Uniform temperature in the rootzone is difficult to achieve with afurnace heating system that circu-lates heat over the top of the crop ora boiler that pumps water through

fin radiation along the sidewalls. Locating the heaton top or under the benches warms the containersfirst before it rises to heat the air, and for mostcrops, soil temperature is more critical to goodgrowth than air temperature.

An added benefit to root zone heating is thepotential savings in fuel costs throughout theheating season. Air temperature in the green-house can be 5-10º F lower than soil temperatureand still achieve excellent growth. This lower airtemperature means the heat loss between insideand outside the greenhouse is less, which resultsin a 10 percent or more reduction in fuel usage.

Research by Bill Roberts and others at RutgersUniversity has shown that about 20 Btus per sq.ft.of bench area is adequate to provide root zone heatwithout drying plants too much or killing tenderroots. In Northern climates, this will provide about25 percent of the total heat a greenhouse needs onthe coldest night. During spring and fall, it willprobably provide all the necessary heat.

The components of a root zone heating sys-tem include a hot water heat source, distribu-tion piping, radiation and a control system.This article will examine these components tosee what the options are.

Select A Hot Water SourceIf you presently have a boiler system heating

the greenhouse, it can probably be modified togive the 100-130º F water needed for root zoneheat. The existing capacity should be adequateas the heat is just being redirected from fins orpipes to the root zone area.

If you have a condensing boiler, one in whichthe boiler can operate safely with a return watertemperature less than 140º F, then the high-limitswitch is set to the highest temperature water thatyou want in the root zone piping. If the boiler willbe used for both high temperature (180º F) and rootzone heat, then a bypass loop and mixing valvewill be necessary to get low-temperature water.

Most greenhouse boilers are the non-condens-ing type and require a bypass loop and mixingvalve to keep the water temperature in the boilerabove 140º F. A 3-way valve is commonly usedto allow water from the boiler to be mixed withreturn water from the root zone and directedback to the boiler or mixed with the return waterand fed into the root zone system.

With the current interest in alternative energysystems, wood and coal boilers are making acomeback. Because fire control is more difficultwith these systems and heat continues after thedamper is closed, a buffer tank is usually installed.This tank, with a capacity of several hundred gal-lons, stores the excess hot water from the boiler.After mixing it with the return water from the rootzone, the water is then distributed back to the rootzone at the desired temperature.

For providing root zone heat to a single hoophouse or small bench area, a domestic hot waterheater will do an excellent job. These heaters fired �

5 4 GPN O c t o b e r 2 0 0 6

greenhouse equipment

Grower 101:

By John Bartok, Jr.

Exploring UnderbenchHeating Options

Root zone heat can benefit crops and help lowerenergy costs, but i t is important to f irst select theright system components for your operation’s needs.

Top: A small boiler is a good source of hot water for the root zoneheating system. Different temperature zones are supplied waterfrom circulating pumps and controlled by remote bulbthermostats. Bottom: For small bench areas up to 2,000 sq.ft., adomestic hot water heater works well. The thermostat on this oil-fired heater is set to the desired 100-120º F water temperature.(Photos: John Bartok, Jr.)

*54_Grower101.qxd 9/14/06 3:51 PM Page 54

by gas, oil or electricity are available in capaci-ties from 30,000 to 40,000 Btus per hr. and willheat up to 2,000 sq.ft. of bench area. Larger com-mercial water heaters and instantaneous waterheaters have also been used with good success.

Select Appropriate PipingThe distribution pipe that carries hot water

from the boiler to the root zone system needs tobe selected carefully. For the bypass loop andpiping near the boiler, metal pipe — either cop-per or iron — should be used, as the water tem-perature is hot. If the water temperature thatwill be distributed in the root zone is less than130º F, PVC is a good choice because it softensand sags at temperatures above 140º F.

The system should be designed so the pipeloops are as short as possible to reduce frictionand heat loss. Locating the boiler near the cen-ter of the bench area to be heated will accom-plished this. Using a 3-pipe, reverse returnsystem will provide the same temperaturewater to all the loops. The system can bezoned so individual benches or areas in thegreenhouse can be heated to different temper-atures. Each zone requires a separate circulat-ing pump and piping. Insulate the high-tem-

perature and large-diameter pipes that carrylarge volumes of water to save energy.

Select Your SystemThere are several systems that are used to

provide heat in the bench area. Containers incontact with the heat source will get conductiveand convective heat. Where there is no contact,the heat is transmitted mostly by radiation.

Bench mats or EPDM tubing spaced 2-3inches apart works well for propagation andcontainers. The water flows through thepipes from the warm-water manifold or pipeand loops back to the cold-water manifold orpipe. The tubing and mats should withstandUV light, fertilizers and high temperatures.

Some growers have attached PEX (cross-linked polyethylene) tubing to the under-side of wire mesh benches. Besides beingresistant to abrasion and chemicals, PEXstops oxygen diffusion, which can causecorrosion in boilers, tanks and plumbing.

