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Types of warm air heating systems:
Gravity systems operate by air convection. Heated air expands, becomes lighter and rises. Cooler air is dense and falls. The difference in air temperature creates the convection or motivation for air movement. The return of a gravity system must be unrestricted. Even a filter is too restrictive. This is necessary to develop positive convection and better distribution. The furnace consists of a burner compartment (firebox) and a heat exchanger. The heat exchanger is the medium used to transfer heat from the flame to the air, which moves through the house. Besides being the medium of heat transfer, the heat exchanger keeps the burned fuels separate from the air. Evaluating the heat exchanger in any warm air heating system is difficult, evaluation is even more difficult in a gravity system. Inspect the cabinet or jacket for burned paint, distortions
and/or scorch marks. Scorch marks may
indicate that the firebox is not performing its designed function and heat is
escaping. Such problems indicate that
the flame is getting beyond the furnace firebox and may be creating fire
safety, economic and functional problems. Heat Exchangers -- Forced Air System As mentioned with gravity warm air heating systems, the heat exchanger is the medium of heat transfer and separates the burned fuel from the air that moves through the house.
The following evidences indicate positive pressure on a heat exchanger.
Blocked Chimneys Or Vent Connectors Blocked or partially blocked vents or chimney flues pose health and safety problems, and are relatively simple to recognize. With the heater on, a blocked or partially blocked flue will create excessive heat and moisture in the basement air, especially at the heater. When combustible gas exhaust backs into the basement from the flue, the humidity of the basement air is dramatically increased because the cooler basement air has less ability to hold moisture. If you cannot operate the heater, you may still recognize present or previously blocked chimney problems. In a gas-fired unit, the area around the bonnet or draft hood will have evidence of rust. This is a result of hot (greater than 400 degrees) exhaust gases contacting the cooler metal around the bonnet or draft hood. You will not see the moisture because it evaporates quickly on the warm metal, however, the rust will be evident. In an oil-fired unit, there may be excessive soot around the burner view door and around the barometric draft control. This is a major health, safety and operational defect. Puffbacks Caused by late or faulty ignition, such as a fouled nozzle or weak ignition arcing. Intermittent puffs of unburned fuel and combustion gases from the burner and combustion area back through the burner and any other open areas. Unburned fuel oil that is deposited in the combustion chamber is likely to ignite when the burner finally comes on. Adjustments and service usually resolves this problem. Spillage Caused when the flow of the combustion products and air in the chimney are reversed due to reduced air pressure. Causes could be whole house fans, exhaust fans, or other combustion appliances. This is similar to a partially blocked chimney, however, it is typically not as bad and it is usually intermittent. The length of the vent pipe is critical. The longer the vent pipe is from the heater to the chimney, the more likely it is for heat to dissipate and develop condensation, which will cause corrosion and vent pipe failure. Ten feet is long; longer than 15 feet is not allowed; and the shorter the pipe is, the better it is. Long, horizontal vent pipes also cause condensation in the chimney. This may explain water coming down the chimney in many situations. Horizontal vent pipes should always slope up from the heater to the chimney. Negative slopes may cause drafting problems, which could endanger the health of the occupants. Check the draft by carefully touching the metal vent pipe. This pipe should be hot when the unit is operating properly. If the pipe is cool three feet to six feet from the heating appliance, there may be problems with drafting. Potential causes may include:
Gas-fired systems have draft diverters to balance the exhaust draft. Oil-fired systems have barometric draft controls to balance the exhaust draft. Lack of a barometric draft control on the vent of an oil-fired heating system may affect the operation of the burner because of possible restricted flow to the chimney. This restricted air, which causes some amount of backpressure to the burner compartment, may alter the flow of air through the burner that is needed for proper combustion of the fuel oil. The repercussion of this condition is usually an inefficient burner and fouled nozzles about one month after proper servicing of the burner. If more than one heating appliance is using one flue, there are some general conditions that apply. Check local codes and safety requirements in your area.
Check the draft at each appliance when both appliances are operating and have had time to stabilize. Limit Controls -- Warm Air Heating Systems Limit controls typically have three settings—two lower settings turn the fan on and off and the high setting turns the burner off. A typical cycle would be as follows:
The thermostat and limit controls generally operate at 24 volts. This is a low voltage system. A typical 120-volt circuit is run to a transformer and the voltage is changed from 120 to 24 volts. These transformers can be found on the outside of the electric service panel or adjacent to the heater One cubic foot of gas requires 10 cubic feet of air to burn. Technicians typically set up gas heating appliances at 13.5 to 15 cubic feet of air to ensure that there is enough air to provide complete combustion, or as complete as the appliance can provide.
The waste consists of water in a vapor state, carbon dioxide, carbon monoxide and traces of aldehydes, such as formaldehyde. Check for sources of combustion air—a louver or vent into the utility room; open ceiling that can draw air from the floor joist system, which may extend throughout the basement; or a louvered or permanently opened window to the exterior. If there are not adequate interior sources, outside air will be necessary. Logic is your best guide. The source must be permanent. Inspection Checklist
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When a heat exchanger cracks or fails, there are two points
to consider, involving air pressure. When cracks and
rust, etc. are located at air pressure points, such as
at turns and/or at the top of the heat exchanger
(where the
direction of the exhaust is not directed toward the
flue), you may
get combustion gases forced into the flow
of the air in the plenum
and ductwork.