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| By Alfred Brusius

Heating your greenhouse: what to be aware of

Tomatoes are grown in almost 90% of all greenhouses. Most of the time it is done without particular greenhouse heating mechanisms. Exotic plants such as orchids or cacti on the other hand, require heating.

But beware: even and especially with tomatoes, greenhouse heating is worth it! Read on to find out why this is so and how to determine the optimal heating requirements.

It is well known that a greenhouse must be well-insulated. For this, double-walled sheets (at least 16mm) of insulating glass and or bubble wrap are often recommended.

Exotic plants always require constant heating. This means that far more energy is required in January than in July since the heating potential of a heat source always remains constant.

On the other hand, the heat required to grow tomatoes (without breeding them) is only necessary for a few months. This is because tomatoes are seldom found in cold winters anyway. Furthermore, no heating is required in June, July and August. This would only be required during a particularly lousy summer with heavy rain and strong cold fronts. April, May, September and October have approximately the same amount of energy use pro-rata. They each account for approx. 20% of the annual requirement of a heating system.

This way, crops have the possibility of being harvested from the beginning of the harvest period in early July, until the end of October. This also brings the additional advantage of preventing certain fungal diseases.

How can you determine the heating demand?

Gewächshaus Heizungsbedarf

To determine the requirements for greenhouse heating, one must calculate the surface of the greenhouse, the type of roofing, be aware of the heat transfer coefficient and identify the desired temperature difference between the outside and inside temperatures. 

The surface is calculated from the sum of all external surfaces of the greenhouse. If there is a pedestal, the surface is only half calculated, as the insulation is better than that of the roofing.

The heat transfer coefficient (= U value, previously k value) is determined by the roofing. For normal glass (3mm), it is equal to a U-value of 6, with double-walled panels 6mm, a U-value of 3.5, with double-walled panels 16mm U = 2.7. For the temperature difference (ΔT) it is possible to assume the lowest value in October - 3.5 ° C and if you want to keep a minimum temperature of 10 ° C in the greenhouse (for the tomatoes) this results in a difference of 13,5 ° C.

An example calculation: If the total external area of the greenhouse is 20m2, the house is equipped with 6mm hollow panels (k-value 3.5) and the desired temperature difference is 13.5 ° C, then the heat demand is calculated: (A x U x ΔT ) = 20 x 3.5 x 13.5 = 945 kcal / h.

Most of the power of the heaters is specified in watts. 1 watt = 0.86 kcal / h. So 945 X 0.86 = 812.7W. So you would need at least 800 watts of power in this example.

Electric greenhouse heating

Elektro Gewächshausheizung

Probably the simplest type of heating is with an electric fan heater. Special greenhouse heaters are made of stainless steel and are above all splash-proof. Each heater must be adjustable, usually in conjunction with a likewise-protected thermostat.

Whether or not electric greenhouse heating is more expensive compared depends on your requirements. But you should not forget the convenience of operation. For example, you can also use the heaters as fans - air movement is important to prevent many fungal and bacterial diseases from taking hold.

Bio Green's electric greenhouse heaters have proven themselves thousands of times over.


Gas greenhouse heating

Gewächshausheizung Gas

Modern greenhouse heaters are safe, thermostatically-regulated and use the preset energy requirements up to 99%. During a combustion cycle, oxygen is consumed and CO2 is released. The CO2 concentration in the greenhouse air promotes plant growth, but flowering plants can suffer damage. An exhaust device can prevent the increased CO2 concentration. This is usually the case with greenhouses that use gas for their heating.

In the world of Bio Green you will find a careful selection of gas greenhouse heaters: A gas heater can have a zero shutdown mechanism (with automatic reclosure) for energy saving and an oxygen deficiency backup. Such greenhouse heaters should be made of stainless steel. Unlike gas appliances for residential areas, gas greenhouse heaters are adjustable from 0 ° C. The combination of the catalyst burner and a temperature controller with zero shutdown means an energy saving of about 25%. The only drawback is that you have to check on them more often, to see if the gas bottle still has enough left


Floor greenhouse heating

Bodenheizkabel Gewächshaus

A "warm foot" promotes growth says the gardener. A warm foot is like a warm root for your plants. Young plants and cuttings appreciate this especially.

Good floor heating can be achieved with heating mats, plates, cables or warm water. It is recommended to operate the floor heating in conjunction with another heating source. Suitable greenhouse floor heating cables can be found in our online store.

Greenhouse heating with petroleum or paraffin

Paraffin Gewächshausheizung

Paraffin heaters are especially suitable for small greenhouses. They also require maintenance, but over much longer periods of time. They are inexpensive to use as frost guards in spring and autumn. Since they are cheap, you can use several of them at once. With the appropriate brand of petroleum for paraffin heaters, you can reduce any odours in your greenhouse, since it burns residue-free and without soot formation. Common petroleum burners have a tank capacity between 3 and 5 litres. That's enough for a week's performance, depending on the type, between 300 and 600 watts.


Hot water greenhouse heating

If the greenhouse is connected to the same source of heating as your house or residence, a hot water heater with heating pipes and a large pipe cross-section may be what you need to ensure good heat distribution. Of course, a licensed heating contractor is required to install this.

Hot water heaters either pump heated water through hoses or the water is heated by the sun in hoses with a larger diameter. They are then arranged throughout the greenhouse to generate warmth. This can be known as vegetation heating if you are laying the hoses around the roots of your plants.. It is even possible to arrange for the water to be heating in conjunction with a solar energy device. However, with vegetation heating, the air in the room either becomes only slightly heated or not at all.

Oil and coal stoves

Oil and coal stoves which can also be used to heat your greenhouse have now become rare. They are now sometimes limited by local or state government laws which forbid their use in certain places. They must always be set up so that the heat is distributed as evenly as possible in the greenhouse.

Additionally, insulating the greenhouse provides even better thermal protection for your plants. Use bubble wrap on the inside and Styrofoam or Styrodur panels on the foundation. Insulation foils usually have special brackets for quick attachment and removal. When installing greenhouse insulation always make sure that the plants still have enough light available.

The original version of this article was published in the greenhouse post, issue 10/2015, text and image: Jörn Pinske.


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