According to the U.S. Department of Energy, natural gas
prices will triple this winter. (For more information see Headlines in the
August 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 your
heating system tuned up for the long, cold winter ahead.
Heating systems are complex compared to materials handling
systems or watering equipment. Everything, including the burner, controls, fuel
supply, heat distribution system and chimney has to be clean and in proper
adjustment to get maximum heat output. An increase in efficiency of only 2 percent
in the heating system for a 30- x 100-foot greenhouse can save 285 therms of
natural gas, 330 gal. of propane or 200 gal. of fuel oil this winter. This will
more than pay for a tune-up.
A competent service person should clean and adjust all
furnaces and boilers at least once each year, preferably before the heating
season begins. The following are the most important things that should be
checked.
Components to Service
Gas Burners. The
combustion process is similar for natural gas and propane. Due to the
differences in heat output of the fuels, different burners are needed. Ideally,
the flame should burn as blue as possible for maximum heat. Yellow tipping of
the flame is caused by insufficient primary air and indicates incomplete
combustion that causes soot formation. A dirty orifice or one that is out of
line can reduce the primary air.
Unstable flames from the burner ports are an indication of
air drafts from leaks in the firebox or a cracked heat exchanger. This can lead
to incomplete combustion and smoke if the flames impinge on cool surfaces. The
leaks or cracks can be a source of flue gases that get into the greenhouse and
injure plants.
Gas Supply. Inlet
pressure and manifold pressure should be checked to be sure they are properly
set. Low pressure may occur when several heating units are supplied from a
single source.
Supply pipe size needs to be adequate for the Btu-per-hour
output of each unit. After visual inspection of the supply piping, turn on the
gas and check for leaks at all fittings using a water/soap solution. During the
winter, maintain adequate gas in propane tanks, as low pressure can lead to an
inadequate supply to burners on cold nights.
Gas valve, overheat control and standing pilot or spark
ignition should be checked for proper operation and safety. The burner manifold
and nozzles should be brushed off with a bristle brush and cleaned with
compressed air. Electrical connections should be tight and clean.
Oil Burners. Only a
few parts typically need maintenance on oil burners. The firebox is designed
for a limited range of heat capacity; therefore, the nozzle should be replaced
with one that has the manufacturer's recommended type, capacity and spray
pattern.
The filter removes particulate matter, mold and slime before
the 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 home
center.
The fuel pump provides the pressure to force the oil through
the nozzle. There are very few problems unless the pump is worn. The pressure
that the pump develops should be between 100 and 120 psi. It can be checked
with a pressure gauge in the bleeder plug. Pressure that is too low will
produce a large droplet size that may cause smoke.
The new 14,000-volt electronic igniter is better than the
older style 10,000-volt transformer for providing a spark source. Both should
produce a 1-inch spark jump between the two contacts. Use an insulated
screwdriver for safety. A gap of less than 1/2 inch usually indicates ignition
problems.
Electrodes provide the spark for igniting the fuel in the
firebox. They need to be cleaned and checked for cracks and should be adjusted
to the position specified in the maintenance manual. The distance from the
nozzle tip and spark gap are critical for smooth ignition.
Flame monitoring devices in the firebox can be either light
or heat sensors. Smaller heating units usually use a cadmium phototube (cad
cell) that sends a signal when activated by light from the flame. Larger units
employ sensors that detect the heat in the fire. Both types need to be
positioned 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 relay
should 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 of
old oil, condensation and bacteria fill the filter quickly with a black slime.
Biocides, filtration or water stripping (drawing off the water from the bottom
of the tank) are effective ways to reduce the problem.
Low oil temperature from outside tanks is another problem
causer. As oil cools below 20º F, its viscosity (resistance to flow)
increases. Water droplets in the oil freeze and paraffin precipitates out
clogging 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 an
adequate draft, the chimney or stack should be at least 2 feet above the peak
of the greenhouse or headhouse. A rule of thumb is that for each 1 foot of
horizontal run of the connector, you need 2 feet of vertical connector. The top
of the chimney should be at least 8 feet above the top of the furnace. Add a
chimney cap to prevent backdrafts.
Chimney size should be the same size as the furnace or
boiler connection. Connections should be airtight and held in place with three
sheet metal screws.
Make-up Air. For
each 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 you
have a separate combustion furnace or boiler, provide a make-up air vent or air
intake pipe near the heating unit. It should be sized to 1 sq. inch per
2,000-Btu-per-hour capacity. The inlet should be above the snow line and screened
to keep out rodents.
Combustion Efficiency Tests
To help in making final adjustments, an efficiency test
should be made. This 10-minute procedure can help to save considerable fuel
over a heating season.
For atmospheric injection gas burners, carbon dioxide level,
flue gas temperature and draft diverter measurements are taken, and gas
pressure and secondary air adjustments are made to get peak efficiency. For
large system power gas burners with forced draft, adjustments are made at the
blower. Testing to meet state statutes, Occupational Safety & Health
Administration standards and insurance company requirements is also required.
On oil burners, measurements of draft intensity, smoke
level, flue gas temperature and carbon dioxide content are usually taken. Draft
intensity varies with outside temperature, wind speed, chimney height and
temperature. For smaller size units, a draft of 0.04-0.06 inches of water
column in the first section of the flue pipe is desirable. Too little draft
increases smoke levels, too much draft reduced efficiency.
A barometric damper in the flue pipe is used to reduce
fluctuations in the draft. Smoke is an indication of incomplete combustion. A
smoke scale reading of one or two is desirable. Higher levels result in
deposits of soot on heat absorbing surfaces.
A flue gas carbon dioxide reading of 10 percent or higher
and a temperature of 600º F or lower will give an efficiency of about 80
percent. Adjustments to the burner motor and air gate opening will help to
obtain the highest efficiency.
Final Thoughts
Two other system components should also be checked. Clean
the sensing element and any exposed contacts in thermostats several times per
year. Locate all thermostats together in the plant zone to give the best
temperature response.
Insulate hot water distribution pipes in unheated areas.
Each uninsulated linear foot of a 2-inch supply pipe will loose about $4 worth
of heat this winter.
Maintenance is an ongoing process with all heating units.
Use a log sheet to record all important operating data, procedures for testing
and corrective measures that were taken.
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