Greenhouse Heating

1. GREEN HOUSE HEATING
Dr. Ajay Singh Lodhi
Assistant Professor
College of Agriculture, Balaghat
Jawahar Lal Krishi Vishwa Vidyalaya, Jabalpur (M.P.)

2.  A good heating system is one of the most important steps
to successful plant production. Any heating system that
provides uniform temperature control without releasing
material harmful to the plants is acceptable. Suitable
energy sources include natural gas, LP gas, fuel oil, wood
and electricity.
 Greenhouse heater requirements depend upon the
amount of heat loss from the structure. Heat loss from a
greenhouse usually occurs by all three modes of heat
transfer: conduction, convection and radiation.
 The heat demand for a greenhouse is normally
calculated by combining all three losses as a coefficient
in a heat loss equation.
GREEN HOUSE HEATING

4.  Heat loss by conduction may be estimated with the
following equation:
Q = A (Ti - To)/R
Where,
Q = Heat loss, BTU/hr
A = Area of greenhouse surface, sq ft
R = Resistance to heat flow (a characteristic of the
material)
(Ti - To) = Air temperature differences between inside
and outside
Heat Loss Calculation

5. FACTORS AFFECTING HEAT LOSS
 Heat loss by air infiltration
depends on the age, condition
and type of greenhouse. Older
greenhouses or those in poor
condition generally have
cracks around doors or holes in
covering material through
which large amounts of cold
air may enter. Greenhouses
covered with large sheets of
glazing materials, large sheets
of fibre glass, or a single or
double layer of rigid or flexible
plastic have less infiltration
Energy loss due to infiltration

6.  The greenhouse ventilation system also has a large
effect on infiltration.
 Inlet and outlet fan shutters often allow a large air
exchange if they do not close tightly due to poor design,
dirt, damage or lack of lubrication.
 The amount of radiant heat loss depends on the type of
glazing and amount of cloud cover.
 Rigid plastic and glass materials exhibit the
“greenhouse effect” because they allow less than 4
percent of the thermal radiation to pass back through to
the outside.

7. FACTORS TO CONSIDER WHILE SELECTING
GREENHOUSE HEATING SYSTEM
 Before determining the type of a system to use, it is
necessary to calculate the amount of heat that will be
required. Remember this should be based on the most
adverse conditions that you reasonably expect to encounter.
 The minimum inside temperature depends on the type of
plants to be grown. Decide whether you just want to save
the plants from the severe injury or if you want normal or
near normal growth to continue. Then determine the
temperature needed to achieve your objective, subtracts
expected minimum adverse temperature for your location
and obtain the differential in °F or which you need to be
prepared.

8.  An easy and fairly accurate method for estimating the
amount of heat required can be obtained by multiplying
the surface area of the greenhouse by the maximum
temperature difference to be maintained, and this
product times a heat transmission factor that depends
on the covering of the greenhouse and is also influenced
by quality of construction.
 A factor of 1.0 to 1.2 is used if the house is covered by a
single layer of polyethylene film (PE) or rigid plastic.
1.0 would assume a well built, tight greenhouse, while
1.2 would assume a little less quality construction and
more air leakage. The same analogy will be true for the
transmission factors used for other materials.

9.  Also, wind velocity affects the transmission factor; the higher
the wind velocity, the greater the heat loss. Use a factor of
0.75 to 0, if the house is covered with a double layer of PE
film with an air space of at least 3/4² but not more than 4².
 Use a factor between 1.1 to 1.4, if the greenhouse is glass
glazed. Add 10 percent to the values obtained if the house is
located in a windy location and there are many leaks for air
infiltration. If the house were very highly constructed and
fairly large panels were used and, if the house were protected
by a wind brake of some kind then you can use a lower heat
transmission factor of about 1.0. Then calculate the heating
input required and use its value to determine the pipe length
in steam and hot water systems or the size of the unit
heaters.

10.  Another factor to consider is the efficiency of the heating
unit. Most manufacturers of heating equipment show
both input and output BTU/hr. Calculations on
equipment size must be based on output capacity.
 In conclusion, successful heating of greenhouses is
dependent upon correct sizing and installation of the
heating system, proper controls and methods of obtaining
uniform heat distribution.
 A type of greenhouse construction, crops to be grown and
temperature levels to be maintained are all important
factors to consider in the selection and design of any
greenhouse heating system.
 Heating system should allow for easy expansion of the
range in the future, and the system should have
sufficient capacity to offset the heat loss from the
greenhouse under most severe conditions. Normally
design for temperatures slightly above minimum 15 to 25
year lows as shown by local weather records.

