FACT SHEETS > GREENHOUSE MANAGEMENT > ROOT ZONE HEATING SYSTEMS

Root Zone Heating Systems
Increased earliness and higher quality crops can be achieved using root zone heating. If theroot zone temperature is maintained at the optimum, air temperature in the greenhouse canbe lowered 5 to 10 deg F reducing heat loss and thereby, energy consumption. Root zone
temperature is more critical than leaf temperature in achieving good plant growth.
System Components
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' and 400' rolls. Select apipe made of virgin plastic rather than one having reconstituted resins. It should have apressure rating of at least 100 psi. Polyethylene will take temperatures up to 130 deg F. Most
growers who use poly pipe operate with a water temperature of 100 deg F to provide 70 to75 deg F soil temperature. Nylon fittings and stainless clamps will minimize the potential forleaks. Fittings that are buried below ground should have double clamps.
Semi-rigid polyvinyl chloride (PVC) is also low cost. It is available in 10' and 20' 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 web. Diameters of 3/8 or ½ have greater heattransfer 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.
To prevent corrosion due to oxygen diffusion of the 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 isalso available for pipes that will be buried below a concrete floor. This is more rigid andstays in place better. In systems where PEX is not used, dissolved oxygen permeates throughthe tubing or pipe walls and causes rusting of ferrous components, such as tanks, fittings andthe boiler heat exchanger. Protection can be achieved by using non-ferrous components

(brass, copper or plastic), installing a non-ferrous heat exchanger, using a glass-lined tank or
water heater or adding a corrosion inhibitor.
System Layout
PVC pipe is the most common material for the supply piping to bring the water from theheater or boiler to the growing area. A reverse return (3 pipe) system is used so that thewater to all the loops travels the same distance. On long runs and in unheated areas, supplyand 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 orplaced 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 to 12 deep willallow roto-tilling of the soil above it. This can be done by plowing a furrow and then layingthe pipe in the bottom or purchasing a pipe-laying chisel that attaches to the drawbar of atractor. For surface installation with bags or troughs, the pipe is laid on top of the groundplastic or weed barrier underneath the plants.
For benches, a 6 to 9 pipe spacing covered by 3 to 4 of sand will provide eventemperature. The sand should be kept wet to transfer the heat and is usually covered with asheet of plastic or weed barrier. An alternative arrangement consists of laying the pipe in thebottom of the bench and covering with wire mesh and a layer of plastic. Some growers haveattached the pipe underneath the bench to get it out of the way and to allow the heat tospread.
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 distancegiving uniform heating. Heat loss from plastic and rubber tubing is relatively slow solengths up to 200' for ½ and 400' for ¾ pipe will give good results with minimum
friction loss.
Sizing the Heater
The size of the loops should be made as large as practical so that the header and pump size
can remain small. Stay within the 200' and 400' recommendations given above. To keep aneven flow of water within the pipes and eliminate air pockets a flow rate of 2 and 2.5
gallons/minute (gpm) is used for the ½ and ¾ 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 that it takes 10 Btu/linear foot of row length. For example,a 30' x 100' greenhouse with 10 rows of plants would require 10,000 Btu/hr of heat (10 rows
x 100' 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 fthr and for beds or benches covered with flats, about 15 Btu/sq ft-hr (Figure 4). This is basedon a water temperature of 100 ° F. Some manufacturers of rubber tubing recommend watertemperature as high as 140 ° F which will increase heat transfer but may cause root damage
on some crops.
Heat Source
A tank-type, domestic hot water heater (30,000 to 40,000 Btu/hr) fired by natural gas orpropane will provide the root zone heat for 3,000 to 6,000 sq ft of growing area. Commercial
water heaters fired by gas or oil are available in larger sizes. As the root zone heating systemdoes 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 theroot zone heat and the air heat. It is best if dual boilers are installed with 1/3 and 2/3
capacities. These can be staged to efficiently handle the heat needs over the entire year. The
boiler water temperature is usually maintained at 180 to 200 deg 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 deg F water for the system. Boilers
are available in sizes from 50,000 Btu/hr and up.
System Plumbing
All closed loop systems require the use of a pre-pressuring 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 thenumber of loops per zone and the size of the piping. For example, a system of 10 - 200'
loops of ½ poly pipe will have a flow of 20 gallons/min (10 loops x 2 gpm/loop = 20gpm). The pump needs to be able to overcome the friction loss in the system. For most rootzone systems, a pump having the calculated capacity at a total of 15 to 20 feet of head willmeet the system needs.
Controls
In the simplest system using a water heater, the thermostat on the tank is set at the desired
root zone water temperature (usually 100 deg 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 theaquastat that controls the output water temperature. Manufacturers guidelines for minimum
water temperature entering the boiler should be strictly followed. Where a boiler is used forspace heating in addition to root zone heat, a higher temperature is usually desired and atempering valve needed. In most areas of the U.S., 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. 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 adegree or two. Mechanical thermostats have a higher differential.
In larger greenhouse systems, the water in the supply lines to the root zone system may becirculated 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 do to a lower air temperature can be as much as 10% and help offset the costof the system.
John W. Bartok, Jr.
Agricultural Engineer
Natural Resources Mgt. & Engr. Dept.
University of Connecticut , Storrs CT
May 2005

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