Central Heating Systems

The most even distribution of heat throughout a greenhouse is accom­plished when the heat is generated from a central system rather than from a localized system. The boiler may be fueled with natural or pro­pane gas, oil, or even wood or coal. It produces either hot water or steam that carries the heat energy into the greenhouses through a series of pipes that give off the heat into the air of the greenhouse. The placement of the pipes has a significant effect on the efficiency of the heat distribu­tion, so placement will vary depending on what is being grown and how the benches are designed and placed. The pipes may be placed beneath the benches for crops grown in or on raised benches. If the potted crops are grown directly on the floor of the greenhouse, then the heat distri­bution pipes may be placed beneath the floor. Central heating systems that use hot water as the heat source can locate the distribution pipes around the perimeter of the greenhouse or overhead as well. A steam system offers an additional advantage. It provides steam for use in pas­teurization of the soil used to support crop growth. Table 19-2 compares unit heaters with the two types of central heating systems.

A Comparison of Greenhouse Heating Systems

Type of System

Advantages

Possible Disadvantages

Steam

Can transport heat throughout large ranges (greenhouse physical plants) without cooling.

Control of greenhouse temperatures not as subtle with steam as with hot water and house may temporarily overheat.

Requires smaller piping than hot water systems, reducing installation costs

Cost of maintenance greater with steam due to damaging effects that high pressure has on pipes.

Makes steam available for heat-treating greenhouse soil

Hot Water

Permits more accurate and responsive thermostatic control of greenhouse temperatures

Larger piping required, increasing cost of installation

Distributes heat more evenly with fewer hot spots to injure plants

Steam still required for soil pasteurization

Less potential for danger to workers if a hot water line ruptures than if a pressurized steam line breaks

Unit heater Adaptable to small greenhouse areas

Fuel cost (gas or oil) may be greater than cost of expanding existing steam or hot water system.

Excellent back-up systems in the event of boiler breakdown or power failure of larger system

Heat distribution uneven but can be improved by attaching plastic sleeves that extend length of house. Bench level temperatures may still be cooler than nearer roof.