Checking Heating System Efficiency

John W. Bartok, Jr.Fall is a good time to have your heating system serviced in preparation for the upcoming cold winter months. The cost is usually more than offset by the 5-10 percent gain in efficiency. The following are a few of the uncommon things that may be overlooked by service personnel.

Efficiency test
This 10 minute test should be done by a technician each time a furnace or boiler is serviced. It consists of measuring the flue gas temperature, carbon dioxide content, smoke level and draft reading over the fire. Adjustment of the air to the burner can improve efficiency and reduce carbon buildup on heat exchangers and lower emissions.

A 2 percent increase in efficiency in fuel usage in a 30- by 100-foot greenhouse saves about 200 gallons of fuel oil during the winter. When done on a regular basis, the efficiency test can indicate when problems are beginning to occur.

Low voltage can cause unit heater motors to burn out by overheating the windings.Adequate voltage to the heating unit
Low voltage causes motors to burn out by overheating the windings. It can also affect the operation of the transformer creating a weak spark. Voltage at the heating unit should be at least 110 volts.

Two causes of low voltage are inadequate power to the electric distribution panel in the greenhouse and too small a wire size that feeds the furnace. Low voltage is quite common in hoop houses where long wires are needed to power heaters that are located at opposite ends.

Barometric damper
The draft on the fire is affected by the height of the chimney, the temperature of the flue gases and the temperature and wind speed of the outside air. Increasing the height, flue gas temperature or wind speed increases the draft. Draft is also greater in the winter due to cold temperatures. Excessive draft increases the heat loss when the heating unit is in the standby mode.

A barometric damper (oil-fired heating units) or draft hood (gas-fired heating units) should be located in the first 12 inches of the stack nearest the burner. Its purpose is to maintain a constant draft on the fire. Gordon Siebring at Siebring Manufacturing has reported that savings of up to 15 percent can be achieved with the installation and proper adjustment of a barometric damper.

Fuel pressure
The normal operating pressure of a fuel oil pump is 100 pounds per square inch. On atmospheric types of burners, the usual ranges of manifold gas pressure are 3 to 5 inches of water for natural gas and 10 to 12 inches for propane. The pressure should be checked and adjusted if necessary. Higher pressures than those recommended result in over-firing and possible damage to the heating units.

Make-up air
Recommendations to tighten up greenhouses for winter may adversely affect the operation of a furnace or boiler. It is not uncommon to hear of greenhouses that freeze up during a cold night even if a heater has been operating. This is usually due to the lack of oxygen for combustion.
 
How much air is necessary for good combustion? With most gas burners adjusted to operate with 50 percent excess air, 20 cubic feet per 1,000 Btu of heater input is generally considered adequate. With fuel oil, 1,400 cubic feet per gallon of No. 2 fuel oil is required for proper combustion and venting.
 
It takes about 250,000 Btu per hour of heater capacity to heat a typical 30- by 100-foot hoop house with a double polyethylene cover to maintain a 65ºF temperature on a night when it is 0ºF outside. A 250,000 Btu per hour gas heater requires about 5,000 cubic feet of air per hour to supply oxygen to the burner. Without any make-up air the 30,000 cubic feet of air in the greenhouse is used up in six hours. The heater only operates at peak efficiency for part of that time and then incomplete combustion occurs and heat output decreases. It may also result in pollutants, such as ethylene gas and sulfur dioxide being released into the greenhouse and causing damage to the plants.
 
A motorized louver or 6-inch PVC pipe that provides make-up air near the burner is necessary. Make sure that the intake is located so that it is not blocked by snow.

Flue pipe and chimney
To maintain adequate draft and reduce the effect of wind turbulence, the top of the chimney should extend at least 2 feet above the roof of the greenhouse or 3 feet above a 10-foot horizontal distance from the roof.
 
Install a cap on the top of the chimney to prevent down drafts and possible air pollution injury to plants. Avoid decreasing the size of the flue connector from the heater to the chimney as this affects the draft and capacity.

Environment control sensors
Thermostats and sensors should be cleaned several times a year. Blow off dust with compressed air. Check for accuracy by placing the sensors in an ice bath or comparing them to an accurate digital thermometer. Be sure that the sun shield is in place and that the location is close to the plants. Aspiration of the sensors can reduce the differential between the high and low setting by several degrees. 

John Bartok Jr. is faculty emeritus, University of Connecticut, Department of Natural Resources Management and Engineering, jbartok@rcn.com.