11.  Provide an adequate system of automatic controls. The
most important control in most heating systems is the
thermostat, which is used to control the operation of the
heating system. For this reason, the thermostat should
be placed at plant level, and shielded from direct rays of
the sun, and it should sense “line air.” Thermostats for
greenhouses should be accurate to within at least 2-3
°F. It is not enough to have good control over the
heating system. An operator must know what degree of
temperature control is necessary for the type of plants
being produced.
 If possible, select a heating system that will allow you
to convert from one fuel source to another.

12. GREENHOUSE HEATING SYSTEMS
 The heating system must provide heat to the
greenhouse at the same rate at which it is lost by'
conduction, infiltration, and radiation.
 There are three popular types of heating systems for
greenhouses, namely
 Unit heater,
 Central heating system and
 Radiation heating.
 The most common and least expensive is the unit
heater system.
 In this system, warm air is blown from unit heaters
that have self contained fireboxes.

13.  These heaters consist of three functional parts, namely,
firebox, metal tube heat exchanger, and heat
distribution fan.
 Fuel is combusted in a firebox to provide heat. The heat
is initially contained in the exhaust, which rises
through the inside of a set of thin walled metal tubes on
its way to the exhaust stack.
 The warm exhaust transfers heat to the cooler metal
walls of the tubes. Much of the heat is removed from
the exhaust by the time it reaches the stack through
which it leaves the greenhouse.
 A fan in the back of the unit heater draws in
greenhouse air, passing it over the exterior side of the
tubes and then out the front of the heater to the
greenhouse environment again.
 The cool air passing over hot metal tubes is warmed
and the air is circulated.

14. Unit Space Heater
 Unit space heaters, either floor mounted or supported, are
normally fuelled with natural or bottled gas or fuel oil and
use fans for heat distribution.
 This system requires a relatively moderate capital
investment, is easy to install, and provides for easy
expansion of facilities
 There are two main types of unit heaters that are used for
space heating in greenhouses: vented and unvented.
 The traditional vented, gas-fired unit heater
transfers heat from the combustion gases to the air through
a heat exchanger, and exhausts the combustion gases
outside the greenhouse through a flue pipe.
 An unvented unit heater burns the gas and exhausts all
combustion gases directly into the greenhouse, so virtually
all the heat from the fuel is used to heat the air.
 There are four types of vented unit heaters; gravity vented,
power vented, separated combustion and high
efficiency condensing heaters.

15. GRAVITY VENTED UNIT HEATERS
 Gravity vented unit heaters
rely on thermal buoyancy and
draw from wind blowing past
the vent pipe to exhaust the
flue gases. These heaters use
inside air for combustion
which accounts for 2% of the
efficiency loss and some
heaters use continuous pilot
lights, which consume a small
amount of additional energy.
Thermal efficiency of gravity
vented unit heater is 80%.
Gravity vented unit heaters

16. POWER VENTED UNIT HEATER
 A power vented unit heater has small
blower that meters the correct amount of
air for combustion and exhaust the flue
gases. The blower operates only when the
heater is firing. This type of unit heater
uses a smaller exhaust pipe that can be run
horizontally through the wall of the
greenhouse reducing installation costs and
acting like a vent damper to minimize
thermal buoyancy losses. Gas fired power
vented heaters often use an intermittent or
electronic pilot, which reduces flameouts
and pilot gas use. The seasonal efficiency of
a power vented unit heater is typically
about 78% with thermal efficiency of 80%.

17. SEPARATED COMBUSTION UNIT HEATERS
 Separated combustion heater is designed for heating
areas that have negative pressure or high humidity,
dusty or corrosive environments. These heaters use a
power vented exhaust and have a separate air intake
duct for combustion air. Modern plastic greenhouses
are tightly constructed with fewer seams than glass
greenhouses and thus have low infiltration rates, it is
possible during times of peak heating for many types of
unit heaters to use enough oxygen in the greenhouse to
cause poor combustion or to cause flue gases to be
drawn into the greenhouse through the flue pipe. Back
drafts of flue gases can be a problem if greenhouse is
located in windy area or if exhaust fans and heaters
are inadvertently used at the same time. Thermal
efficiency of these heaters is 80%.

18. HIGH EFFICIENCY CONDENSING HEATER
 High efficiency condensing heaters use a power vented
exhaust and a separate air intake, so heated
greenhouse air is not used for combustion and they
require a drain or other way to dispose of the acidic
condensate. Both the thermal and seasonal efficiencies
are typically about 93%. High efficiency condensing
heaters are more expensive than other types but they
provide more heated air per unit of fuel.

19. PORTABLE UNIT HEATERS
 Portable unit heaters are often
used for temporary or
emergency heating and operate
on kerosene heating oil or LP
gas. These unvented units are
not suited for use in enclosed
structures because they do not
have air intake venting. If
using portable units for
emergency or temporary
greenhouse heating, use only
LP gas fired units and open a
vent or prop open a door to
replace the oxygen burned by
the heater.

