After the cost of plants and labour the next item of expenditure in growing chrysanthemums, carnations and pot plants is fuel. Owing to this it is important that the heating system is carefully chosen and that it is able to maintain the required temperatures throughout the glasshouses at crop level. It needs to be flexible to operate, reliable at all times of the year, responsive to thermostat controls. The heating pipes (hot water system) or ducts (hot air system) must be in a position where work on crops is not obstructed and will not hinder light levels.

It is essential to control the heating system in the glasshouse so that the temperature is maintained at the desired level for a particular crop.

Paraffin heaters. These are often used on small scale, in small amateur greenhouses, although larger units are available which are used to heat larger structures. They are also useful not so much for heating greenhouses, but for keeping the frost out; this is called FROST PROTECTION, and is especially useful in the production of bedding plants, where plants have to be grown hard, especially in the late stages, so only a minimum heat output is required. Accurate control of the greenhouse temperature is not possible with this method of heating, but by choosing the correct size heater for a given greenhouse the correct level of heat to keep frost out can be generated.

There is an important problem with heating by paraffin. Since the paraffin is COMBUSTED inside the greenhouse there is the possibility that TOXIC GASES such as sulphur dioxide may be produced due to incomplete combustion of the paraffin. Sulphur dioxide can seriously damage foliage turning it white and even kill plant material. It is important therefore that this problem is eliminated when using paraffin heaters; this can be achieved by following these simple guidelines:

1. Use the correct size of heater for a particular greenhouse.

2. Ensure that correct maintenance is carried out on the heater as and when required.

3. Use only HIGH GRADE PARAFFIN with a low sulphur content. Premium grade paraffin has a sulphur content which is guaranteed not to exceed 0.06%. Typical samples should be between 0.03 to 0.04%.

4. Take care to keep the wick clean.

5. Provide ventilation – but this inevitably lets some of the precious heat out!

6. Have a proper chimney to direct the flue gasses out of the glasshouse air.

(One advantage of using paraffin to heat a greenhouse is that carbon dioxide gas is given off; this helps to increase the amount of C02 in the atmosphere, thus increasing the rate of photosynthesis in daylight hours.)


The temperature of soil or growing media is a major element affecting plant growth particularly in the root zone. This has direct control in three basic areas:

(a) seed germination,

(b) root formation development, and

(c) root function.

Disadvantages – Slow response to controls. Very poor circulation should pump fail. Plus the cost implications of vast quantities of hot water not required when the sun comes out or when the heat is no longer required – ie the desired air temperature reached.

Although still commonly found in domestic glasshouses, the thermal syphon system is now largely being superseded by more modern systems in which very hot water is pumped under pressure and at high speed through small bore pipes (38 – 50mm diameter). This HIGH SPEED HOT WATER SYSTEM is often based on either steam-heated hot water or a steam boiler.

The advantages of this system – It is possible to control the heat output accurately. Quick response to rises and falls in temperature. Cheaper small bore pipes. Good temperature distribution. Much less water has to be heated up, saving on fuel costs.

Disadvantages – Little reserve of heat on failure of boiler or pumps. The steam boiler required for this system is expensive.

Steam. In this system, steam itself passes through the pipes heating them as it goes. Small bore pipes are used – 25mm, 38mm, 50mm. Condensate traps are placed at the end of each pipe run, and the condensed steam is returned to the boiler as water.

Advantages – Easily and accurately controlled. Responds well to quick rise and fall in temperature. Cheaper, easily assembled pipe system. Steam carries much more heat than hot water. It is a cheaper system on the very largest scale e.g. 1 hectare of glass.

Disadvantages – Fierce heat, sometimes causing damage to plants. No reserve heat.

Hot air. Hot air, produced by either electrical or oil heater or hot water/or steam calorifier unit, is blown directly into the glasshouse via a vent in the heater, or blown by fans into perforated polythene ducts.

Advantages – Relatively cheap. Easily adapted. Heat can be accurately controlled, although there may be problems with heat distribution. It may help with the reduction of high levels of relative humidity.

Disadvantages – Drying atmosphere. No reserve of heat. Costly to run.

Advantages of the thermal syphon system – Simple system. Relatively cheap and simple boiler. Gentle heat. Good reserve of heat should a breakdown occur. The commercial growers with very large systems have ways of burning coke and small coals in huge chain-grate boilers, which work best flat out all the time to provide a basic level of heating. The growers then top up the extra heat which may be needed by burning the more flexible fuels like natural gas or oil. These fuels need not be burning all the time, only when circumstances require it, so that an obvious economy is made. So, for example, when the sun is out and the glasshouses are up to the required temperature, the additional heating is not necessary.

Disadvantages – Cumbersome pipes. Boiler often in a pit. Poor heat circulation. No accurate control of heat. More costly and labour-intensive to operate than modern systems.

An advancement on this system is called CIRCULATED HOT WATER. This type of system can be adapted from the thermal syphon system by adding a PUMP.

Advantages of the circulated hot water system – There is some measure of control of heat. Good heat circulation. Easy access to the boiler. Good reserve of heat. Rise and fall of pipes is eliminated; (compared with syphon system).

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