Is it time to replace your greenhouse heating system?

After labor, heating costs are the largest production item for most growers. New technology, energy efficiency measures and alternate fuels can help to reduce this cost. Heating system technology is continually evolving with new products coming onto the market each year. With fuel prices changing frequently, growers need to evaluate the many options available before making a decision on purchasing new or upgrading existing equipment.

Heating system upgrade

A starting point is age. A furnace or boiler that is more than 10 years old is a good candidate for repair or upgrade. It depends somewhat on how much use it has had and whether regular maintenance has been provided. Repair and replacement parts are generally available for these units.

Efficiency testing of a furnace or boiler involves a simple 10-minute procedure and, if done on a regular basis, can indicate when problems are beginning to occur. Records of temperature and carbon dioxide levels of the flue gases taken at weekly intervals may indicate that carbon is building up on the heat surfaces or air leaks are developing in the combustion chamber.

Before the heating season begins, the furnace or boiler should be cleaned and serviced. The burner blast tube, fan housing and blower wheel should be free of dirt. Leaks into the combustion chamber, especially joints between cast iron boiler sections and around the fire door, should be sealed. The oil filter should also be replaced and carbon removed from heat-transfer surfaces. Manufacturers’ recommendations should be followed in replacing the nozzle and in adjusting the ignition electrodes. A new burner may also be the best option.

On gas burners, servicing consists of cleaning the orifice, the burner, the heat-transfer surfaces and the controls. Gas valves are checked for operation and leaks. Gas pressure is adjusted for the type of fuel used. The pilot light or ignition system is cleaned, soot is removed and the fan and limit controls are checked for safety. Worn parts should be replaced.

Heating system replacement

A furnace or boiler should be replaced when it is no longer safe, when it has an efficiency of less than 70 percent or when its emissions are more than 10 percent above recommended EPA standards.

New oil-fired furnaces and boilers have an efficiency of about 85 percent and new gas ones can get up to 96 percent. This increase in efficiency over existing equipment can go a long way toward paying for the new unit. The condensing unit heater or boiler is a good choice due to its higher efficiency.

Changing to a different fuel can also result in a good payback. Expansion of natural gas service areas is allowing growers to change to a lower-cost fuel. Be aware that if you only operate on a seasonal basis, that you may have to pay the monthly service charge during the summer for having the gas available. Gas costs tend to increase during the winter when the residential use is greatest.

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 hoop houses 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.

Another option that should be considered is the heat storage buffer tank. 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 and coal-fired boilers work particularly well with a buffer tank as they absorb the variable heat output from the combustion process, which is not as easy to control as with fossil fuels.

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.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to sister publication Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com