Greenhouse Heating – Hot Water Systems

Greenhouse Hot Water Systems

Large bore pipes

These were once normal installation for all greenhouses, but are not now considered economical for large units. Pipes are usually of 10cm (4in) diameter and the water circulated by gravity, provided there is a gradual rise from the boiler to the highest point in the unit and a gradual fallback to the boiler. There are design problems here, in avoiding obstruction to doors and in siting the boiler or heat source at a sufficiently low point—often below ground-level unless the heating pipes in the greenhouse are fairly high.

It is a slow and costly operation to heat up large volumes of water, but once hot it retains and radiates its heat for a considerable period. This, however, in combination with solar radiation often causes the temperature of the air in the greenhouse to rise excessively, which is not only wasteful but may be injurious to the plants, although it can help to encourage air currents through a crop. Once solar radiation ceases there may be a quick demand once again for heat, and the whole process of re-heating the now cool large volume of water has to be begun again.


Advantages of small bore pipes

Smaller bore pipe systems of 2.5-5cm (1-2in) diameter are invariably installed now in greenhouses: there is much less water to heat, and thus the total bulk cools more rapidly. A further advantage of small bore pipes is that as they take up less room they can be readily taken to the crop and spread over the cropping area or under benches, between plants, and placed on the soil — especially important for plants such as early tomatoes which must be planted in warm soil.


Accelerated hot water systems

The main disadvantage of small bore pipe systems is that as the diameter of the pipe decreases the friction increases, and it may be found that the hot water does not circulate rapidly enough or may not be capable of completing its circuit back to the heat source again. This can be overcome by the use of an accelerator pump — a matter for consultation with a heating engineer. Existing gravity systems can, in many cases, be greatly improved by the fitting of such a pump, although this will seldom be necessary on smaller installations.


High speed hot water system

Accelerator pumps merely assist the circulation of the water but still allow some natural circulation; high speed hot water systems, usually of small bore design, depend entirely on pumps for the circulation of the water, allowing no natural circulation. They include alkathene soil warming pipes underground, and ‘mini-bore’ bench warming systems. Problems do exist with high speed water systems unless an entirely automatic system of firing is involved. The temperature of the greenhouse is usually controlled by a thermostat which operates the pumps and if a non-automatic solid fuel boiler is being used and the pump is shut off by the thermostat (due to a rise above the desirable temperature in the greenhouse) there can be serious overheating of the boiler. With automatically fired boilers, especially with oil or gas where there is no great residual heat involved, this situation does not arise. The problem can be overcome by using a non-return valve on a bypass which opens when the pump is non-operative.

In high speed hot water systems, and to a limited degree with accelerated systems also, it is important to ensure that the pump is the correct size for the system. An inadequate size of pump would fail to overcome the frictional resistance of the pipes, but too powerful a pump would not only circulate the water, but could force it out of the system through the header tank. The design of pumps and accelerators and their selection for a specific task is a highly technical matter best taken up with a specialist supplier who will specify a particular size for the heating system involved after taking into account the length of pipe, its diameter, the calorific rating of the system, the acceptable temperature drop between flow and return pipes, and the design of the whole system.

Formulae exist which allow the ready calculation of pump sizes which are rated on what is called their ‘circulating head’ and it should be repeated that a pump with a larger circulation head than is necessary may give rise to problems. There are several ways of overcoming this, such as fitting it on the flow pipe instead of, as is normal, on the return, or ensuring that the header tank is on the suction side of the pump, or alternatively raising the height of the header tank. Pumps with variable circulating heads which allow some adjustment are available and may be the answer for smaller domestic greenhouse systems.


Types of piping

Cast iron piping is now seldom used for greenhouse heating, although there are still some older systems in existence. Steel pipes are invariably used and this has the distinct advantage of allowing screwed couplings or alternatively welding to be readily carried out — not possible with cast iron pipes. Pipes rust readily and while it was stated for a while that aluminium based paint should not be used as this reduced heat output, research has shown the reduction in heat transmission to be insignificant. Aluminium paint is therefore advised to reduce external rusting and general deterioration. Bitumastic paint should not be used as this gives off fumes when the pipes are warm. Newer types of vegetable oil paint have also been developed.


Transmission of heat from different pipe sizes

The transmission of heat from pipes varies according to their diameter, the temperature of the water in them, and the external temperature of the air surrounding them.

Heat output: Approximate heat output from different pipe parameters

pipe temperature


The above chart shows the respective output of pipes of different diameters at an air temperature of 13°C (55°F), the variation of output for different air temperatures being shown in the accompanying table. To use this chart read upwards from the base to the line indicating pipe size, then move left to ascertain the output per 30cm (12in) of pipe. As the temperature of the water in a heating system varies, as indeed does the air temperature, it is necessary to accept a workable figure for design purposes.

It can be seen, however, from the figure that the output from pipes can be considerably greater with a higher water temperature, especially when the water temperature rises above boiling point . With smaller greenhouse systems one must be realistic and, considering the difference between the flow and return temperatures, the following figures are reasonably workable. Pipe emission at 37°C (100°F) temperature difference to nearest round figures per 30 cm (12in).


