Grower 101: Heating Systems–Maintenance Pays
According to the U.S. Department of Energy, natural gasprices will triple this winter. (For more information see Headlines in theAugust 2003 issue of GPN.) With the continuing unrest in the Middle East,propane and fuel oil will probably follow suit. Now is the time to get yourheating system tuned up for the long, cold winter ahead.
Heating systems are complex compared to materials handlingsystems or watering equipment. Everything, including the burner, controls, fuelsupply, heat distribution system and chimney has to be clean and in properadjustment to get maximum heat output. An increase in efficiency of only 2 percentin the heating system for a 30- x 100-foot greenhouse can save 285 therms ofnatural gas, 330 gal. of propane or 200 gal. of fuel oil this winter. This willmore than pay for a tune-up.
A competent service person should clean and adjust allfurnaces and boilers at least once each year, preferably before the heatingseason begins. The following are the most important things that should bechecked.
Components to Service
Gas Burners. Thecombustion process is similar for natural gas and propane. Due to thedifferences in heat output of the fuels, different burners are needed. Ideally,the flame should burn as blue as possible for maximum heat. Yellow tipping ofthe flame is caused by insufficient primary air and indicates incompletecombustion that causes soot formation. A dirty orifice or one that is out ofline can reduce the primary air.
Unstable flames from the burner ports are an indication ofair drafts from leaks in the firebox or a cracked heat exchanger. This can leadto incomplete combustion and smoke if the flames impinge on cool surfaces. Theleaks or cracks can be a source of flue gases that get into the greenhouse andinjure plants.
Gas Supply. Inletpressure and manifold pressure should be checked to be sure they are properlyset. Low pressure may occur when several heating units are supplied from asingle source.
Supply pipe size needs to be adequate for the Btu-per-houroutput of each unit. After visual inspection of the supply piping, turn on thegas and check for leaks at all fittings using a water/soap solution. During thewinter, maintain adequate gas in propane tanks, as low pressure can lead to aninadequate supply to burners on cold nights.
Gas valve, overheat control and standing pilot or sparkignition should be checked for proper operation and safety. The burner manifoldand nozzles should be brushed off with a bristle brush and cleaned withcompressed air. Electrical connections should be tight and clean.
Oil Burners. Only afew parts typically need maintenance on oil burners. The firebox is designedfor a limited range of heat capacity; therefore, the nozzle should be replacedwith one that has the manufacturer’s recommended type, capacity and spraypattern.
The filter removes particulate matter, mold and slime beforethe oil reaches the nozzle. It needs to be replaced once or twice per year.Filters can usually be purchased locally from a heating supply store or a homecenter.
The fuel pump provides the pressure to force the oil throughthe nozzle. There are very few problems unless the pump is worn. The pressurethat the pump develops should be between 100 and 120 psi. It can be checkedwith a pressure gauge in the bleeder plug. Pressure that is too low willproduce a large droplet size that may cause smoke.
The new 14,000-volt electronic igniter is better than theolder style 10,000-volt transformer for providing a spark source. Both shouldproduce a 1-inch spark jump between the two contacts. Use an insulatedscrewdriver for safety. A gap of less than 1/2 inch usually indicates ignitionproblems.
Electrodes provide the spark for igniting the fuel in thefirebox. They need to be cleaned and checked for cracks and should be adjustedto the position specified in the maintenance manual. The distance from thenozzle tip and spark gap are critical for smooth ignition.
Flame monitoring devices in the firebox can be either lightor heat sensors. Smaller heating units usually use a cadmium phototube (cadcell) that sends a signal when activated by light from the flame. Larger unitsemploy sensors that detect the heat in the fire. Both types need to bepositioned accurately and cleaned to remove soot.
Safety devices are an important part of all heating systems.Fan and limit switches, aquastat, combustion relay and safety timing relayshould be checked for positive operation using a test light or multimeter.
Fuel Oil Supply.Twenty percent of service calls are a result of dirty fuel. A combination ofold oil, condensation and bacteria fill the filter quickly with a black slime.Biocides, filtration or water stripping (drawing off the water from the bottomof the tank) are effective ways to reduce the problem.
Low oil temperature from outside tanks is another problemcauser. As oil cools below 20º F, its viscosity (resistance to flow)increases. Water droplets in the oil freeze and paraffin precipitates outclogging the filter. Moving the tanks inside, adding a fuel treatment(kerosene), applying heat tape (check building codes to see if this is allowed)and raising pump pressure slightly will reduce the problem.
Chimneys. To get anadequate draft, the chimney or stack should be at least 2 feet above the peakof the greenhouse or headhouse. A rule of thumb is that for each 1 foot ofhorizontal run of the connector, you need 2 feet of vertical connector. The topof the chimney should be at least 8 feet above the top of the furnace. Add achimney cap to prevent backdrafts.
Chimney size should be the same size as the furnace orboiler connection. Connections should be airtight and held in place with threesheet metal screws.
Make-up Air. Foreach gallon of fuel oil burned, 800 cu.ft. of air is needed. With modern,airtight greenhouses, a separate source of make-up air should be provided,otherwise incomplete combustion can occur with reduced heat output. Unless youhave a separate combustion furnace or boiler, provide a make-up air vent or airintake pipe near the heating unit. It should be sized to 1 sq. inch per2,000-Btu-per-hour capacity. The inlet should be above the snow line and screenedto keep out rodents.
Combustion Efficiency Tests
To help in making final adjustments, an efficiency testshould be made. This 10-minute procedure can help to save considerable fuelover a heating season.
For atmospheric injection gas burners, carbon dioxide level,flue gas temperature and draft diverter measurements are taken, and gaspressure and secondary air adjustments are made to get peak efficiency. Forlarge system power gas burners with forced draft, adjustments are made at theblower. Testing to meet state statutes, Occupational Safety & HealthAdministration standards and insurance company requirements is also required.
On oil burners, measurements of draft intensity, smokelevel, flue gas temperature and carbon dioxide content are usually taken. Draftintensity varies with outside temperature, wind speed, chimney height andtemperature. For smaller size units, a draft of 0.04-0.06 inches of watercolumn in the first section of the flue pipe is desirable. Too little draftincreases smoke levels, too much draft reduced efficiency.
A barometric damper in the flue pipe is used to reducefluctuations in the draft. Smoke is an indication of incomplete combustion. Asmoke scale reading of one or two is desirable. Higher levels result indeposits of soot on heat absorbing surfaces.
A flue gas carbon dioxide reading of 10 percent or higherand a temperature of 600º F or lower will give an efficiency of about 80percent. Adjustments to the burner motor and air gate opening will help toobtain the highest efficiency.
Two other system components should also be checked. Cleanthe sensing element and any exposed contacts in thermostats several times peryear. Locate all thermostats together in the plant zone to give the besttemperature response.
Insulate hot water distribution pipes in unheated areas.Each uninsulated linear foot of a 2-inch supply pipe will loose about $4 worthof heat this winter.
Maintenance is an ongoing process with all heating units.Use a log sheet to record all important operating data, procedures for testingand corrective measures that were taken.