It is also possible to suspend bare iron pipeor low-output fin pipe about 18 inchesbeneath the bench. Usually the pipe is hungby a chain or hooks to the bench supportframes. Filled with 120º F water temperature, �

O c t o b e r 2 0 0 6 GPN 5 7

equipment

Subirrigation Delivery Of Bonzi

Bonzi is very effective when delivered to themedia as a substrate drench. The drench treat-ments provide better and longer lasting control ofvigorous crops and have little effect on flower sizeand timing compared to sprays.

Bonzi is approved for application via irrigationsystems (chemigation), including drip tubes, irriga-tion booms, hand-held wand with an injector andsubirrigation (ebb and flow benches, flood floorsand water collecting saucers). Users should consultthe Bonzi label for specific requirements of systemsused for chemigation.

Applying Bonzi via subirrigation is a relativelynew procedure and different from other, tradition-al drench techniques. It is common, at first, forusers to be a l ittle apprehensive about thismethod. While users do need to be very aware ofthe details involved with using Bonzi in subirriga-tion, this method offers significant advantages,such as treating a large number of plants with min-imal labor and providing an easy method forachieving uniform treatment of all plants. Also,distribution within the media is more uniform,which is important where there are multiple plantsper container. Subirrigation allows “drenching” ofsmaller pots or packs where a traditional drench isdifficult or impossible.

Bonzi is also more active when applied in subir-rigation than it is in a traditional drench to the sur-face of the media because there may be moreroots near the bottom of the container whereBonzi is concentrated when applied by subirriga-tion. Therefore, the optimum Bonzi rates for aone-time application in subirrigation should beevaluated starting at 50-75 percent of the opti-mum rates used in a traditional drench.

Another emerging strategy in subirrigation is touse continuous or multiple applications. For this, theoptimum Bonzi rates are 10-30 percent of the tradi-tional drench rates. This allows more flexibility, asthe Bonzi can be metered out more uniformly overthe crop during periods of rapid growth, therebycontrolling and delivering Bonzi at several stages inthe development and growth of the plants.

Bonzi® tip of the month

Maximize Profitability. Right-size Your Plants.

©2006 Syngenta. Syngenta Professional Products, Greensboro, NC 27419. Bonzi® is a trademark of a Syngenta Group Company.

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Rubber tubing and PVC pipe are used to supply this flat-top root zone heat mat. PVC pipe can handle up to 130º F waterwithout sagging.

Pots are warmed from the bottom on this small-diameter EPDM rubber tubing system.

*54_Grower101.qxd 9/28/06 12:01 PM Page 57

an 11⁄2-inch bare steel pipe will give off about 50Btus per linear foot. A three-fourth-inch-diameteraluminum pipe with two 1-inch fins will give offthe same number of Btus and hold only one-thirdthe volume of water. This reduces the waterneeded in the system and the overshoot in tem-perature once the system shuts off. Under a 6-ft.-wide bench, a loop of either the bare pipe or finwill provide the needed 20 Btus per sq.ft.

Circulating Pump And ControlA heating system circulating pump is typically

used to move heated water through the radiation.In a system with multiple zones, either one pumpper zone or a single larger pump with zonevalves is used. The pump needs to be sizeddepending on the number of loops and the fric-tion loss in the piping. Being a closed system,there is no head loss due to pipe elevation.

The sensor that controls the flow of hot water

to the root zone radiation should be placed in arepresentative pot or flat in the middle of one ofthe benches. The simplest control is a thermostatwith a remote sensor bulb. The root zone systemcan also be connected as the primary heat zone tomany controllers and computers.

Operating HintsIf you install the radiation under the bench,

attaching an 18-inch skirt to the sides of the benchwill trap the heat and keep it from escaping andheating the air in the greenhouse. Weed barriermats or plastic sheets work well for this.

To prevent a chimney effect from losing theunder-bench heat, keep the bench full of plants,lay a weed barrier mat on the bench top beforeplacing the pots down or cover any sectionswhere plants have been removed with plasticsheeting or other material.

Root zone heat on benches can provide more

uniform temperature control than a heating sys-tem used to maintain air temperature in agreenhouse. This results in better germinationof seed, faster rooting of cuttings and betterplant growth and disease control in pottedplants. The system will pay back quickly in fuelsavings due to the lower air temperature thatcan be maintained.

John Bartok, Jr. is an agricultural engineer andextension professor emeritus in the NaturalResource Management and EngineeringDepartment at the University of Connecticut,Storrs, Conn. He may be reached atjbartok@rcn.com.

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5 8 GPN O c t o b e r 2 0 0 6

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Left: Low-output fin radiation is easy to install and out of the way of plant handling. Rubber tubing connects the separate runs of aluminum pipe. Right: A plastic weed mat material attached to the benchsides contains the heat underneath the bench and allows the air temperature to remain cooler.

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