Root zone heating is an effective option for greenhouses that provides heat directly to the growing media rather than heating the air of the greenhouse. This approach provides a triple benefit for greenhouse growers: faster production, higher quality crops and energy savings. If the root zone temperature is maintained at the optimum, air temperature in the greenhouse can be lowered 5 to 10 degrees F., reducing heat loss to the outside and therefore, reducing energy consumption. This is possible because root zone temperature is more critical than leaf temperature for achieving good plant growth.
A typical hot water root zone heating system contains a water heater or boiler, circulating pumps, piping, and controls.
The least expensive pipe is polyethylene, which is available in 100-foot and 400-foot rolls. Select a pipe made of virgin plastic rather than one having reconstituted resins. It should have a pressure rating of at least 100 psi. Polyethylene will take temperatures up to 130 degrees F. Most growers who use poly pipe operate with a water temperature of 100 degrees F. to provide 70 to 75 degrees F. soil temperature. Nylon fittings and stainless clamps will minimize the potential for leaks. Fittings that are buried below ground should have double clamps.
Semi-rigid polyvinyl chloride (PVC) is also low cost. It is available in 10-foot and 20-foot lengths which makes installation easy. Fittings are connected with pipe cement.
Commercially available systems are available that use EPDM rubber tubing either as single tubes or as two or four tubes attached to a rack. Diameters of 3/8 inch or ½ inch have good heat transfer and eliminate some problems from chemical coating and sedimentation blocking. The tubing is connected to plastic or copper headers with plastic inserts or brass fittings. Some manufacturers offer custom-made, ready-to-install modules with headers sized to fit the row spacing.
If you are concerned about corrosion due to oxygen diffusion through the rubber tubing that can end up damaging ferrous components in a closed loop heating system, cross-linked polyethylene tubing (PEX) should be used. This pipe contains an oxygen diffusion barrier. A multi-layer composite tubing with an aluminum center core is also available for pipes that will be buried below a concrete floor. This is more rigid and stays in place better. In systems where PEX is not used, dissolved oxygen permeates through the tubing or pipe walls and causes rusting of ferrous components, such as tanks, fittings and the boiler heat exchanger. Protection can be achieved by using nonferrous components (brass, copper or plastic), installing a nonferrous heat exchanger, using a glass-lined tank or water heater or adding a corrosion inhibitor.
PVC pipe is the most common material for the supply piping to bring the water from the heater or boiler to the growing area. On long runs and in unheated areas, supply and return pipes should be insulated to save energy.
For EPDM rubber installations follow the manufacturer's recommendations for spacing, length of run, and circulating pump size. The tubing can be buried in sand on the floor or placed on top or underneath the bench. Some manufacturers supply a slotted insulation board for placing the tubing on top of the bench.
For soil-grown crops such as tomatoes or cucumbers, placing the pipe 8 inches to 12 inches deep will allow rototilling of the soil above it. This can be done by plowing a furrow and then laying the pipe in the bottom or purchasing a pipe-laying chisel that attaches to the drawbar of a tractor. For surface installation with bags or troughs, the pipe is laid on top of the ground plastic or weed barrier underneath the plants.
For benches, a 6-inch to 9-inch pipe spacing covered by 3 inches to 4 inches of sand will provide even temperature. The sand should be kept wet to transfer the heat and is usually covered with a sheet of plastic or weed barrier. An alternative arrangement consists of laying the pipe in the bottom of the bench and covering with wire mesh and a layer of plastic. Some growers have attached the pipe underneath the bench to get it out of the way and to allow the heat to spread.
The pipe is installed as loops fed by a supply header with the other end connected to a return header. Using a reverse return system, the flow through each loop travels the same distance, giving uniform heating. Heat loss from plastic and rubber tubing is relatively slow, so lengths up to 200 feet for ½ inch and 400 feet for ¾-inch pipe will give good results with minimum friction loss.