20. CENTRAL HEATING SYSTEM
 A second type of system is central heating system, which
consists of a central boiler that produces steam or hot water,
plus a radiating mechanism in the greenhouse to dissipate
the heat.
 A central heating system can be more efficient than unit
heaters, especially in large greenhouse ranges.
 In this system, two or more large boilers are in a single
location.
 Heat is transported in the form of hot water or steam
through pipe mains to the growing area, and several
arrangements of heating pipes in greenhouse are possible.
 The heat is exchanged from the hot water in a pipe coil on
the perimeter walls plus an overhead pipe coil located across
the greenhouse or an in-bed pipe coil located in the plant
zone. Some greenhouses have a third pipe coil embedded in a
concrete floor.
 A set of unit heaters can be used in the place of the overhead
pipe coil, obtaining heat from hot water or steam from the
central boiler.

21. HOT WATER SYSTEM
 Hot water systems utilizing
piping that can be perimeter,
under benches, or overhead fan
forced unit heaters can be used.
These require a boiler, valves,
and other necessary controls.
However, a hot water system is
simpler to install and normally
requires less maintenance than
a steam system. There are
slower heating and cooling of
pipes, but temperatures are
normally more uniform. Hot
water systems are mainly used
in smaller ranges.

22. STEAM HEATING SYSTEM
 A steam heating system needs a boiler,
valves, traps and other controls
depending upon the size and type of
boiler used. Steam provides rapid
heating and cooling of the steam lines
and usually pipe length needed is less.
Lines may be smooth or finned, and
about l/3 of the heat should be overhead
and about 2/3 along the side walls.
Lines can also be arranged under
benches or with overhead fan-forced
unit heaters. A steam system also
allows the use of steam for soil
pasteurization. A steam system requires
a high initial investment; however, it
has a long life expectancy. Steam
heating systems are most often used in
large ranges as steam can be
transported long distances efficiently.

23. Pipe rail heating distribution

24. Under Bench Heat Distribution

25. Overhead Heat Distribution

26. RADIATION HEATING
 The third type of system is radiation heating system.
In this system, gas is burned within pipes suspended
overhead in the greenhouse. The warm pipes radiate
heat to the plants.
 Low intensity infrared radiant heaters can save 30%
or more of fuel compared to conventional heaters.
Several of these heaters are installed in tandem in
the greenhouse.
 Lower air temperatures are possible since only the
plants and root substrate are heated directly by this
mode of heating.

27. UNIT RADIANT HEATERS
 Radiant heater, as opposed to warm air systems (such
as a forced air unit heaters), delivers the source of heat
to the floor level, not the ceiling.
 Radiant energy is totally pure radiation and is absorbed
by an object without physical contact with the heat
source or by heating the surrounding air, as is the case
with convective, forced air systems. Radiant heaters are
the most efficient and effective method in which to
deliver "heat" under the diverse conditions present in
warehouses, garages, storerooms as well as the largest
facilities imaginable.
 Hot gases are moved through the radiant tube either by
vacuum (negative) or power (positive) pressure. The
radiant energy produced is then directed downward by
the reflectors positioned above the radiant tubes.

28. Radiant heaters

29. SOLAR HEATING SYSTEM
 The fourth possible type of system is the solar
heating system, but it is still too expensive to be a
viable option.
 Solar heating systems are found in hobby
greenhouses and small commercial firms. Both water
and rock energy storage systems are used in
combination with solar energy.
 The high cost of solar heating systems discourages
any significant use by the greenhouse industries.

30. SOLAR HEATING SYSTEM
Solar heating system used for greenhouse heating

31.  This consists of a flat black plate (rigid plastic, film plastic, sheet
metal, or board) for absorbing solar energy. The plate is covered on
the sun side by two or more transparent glass or plastic layers and
on the backside by insulation.
 The enclosing layers serve to hold the collected heat within the
collector.
 Water or air is passed through the copper tubes placed over the
black plate and absorb the entrapped heat and carry it to the
storage facility.
 A greenhouse itself can be considered as a solar collector. Some of
its collected heat is stored in the soil, plants, greenhouse frame,
floor, and so on.
 The remaining heat is excessive for plant growth and is therefore
vented to the outside. The excess vented heat could just as well be
directed to a rock bed for storage and subsequent use during a
period of heating. Collection of heat by flat-plate collector is most
efficient when the collector is positioned perpendicular to the sun
at solar noon.
 Based on the locations, the heat derived can provide 20 to 50% of
the heat requirement.

32. Thank You