked roof is larger than the footprint.
If the roof is much larger than length times width, raise the area by typing a larger length or width, or accept this as a low estimate. Infiltration on a windy night can matter as much as the U-factor.
04
Equipment
The heater has to cover this loss.
Dengaro arranges financing for a heater. BTU per hour is not a loan amount.
Important questions
Know what to expect.
What U-factor should I use?+
The one for the skin you have. 1.1 is a single-poly planning default, not a measured value.
Does this include the sun?+
No. Daytime solar gain is not subtracted. The number is the conduction loss for the temperature difference you hold.
Is the floor heated?+
The floor is not in the exposed area. A heated floor or a bench is a separate load.
How do I turn BTU into fuel?+
Divide by the fuel's BTU content and by the heater efficiency. This page stops at BTU per hour.
Financing built into the sale