The loops should be made as long as practical so that the header and pump size can remain small. Stay within the 200 feet and 400 feet recommendations given above. To keep an even flow of water within the pipes and eliminate air pockets a flow rate of 2 and 2.5 gallons per/ minute (7.5-9 liters per minute) is used for the ½-inch and ¾-inch pipe, respectively.
For tomatoes or cucumbers grown in rows in the soil or in bags with a single line of pipe under each row, you can estimate it takes 10 Btuh/linear foot of row length (10 watts per meter). For example, a 30-foot-by-100-foot greenhouse with 10 rows of plants would require 10,000 Btu/hr (3kW) of heat (10 rows x 100-foot length x 10 Btu/hr/linear ft). Add about 10% to this total for heat loss from the supply pipes. The soil around the pipes needs to be kept moist to get good heat transfer.
Heat loss from beds or benches covered with plants growing in the soil is about 20 Btu/sq ft/hr (50 watts/square meter) and for beds or benches covered with flats, about 15 Btu/sq ft/hr (Figure 4). This is based on a water temperature of 100 degrees F. Some manufacturers of rubber tubing recommend water temperature as high as 140 degrees F., which will increase heat transfer but may cause root damage on some crops.
A tank-type, domestic hot water heater (30,000 to 40,000 Btu/hr) fired by natural gas or propane will provide the root zone heat for 3,000 to 6,000 square feet of growing area. Commercial water heaters fired by gas or oil are available in larger sizes. As the root zone heating system does not provide all the heat needed to keep the greenhouse warm on cold nights, a unit heater or other source of heat is needed.
In larger greenhouses, a boiler is usually installed that is large enough to provide both the root zone heat and the air heat. It is best if dual boilers are installed with one-third and two-thirds capacities. These can be staged to efficiently handle the heat needs over the entire year. The boiler water temperature on large systems is usually maintained at 180 to 200 degrees F. during the coldest part of the year. A tempering valve installed in the supply line mixes the hot water and the returning cool water from the root zone piping to provide the 100 degrees F. water for the system. Boilers are available in sizes from 50,000 Btu/hr and up.
All closed loop systems require the use of a pre-pressurized diaphragm expansion tank, an air eliminator and vent installed on the supply pipe as close to the hot water source as possible. Valves needed include a pressure relief valve, flow balancing valves, gate valves to isolate parts of the system, pressure-reducing valves to fill the piping and zone valves to control individual sections of the system independently.
Water is moved through the system with circulating pumps. The flow rate is based on the number of loops per zone and the size of the piping. For example, a system of 10 to 200-foot loops of ½-inch poly pipe will have a flow of 20 gallons/min (10 loops x 2 gpm/loop = 20 gpm). The pump needs to be able to overcome the friction loss in the system. For most root zone systems, a pump having the calculated capacity at a total of 15 to 20 feet of head will meet the system needs.
In the simplest system using a water heater, the thermostat on the tank is set at the desired root zone water temperature (usually 100 degrees F.). Return water from the loops goes back to the tank to be reheated. The same system can be used with most boilers by setting the aquastat that controls the output water temperature. Manufacturers' guidelines for minimum water temperature entering the boiler should be strictly followed. Where a boiler is used for space heating in addition to root zone heat, a higher temperature is usually desired and a tempering valve needed. In most areas of the United States, root zone heat will provide less than 25% of the total greenhouse heat needs on the coldest night so an additional heat distribution system is needed to heat the air in the greenhouse. This can be fin or pipe radiation, water to air heat exchangers or hot air furnaces.
Activation of the circulating pump is done with a sensor inserted in the soil or growing bag. An electronic thermostat is a good choice as the differential between on and off is only a degree or two. Mechanical thermostats have a higher differential.
In larger greenhouse systems, the water in the supply lines to the root zone system may be circulated continuously. This maintains warm water near the growing area. Solenoid valves on each zone, activated by a sensor in the bed, control the flow to that zone.
Root zone heat has proven to be an effective way to get better propagation and production. Energy savings due to a lower air temperature can be as much as 10% and help offset the cost of the system.