An underbench heating system can reduce fuel costs 10 percent or more. Air temperature in the greenhouse can be five to 10 degrees lower than soil temperature and still achieve excellent growth. This lower air temperature means that the heat loss between inside the greenhouse and outside will be less.
Maintaining optimum root temperature results in increased growth and reduced time to reach maturity for many plants. It also affects the microclimate of the plants by eliminating temperature stratification and lowering humidity in the plant zone.
A uniform temperature in the root zone is difficult to achieve with unit heaters that circulate heat over the top of the crop or a boiler that pumps water through fin radiation along the sidewall. Locating the heat under the benches warms the containers first before it rises to heat the air. For most crops, soil temperature is more critical to good growth than air temperature.
How much heat is needed?
Research at Rutgers University has shown that 15 to 25 Btu’s per sq. ft. of bench area is adequate to provide the root zone heat without drying the soil too much or killing the tender roots. In northern climates, this will provide about 25 percent of the total greenhouse heat needs on the coldest night. During the spring and fall, and in southern climates, it may provide most of the heat needed.
The components of a root zone heating system include a hot water heat source, distribution piping, radiation and a control system.
Select a hot water source.
If you have a boiler system in the greenhouse, it can probably be modified to give the 100ºF to 120ºF water needed for the root zone heat. The existing capacity should be adequate as the heat is just being redirected from fin or pipe radiation to the root zone area.
If you have a condensing boiler, one in which the boiler can operate safely with a return water temperature less than 140ºF, then the high limit switch is set to the highest temperature water that you want in the root zone piping. If the boiler will be used for both high temperature (180ºF) and root zone heat then a bypass loop and mixing valve will be necessary to get the low temperature water.
For providing root zone heat to a single hoophouse or small bench area, a domestic hot water heater will do an excellent job. These heaters fired by gas, oil or electricity are available in capacities of from 30,000 to 40,000 Btu/hr and will heat up to 2,000 sq. ft. of bench area. Multiple heaters or larger size commercial water heaters and instantaneous water heaters have also been used with good success.
Distribution pipe to carry the hot water from the water heater or boiler to the root zone system must be selected carefully. For the bypass loop and piping near the boiler, metal pipe (either copper or iron) should be used. If the water temperature that will be distributed in the root zone is less than 130ºF, then PVC pipe is a good choice. It should be supported to prevent sag. Insulate the high temperature and large diameter supply pipes to save energy.
The system should be designed so that the pipe loops are as short as possible to reduce friction losses and heat loss. Using a three-pipe, reverse return system will provide the same temperature water to all the loops. The system can be zoned so that individual benches or areas in the greenhouse can be heated to a different temperature. Each zone requires a separate circulating pump and piping.
Select the root zone radiation to give uniform temperatures.
Several systems are used to provide heat in the bench area. A common system uses a small diameter aluminum pipe with two fins (BioTherm Duo Fin or Delta Fin). This pipe will give off about 70 Btu’s of heat per linear foot at 120ºF water temperature. Under a 6-foot wide bench, using two loops (four runs) of the fin pipe is usually required to get uniform temperature. The fin pipe is usually suspended from the bench frame about 18 inches beneath the bench top.
Some growers have attached PEX (cross-linked polyethylene) tubing to the underside of wire mesh benches. Besides being resistant to abrasion and chemicals, PEX stops oxygen diffusion which can cause corrosion in boilers, tanks and plumbing. The heat transfer from the plastic pipe is lower, so more loops are required.
Circulating pump and control.
A circulating pump is needed to move the heated water through the pipes. In a system with multiple zones, either one pump per zone or a single larger pump with zone valves can be used. The pump should be sized on the number of loops and the friction loss in the piping. Being a closed system, there is no heat loss due to elevation in the pipes.
The sensor that controls circulating pump and the flow of warm water to the root zone radiation should be placed in a representative pot or flat in the middle of one of the benches. The simplest control is a thermostat with a remote sensor bulb. The root zone system can also be connected as the primary heat zone to many controllers and computers.
Attaching an 18-inch skirt to the sides of the bench will trap the heat under the bench and provide a more uniform temperature. Weed barrier mat or plastic sheeting works well for this.
To reduce the chimney effect from losing the heat under the bench, keep the bench full of plants, lay a weed barrier mat on the bench top before placing the pots down or cover any sections where plants have been removed with plastic sheeting.
Underbench heat can give more uniform root zone temperature than the heating system used to maintain air temperature in a greenhouse. This results in better germination of seed, faster rooting of cuttings and better plant growth and disease control in potted plants. The system will pay back quickly in savings in fuel due to the lower air temperature.