With fuel costs rising rapidly, root zone heating is becoming popular as an energy saving technique. Supplying the heat under the crop reduces the heat needs by allowing the air temperature to be maintained 5 to 15ºF cooler. It also maintains a more uniform heat pattern than can be obtained with perimeter or unit air heaters. Root zone heat can be provided in the floor or under the crop on benches.
The basic floor system consists of pipe embedded in a layer of sand or concrete. Warm water, pumped through the pipes, conducts the heat to the plants placed on the floor. The sand or concrete distributes the heat evenly across the floor surface.
In the bench system, aluminum fin or bare steel pipe placed under the bench radiates heat up to the root zone. Another system utilizes rubber tubing or mats placed on the bench top under the plants.
Depending on climate, the root zone heating system will provide 25 – 75% of the total heat needs of the greenhouse. The remaining is usually made up with perimeter radiation or air heaters. Research has shown that about 15 to 30 Btu/hr per square foot of floor or bench can be obtained from the root zone heating system.
To get good service from these systems, they have to be installed correctly. Cutting corners usually doesn't pay. Here are a few installation techniques that may help.
For heating small areas, less than 3000 sq ft, a low-cost, domestic hot water heater is usually the best choice. These are available in natural gas, propane and electric models in sizes to about 45,000 Btu/hr. Commercial water heaters available with output to 300,000 Btu/hr can be used for larger areas. Select a heater with a glass lined tank. Gas fired models frequently require only a plastic flue pipe rather than a metal or masonry chimney. The installation is simple in that besides the water heater all that is needed is an expansion tank, air eliminator, pressure – temperature relief valve (PTR), circulating pump and remote bulb thermostat. The thermostat on the water heater is usually set at 100 to 110ºF and the circulator thermostat in the soil of a representative container is set at the desired soil temperature.
Hot water from an existing or new boiler can also be used. One or more circulators and tempering valves are needed to feed the root zone heat. Because the return water from the root zone is cool, a non-condensing boiler should not be used. Cool water, less than 135ºF can cause condensation that is highly acidic and can damage the boiler. A tempering valve that protects the boiler could be installed to warm the return water. A better option is to have a more efficient, condensing boiler that uses the heat from the flue gases to warm the return water.
Another installation that works well is to install a heat exchanger between the boiler and the root zone heat. A heat exchanger isolates the boiler water from the root zone tubing water. A circulating pump moves the hot boiler water on one side of the heat exchanger and a second pump passes the water through the other side. Heat exchangers are used if the root zone tubing is filled with glycol solution in a greenhouse that is shut down during cold weather. It is also common in a system that has been installed to deal with the problem of oxygen diffusion when the tubing does not have an oxygen barrier.
Heat distribution in the root zone
Oxygen diffusion can corrode heating systems. Dissolved oxygen molecules are present in all fresh water. These molecules can attack ferrous components in the heating system causing rust. Plastic or rubber tubing that does not have a diffusion barrier will allow oxygen to enter the water and sludge and rust to accumulate restricting flow. It is best to use a material such as PEX, a cross-linked polyethylene tubing that has an oxygen diffusion barrier. PEX tubing is available is sizes from 3/8" to 2" and in roll lengths to 1000'. Typical size for floor systems is ½" for loops up to 200' and ¾" for loops up to 400'. Tube spacing is usually 9" to 12" on center. If you use a non-oxygen diffusion barrier tubing and the water flows through the boiler, be sure that all the pipe and fittings are copper or brass. Also add a water treatment that balances the pH and removes the free oxygen from the water.
Some growers have installed low-cost, Schedule 80 polyethylene pipe in sand floor or under soil grown crop, such as tomatoes or cucumbers with good results. With a glass lined hot water heater and no ferrous components, the life has been good.
EPDM rubber tubing is common for on bench heat and low-output fin or bare steel pipe is used under benches. These systems provide uniform heat if the bench is kept full of plants or a weed barrier mat is placed on the bench to spread the heat. A gap in the plant canopy creates a chimney effect allowing heat to escape and can make control of the temperature difficult. Skirts (18" high) placed around the side of the bench can be used to reduce the heat escaping from under the bench.
The circulating pump is the heart of the root zone system. Centrifugal pumps are used as a good flow is created without much energy. High pressure is not needed as the system is closed and the water is not lifted very high. Inline pumps are the most common. A wet rotor circulator having the advantages of no seals and low cost could also be used. Place a shut-off valve and a union on both sides of the pump so that it can be serviced or replaced easily.
When sizing the pump, remember that the flow should be about 2.5 feet/min for ½" or ¾" tubing. This keeps the temperature difference between the supply and return ends of the loop to between 5 and 10ºF. The head or pressure loss is determined by the number and length of loops in the system and the tubing size. The pump is best located near the expansion tank to reduce pressure differences.
A remote bulb thermostat is the most common control. Placed in a pot or flat, it senses soil temperature and activates the circulating pump when heat is needed. When the water is supplied by a boiler, controls that modulate water temperature or a variable speed pump may be used.
Root zone heat makes sense as it provides a uniform temperature under all the plants. It is difficult to get the ideal root temperature of 70 - 75ºF required for optimum growth by most plants with an air heat system.
John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs CT – 2013