Heating in Greenhouses is one of the first and oldest horticultural revolutions that took place. In the Netherlands wide use of heating systems started in the sixties. In the eighties the second horticultural revolution took place which was the wide use of computerized control systems, in the mid nineties grow lights were added to the equation. At the moment we are in the middle of the closed greenhouse revolution. In this article we will talk about the various heating systems we have in greenhouses and the benefits and disadvantages. For heating your produce or flowers in the greenhouse for crops like roses, chrysanthemum, carnation, tomatoes, cucumbers and bell pepper you have a choice, between a central heating system or a localized heating system or no heating at all.
Central heating: hot water or steam boiler with pipe system
- large investment, large capacity
- less expensive fuels can be used
- breakdown or failure could be critical if no backup
- inefficient if not run near capacity
- bottom heating is more practical
Localized heaters often called Forced Air heaters: gas-fired unit heaters or furnaces, possibly with convection tube; infrared heaters
- lower first investment
- can be for any area, easily expanded or contracted
- if the unit fails, others can carry the load
- more difficult to efficiently distribute the heat
If we take a closer look at central heating there are two options:
Hot Water A central boiler is used to heat water between about 120 F and 180 F. The water is distributed through pipes in the greenhouse. Pumps and mixing valves are used to uniformly distribute the heat and to control how much heat goes out to the greenhouse. Gives good control of temperature, more gradual changes in temperature, may be slow to respond to sudden drops in temperature. Large volumes of water are used.
Steam A steam system uses a smaller boiler, less plumbing and no circulating pumps. It is more difficult to control the flow of steam or heat into the greenhouse. High heat input and quicker changes in temperature are possible. Generally low pressure systems are used in greenhouses (5 to 15 pounds). Water expands as it is converted to steam which causes pressure and forces the steam from the boiler, through the pipes in the greenhouses. As it condenses, volume is lost and the condensate drains back to the boiler by gravity flow. One cubic foot of water will expand to 58.8 cubic feet of steam. Steam pipe is generally smaller than hot water pipe since there is less resistance to flow and a high heat loss per foot of pipe.
The boiler has to distribute the steam or hot water into the greenhouse there are various ways of doing this:
Pipe/Rail Heating distribution
Pipe rail heating systems are widely used and important to get the humidity out of the greenhouse. In the morning they turn on the pipe rail system which is mostly called “minimum pipe” temperature, in this way they get the access humidity out so a better climate is achieved. So apart from temperature control this is one of the most important tools in humidity control. In the picture below you so a similar system for chrysanthemums these growers use a special system which is called a hoist heating system because they need to improve the soil by steaming, plowing and adding organic material. In this system the grower is able to lift the system and do the work properly
Under bench Heating distribution
When you grow the pot plants on table you need a heating system very similar to the pipe rail system but in this system it’s located under the table.
In-Floor Heating distribution
In the picture you see the tubes of an in-floor heating system, this system hasn’t been finished yet since concrete needs to be poured on top of it. This photo gives a good overview of how the system works. Sometimes this heating distribution system is called a radiant heating system. This is system is mostly used for pot plants grown directly onto the floor. The floor heats up, this causes air movement which has a positive effect on the humidity around the plant since that can be moved away. Since the system is in the floor it gives a lot of freedom to work it won’t disturb any worker.
Overhead Heating distribution
Overhead heating is very important in areas were they have a cold winter, since it provides additional heating. The systems are very nice compatible with hanging baskets. A disadvantage of this heating system is that is takes a lot of light away from the plants.
Perimeter Heating distribution
In the very cold winter months when the normal heating system can’t cope on the north faced wall of the Greenhouse additional perimeter heating distribution will help. In the picture above you can see top fin heating pipes.
Types of steam valves
- pneumatic: air pressure controlled by the thermostat opens and closes the valve
- modulating valve: electric motor opens or closes the valve
- approximately 80% efficiency
- power vented
- direct Spark Ignition
- Big disadvantage of these heaters are that they bring a lot of humidity in the greenhouse, which makes them unsuitable to use in many climates for bell peppers and tomatoes the biggest vegetable greenhouse crops in the world.
Thermostatics Units of heat quantity
- British thermal unit (Btu): amount of heat energy required to raise the temperature of 1 pound of water 1 °F
1 Btu = 252 cal
- Horsepower (hp): another measure of energy; boiler heat output is reported as hp
1 hp = 33,475 Btu