heating greenhouses - information on greenhouse heater requirements

Heating Greenhouses - Greenhouse Heaters and Requirements

heating greenhouses

How Many BTU Do You Need? Here Is A Tip!
Multiply the width x the length of your greenhouse, this equals the sq/ft. Multiply the sq/ft by 77.
The result is approximately how many BTU you need to operate your greenhouse.

HEATING GREENHOUSES

Many types of heaters and heating systems are satisfactory for greenhouses. You must decide which system best suits your greenhouse operation. Consider the initial cost, economy of operation, and available fuel. You can heat your greenhouse efficiently with coal, electricity, gas, and oil. Heating equipment can be a space heater, a forced air heater, a hot-water or steam system, or electric heaters. Radiant heat lamps over plants and soil cables under plants can also be used.

The capacity of your system will depend on the size of your greenhouse, whether it is covered with a single layer or a double layer of plastic or glass, and the maximum difference between inside and out- side temperatures. Heat systems are rated in British thermal units (BTU) per hour. The firm from which you buy your greenhouse can tell you what size of heater you will need; or you can estimate the size as follows:

•First, find the temperature difference. This is the difference in degrees Fahrenheit between the lowest outside temperature and the temperature you want to maintain inside your greenhouse. For instance, if you want to maintain a minimum inside temperature of 60 degrees and the coldest night temperature you expect is -10 degrees, your temperature difference is 70 degrees

•Next, find the number of square feet of exposed glass or plastic in your greenhouse. Don't forget to add the areas of the sides and ends to the area of the roof.

•Multiply the temperature difference by the number of square feet. For example, suppose you have a 20 x 100 foot greenhouse with a total of 3,400 square feet of exposed plastic. You would multiply 3,400 by 70 (the temperature difference). This would give you 238,000.

•Now, if your greenhouse is covered with 2 layers of plastic or glass, multiply the 238,000 by 0.8. If it is covered with only 1 layer, multiply by 1.2. This will give you the required BTU per hour capacity of your heater.

In the example, a 2-layer greenhouse would be: 238,000 x 0.8 = 190,400 BTU per hour. The 1-layer greenhouse would be: 238,000 x 1.2 = 285,600 BTU per hour.

BTU Quick Grid
Temp Rise	Greenhouse size
Temp Rise	4x8	6x4	6x8	6x12	8x12	8x16
5	682.5	710.5	952	1228.5	1379	1648.5
10	1365	1421	1904	2457	2758	3297
15	2047.5	2131.5	2856	3685.5	4137	4945.5
20	2730	2842	3808	4914	5516	6594
25	3412.5	3552.5	4760	6142.5	6895	8242.5
30	4095	4263	5712	7371	8274	9891
35	4777.5	4973.5	6664	8599.5	9653	11539.5
40	5460	5684	7616	9828	11032	13188
45	6142.5	6394.5	8568	11056.5	12411	14836.5
50	6825	7105	9520	12285	13790	16485
55	7507.5	7815.5	10472	13513.5	15169	18133.5
60	8190	8526	11424	14742	19782
65	8872.5	9236.5	12376	15970.5	17927	21430.5
70	9555	9947	13328	17199	19306	23079

The type of system you choose will depend on how much you want to spend.

The four types are:

Space heaters - For low-cost heating for small greenhouses, use one or more ordinary space heaters.

WARNING: If you use a gas, oil or coal heater, be sure to have a fresh air supply to avoid carbon monoxide buildup due to restricted oxygen supply. Fans are also needed to improve circulation. Use high grade (low sulfur) kerosene to avoid sulfur dioxide damage; high ignition temperature to avoid carbon monoxide and ethylene buildup is important.

Forced-air heater - The best system for heating a small greenhouse is a forced-air furnace with a duct or plastic tube system to distribute heat. You can use a thermostat to control the temperature in the greenhouse.

Hot-water or steam heater - A hot-water system with circulator or a steam system linked with automatic ventilation will give adequate temperature control. In some areas, coal or natural gas is readily available at low cost. This fuel is ideal for hot-water or a central steam system. Steam has an advantage in that it can be used to sterilize growing beds and potting soils.

Electric heaters - Overhead infrared heating equipment combined with soil cable heat provides a localized plant environment, which allows plants to thrive even though the surrounding air is at a lower than normal temperature. Electric resistance-type heaters are used as space heaters or in a forced air system.