Greenhouse Heating

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Greenhouse Heating

• Principles and Hardware

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Important Heat Units

• Btu British thermal unit amount of
  heatrequired to raise 1 lb of water 1 ºF
• Boiler horsepower 33,475 Btu
• Calorie is the amount of heat required to
  raise1 g of water 1 ºC
• 1 Kcal 1000 cal 3.968 Btu
• 1 Joule .239 cal .00095 Btu
• 1 Watt 1 J per second

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Heating Basics

• You must add heat at the rate it is lost
• Heat is lost by CONDUCTION, Infiltration, and
  Radiation
• Many different factors play a role in the
  amountof heat that is lost (a few are listed
  here)
• Type of glazing
• Frame material choices
• Local climate

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Glazing Heat Loss

• Worst single layer of polyethylene film or
  single layer of glass
• Best 3 layer polycarbonate panels
• Double Layer of Poly with dead air space brings
  the heat loss down to .70 Units

Heat Loss Units
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Greenhouse Air Infiltration

• Varies with glazing material choice
• You must also consider the quality and age ofthe
  construction
• Variability among greenhouses is high
• EXAMPLE  An older glass greenhouse mayhave air
  infiltration rates 8 times that of a newhigh
  quality double-layer plastic film coveredhouse.

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Types of Heating Systems

• Unit Heater
• Central Heat
• Radiant Heat
• Solar Heat

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Important Heating Notes
CH2O(s) O2(g) -----gt CO2(g) H2O(g)
Incomplete Combustion
CO
Sulfur Dioxide
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Unit Heaters

• Cost including installation labor is 0.50 -
  1.00/sq. ft.
• More ideally suited for smaller greenhouses.
• As greenhouse operations get larger
  centralheating systems become more
  economicallyfeasible.
• Components
• Fuel burner
• Heat exchanger
• Distribution
• Controls

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Unit Heaters

• Heat supplied as needed
• Available in horizontal and vertical designs
• Must have a constant supply of oxygen
• Exhaust stack must be tall enough above
  thegreenhouse roof to permit dissipation of
  thesmoke without re-entry into the greenhouse.
• Openings for fresh air must be provided for unit
  heaters---or else a heater can use up all the
  oxygen in a house quickly.

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Common Fuelsfor Unit Heaters

• No. 2 oil
• Kerosene
• LP gas
• Natural gas
• Many types of unit heaters can be refitted to
  burn a different type of fuel.

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Unit Heaters

• You go outside to your doubly poly greenhouse
  early one winter morning and realize that the
  heater is off. You are naturally concerned about
  your Poinsettias as the Holidays are approaching.
  What should you do first? and Why?

ALLOW FRESH AIR IN THE GREENHOUSE
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HAF (Horizontal Air Flow)

• Horizontal Airflow
• Utilized to establish uniform temperatures in
  greenhouses
• For individual greenhouses two rows of fans
  installed along the length of the greenhouse
• Fans should be 2-3 feet above the plants
• For ridge and furrow fans should be installed
  along the center of the greenhouse

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Central Heat

• Consists of one or more boilers in a central
  location (steam or water)
• Cost 2.50-3.25 / sq. ft. of floor space
• Boilers can burn cheaper fuels than unit
  orradiant heaters
• Engineer Redundancy into the system whenever
  possible.

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Suitable Boiler Fuels

• Wood Chips
• Logs
• Coal
• No. 4,5, and 6 fuel oil
• Many boilers can be retrofit to burn other fuels
  at a nominal cost.

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Other Boiler Notes

• Hot water boilers most popular worldwide
• Temperature uniformity is greater than in steam
  systems
• Larger reserve of heat
• Due to lower operating temperatures you can place
  the pipes closer to the plants
• For some crops Boilers can also be a good source
  of CO2 for greenhouse atmosphere enrichment.

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Boiler Components

• FIREBOX where fuel is burned
• Flue provides a way for smoke, from
  thefirebox, to vent to the outside air
• Heat exchanger network of tubes either filled
  with or surrounded by water.

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Heat Distribution

• Pipe size and quantity
• American greenhouse 2 Fe pipe 180 F
• Dutch greenhouse 2 Fe pipe 203 F
• Steam delivery smaller pipe 215 F
• Amount of pipe necessary depends on greenhouse,
  crop, local climate, and type of pipe to be
  utilized
• Tables 3-3 and 3-4 give you the info necessary to
  estimate pipe length once you have calculated the
  necessary total heat requirement for the
  greenhouse.

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Radiant Heater System

• May be cheaper to operate than unit heatersEmit
  infrared radiation
• Plants and other objects are heated first
  beforethe air, so air temperatures can be 5 to
  10 ºFcooler than a conventional house
• Available in sizes from 20,000 to 120,000 Btu/hr

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Radiant Heat Systems

• Place above plants in lines running along the
  length of the greenhouse
• Can be spaced as far as 40 ft apart Composition
  of reflector is important

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Radiant Heat Advantages

• Newer systems do not emit visible red light
• More efficient fuel combustion
• Cooler air temps reduce DF why does this
  saveyou money?
• Only 25 electrical consumption of othersystems.

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Solar Heat

• Tax incentives for solar heating are gone, until
  this winter petroleum fuel product costs were
  low.
• There are commercial greenhouses with solar power
  however some real costs may not have been
  included
• Since fewer structures like this are built
  construction costs are generally higher.

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Bench Heating
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Heating Calculations