Upgrading the Greenhouse Heating System

Heating system technology is continually evolving with new products coming on the market each year.  With fuel prices increasing rapidly, growers need to evaluate the many options available before making a decision on purchasing new or replacement equipment.  This column looks at some of the more recent concepts and equipment that are now on the market.

Heating system replacement
A furnace or boiler should be replaced when it is no longer safe, when it has an efficiency of less than 75% or when its emissions are more than 10% above recommended EPA standards.  Design and installation of a new system should be guided by a professional that is familiar with greenhouse conditions and environment control.  You should also expect startup training and maintenance instructions.

System selection
When selecting equipment for a gutter-connected greenhouse or multiple freestanding greenhouses, consider installing a central boiler system.  This provides many options including easy distribution of heat through water to air heaters, sidewall fin piping or root zone radiation.  For individual hoophouses that are operated mainly during the fall and spring growing season, hot air furnaces or unit heaters are still the best choice in most cases as the heating system does not require draining for the winter.  Air circulation is needed with both systems to provide uniform temperature.

Condensing boilers and heaters
Water vapor is a byproduct of the combustion of gas or oil. This water vapor which has a temperature of 400 to 500ºF along with the other byproducts normally goes up the stack and is exhausted into the atmosphere.  A condensing boiler incorporates an extra heat exchanger in the flue gas exhaust system so that the water vapor condenses back to a liquid. This process captures up to 8000 Btu of heat per gallon of condensate which is as much as 13% of the original fuel energy.  The condensate heats the incoming air (if an air to air heat exchanger is used) or pre-heats the water (if an air to water heat exchanger is used). After the heat is removed, the low-temperature condensate water can be drained through a PVC pipe.

Condensing boilers work best where the return water is cool such as in a root zone system where the water temperature is less than 100ºF as compared to a fin radiation system where the water temperature may be 140ºF or higher.  More of the heat can be transferred to the water. The efficiency is also affected by the humidity of the air.  Under the right operating conditions, condensing boilers can achieve an efficiency of 95% or more.

Gas fired condensing unit heaters are now available with an efficiency of 93%.  These remove excess flue gas heat and put it in the greenhouse. They are available in several heat outputs.

Condensing boilers and heaters are more expensive than conventional boilers but with fuel prices escalating, the payback period continues to decrease.
 

Low-mass boilers
The amount of water storage in today's boilers continues to decrease.  This results in a unit with smaller physical dimensions that takes up less space in the boiler room.  It also means that there is less heat loss when it is on standby.

Although cast iron gives a longer life, many manufacturers now use steel for the firebox, heat exchanger to reduce the mass.  Some also incorporate an extra pass for the flue gases to reduce the flue gas temperature more.  With low mass boilers, there is more concern with thermal shock where the water returning from the piping system cools the firebox too much.  This can put a stress on welded joints and castings.

Combustion technology
In a conventional burner the fuel is injected continuously under pressure.  It mixes with the air to the correct fuel-air ratio.  Pulse units have been developed that operate similar to a combustion engine.  The fuel-air mixture in a pulse burner is injected into the firebox at 60 to 70 times a second.  An ignition spark is used to start the burning process but once the unit heats up, it operates without the spark.  Pulse burners tend to have a higher efficiency due to the greater uniformity of the fuel-air mixture and the finer atomization.  Efficiencies of over 90% are common.

Combustion of fuel requires air to provide the oxygen.  About 10 cubic feet of air per minute is needed per boiler horsepower (33,475 Btu).  In a conventional furnace or boiler this air is generally supplied from cracks around the doors or vents or from a louver located in a sidewall near the heating unit.  This cold air can create drafts that affect plant growth.  The general recommendation is to provide 1 square inch of opening per 1000 Btu/hr capacity of the heating unit. 

Separated air burners get all of the make-up air from outside the greenhouse through a wall vent and piping that is directly connected to the burner.  This supplies dry air which is less corrosive than the normal moist greenhouse air.  It can also increase efficiency slightly.

Heat storage buffer tank
This technology has been around since the 1970's when it was part of many solar collection systems.  It is now being applied to greenhouses and industrial plants.  A large insulated water tank is made part of the heating system.  Hot water from the boiler is circulated through a heat exchanger to heat the water in the buffer tank.  At night when heat is needed, the hot water from the tank is circulated through the heat pipes or unit heaters in the greenhouse.  This system will allow a smaller boiler to be installed as it can be operated continuously day and night.  Wood fired boilers work particularly well with a buffer tank as they absorb the variable heat output from the combustion process that is not as easy to control as with fossil fuels. 

Controls
Solid state controls are now part of most heating systems.  They are more accurate and can integrate more control functions.  Water temperature modulation should be incorporated into most boiler systems as it allows a lower temperature water to be circulated through the radiation as the greenhouse heat needs decrease during the day.  This reduces overheating with a subsequent energy savings.  Adjusting the water temperature based on the outdoor temperature and predicted weather conditions can save considerable fuel over the heating season.  Purchase a boiler with a turn-down ratio of at least 5 to 1.

Upgrading the heating system should pay for itself with less fuel usage.  Review the options in equipment before making a choice.

John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs, CT – 2013.