2.5cm (1in)

23 w/hour/°C

(80 Btu)

3.1cm (1-1/4in)

29 w/hour/°C

(100 Btu)

3.8cm (1-1/2in)

32 w/hour/°C

(110 Btu)

5cm (2in)

38 w/hour/°C

(130 Btu)

10cm (4in)

67 w/hour/°C

(230 Btu)


Gilled pipes are available and these permit a much greater heat emission according to the design (up to four or five times as much as plain pipes).


Designing a hot water heating system

To design a pipe heating system the first stage is to determine the heat loss of the greenhouse and thereafter to calculate the length of pipe needed to supply the necessary heat in a form which will conveniently fit in with the type of cropping being practised. To return to the 2.4 x 1.8m (8 x 6ft) greenhouse, and assuming a 22°C (40°F) lift, we are concerned with the supply of some 3810 watts (13,000 Btu). If a 10cm (4in) system was selected the normal arrangement of pipes will be a double run of pipe along each side and across the end, which comes to 13m (44ft), some 3.3m (12ft) short of the calculated figure. Assuming slightly hotter pipes in view of the nearness of the boiler, it is fair to assume that the required input of heat could be supplied, bearing in mind that this allows for 16°C (60°F) in the greenhouse at —7°C (20°F) outside temperature.

If, however, it is decided to use 3cm (1-1/4in) pipes, the required 39m (130ft) will necessitate either spreading or banking on the sides requiring six lines of pipes along each side and across the end. There is virtue, therefore, for greenhouses of this small size, in using larger bore pipe, especially as to do this would also facilitate circulation without the fairly costly installation of a pump. As a compromise 28m (approx 90ft) of 5cm (2in) pipe would be required — four lines on the sides and at the end.

It will be noted that no provision has been made in this instance for heating in the region of the door and it would be difficult to allow for this where circulation depends entirely on gravity. Where a pump and small bore pipes are being used, these could conveniently be taken either below or above the door. Pipes can all be installed at high level and this has virtues for flower crops. Where larger greenhouses are concerned and spread heat is desirable for an early tomato crop, the pipes should be spaced out over the greenhouse.

Commercially, much use is made of flexible rubber couplings to allow the pipes to be raised or lowered as desired. When the soil is to be heated prior to planting tomatoes, the pipes are allowed to actually lie on the ground, and subsequently when the soil is warm and more convection heat is necessary to warm the air, the pipes can be raised on bricks. On the Continent, and increasingly in the UK, much use is made of under-plant alkathene pipes. A frequent practice in smaller houses is to install benches for the earlier part of the year for intensive propagation, removing these in the spring to make way for a groundgrown crop of tomatoes or chrysanthemums. In this case rubber coupled pipes capable of movement could be kept up a short distance below the bench to provide local heat for propagation, then dropped down to the ground to warm the soil for tomatoes or other crops.


Low pressure steam heating systems

These have little place in small installations, a steam boiler being an expensive item, only economical between about 363 — 454kg (800-1,000lb) of steam per hour, this being capable of supplying heat for a greenhouse of about 1,000m2 (one-quarter of an acre). As with medium pressure hot water systems less heating pipe is required.


Medium pressure hot water systems

In these systems hot water can be circulated through the heating pipes, if necessary above boiling point, the water being prevented from boiling because it is under pressure. The pressure is provided either by an ancillary unit or by a steam ‘pad’ in the boiler. Hot water boilers can be used where an ancillary unit is to be used, whereas with the pad system a steam boiler is necessary. As water temperatures in the pipes can be as high as 110-121°C (230-250°F), less radiation surface is needed in the greenhouse, so reducing the total length of pipe required, although the pipe layout will follow a similar pattern to a hot water system. A circulating pump is required to move the water round quickly.

One advantage of the steam pad boiler system, where the heated water is taken off well below the steam level, is that steam for sterilization is available simultaneously with the output of pressurized water for heating. Such systems have little place in smaller installations.


Low pressure steam systems (using steam boiler)

A problem with low pressure steam heat- ing systems is how to achieve good control of pipe temperature. The flow of steam into the pipe, while generally controlled by an electrically operated motorized valve, merely allows for the heat being either ‘on’ or ‘off . Temperature gradients are also liable to develop, as it is warmer nearer the boiler, though this can be overcome if the length of heating pipe is kept short on each line and there is a large reserve of steam. It is also best to keep the diameter of the pipe as small as possible to keep the thermal inertia low. The steam condenses and is allowed to flow back to a condenser tank for pumping back into the boiler. Steam is of course also available for sterilization purposes.

Other systems centre round the injection of steam into water contained in 4in pipes, somewhat noisy but nevertheless quite effective.

Calorifiers or heat exchanger systems can also be used whereby steam is put through the calorifier, which in turn heats the water which is then pumped round the heating circuit. Low pressure steam injection systems have also been developed in recent years and have proved fairly efficient.


24. March 2011 by Dave Pinkney
Categories: Greenhouse Equipment, Greenhouse Gardening | Tags: , | Comments Off on Greenhouse Heating – Hot Water Systems


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