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 Mobile View HEATING SYSTEMS ANTI SCALD VALVES BOILERS, HEATING BOILER OPERATION DETAILS BOILER CONTROLS & SWITCHES Air Bleeder Valves Aquastat Functions BOILER LEAKS CORROSION STAINS BOILER OPERATING PROBLEMS BOILER PARTS LIST Cad Cell Relay Switch Flame Sensors Circulator Pumps & Relays Expansion Tanks Limit Switches Mixing Valves Relief Valves - TP Valves Spill Switches Stack Relay Switch Zone Valves CARBON MONOXIDE/DIOXIDE CARBON MONOXIDE WARNING CHIMNEY INSPECTIONS CHIMNEYS & Flues - Asbestos Transite Pipe DRAFT HOODS - gas fired DRAFT REGULATORS - barometric dampers DUCT SYSTEMS DUCT INSULATION, Asbestos Paper DUCTS, Asbestos Transite Pipe DUST FROM HVAC? ELECTRICAL POWER SWITCH FOR HEAT FURNACES DUCT SYSTEM DEFECTS FAN LIMIT SWITCH FURNACE CONTROLS & SWITCHES FURNACE OPERATION DETAILS HEAT EXCHANGER LEAKS STACK RELAY SWITCHES GAS PIPING, VALVES, CONTROLS GAUGES ON HEATING EQUIPMENT HEAT LOSS INDICATORS HEATING COST SAVINGS HEATING LOSS DIAGNOSIS-BOILERS HEATING LOSS DIAGNOSIS-FURNACES HEATING OIL CLOUD WAX GEL POINT HEATING OIL SLUDGE HEATING SYSTEM INSPECTION DIAGNOSIS REPAIR HEATING INSPECTION CONCEPTS HEATING INSPECTION PROCEDURE INSULATION MIXING VALVES NO HEAT - BOILER / FURNACE DIAGNOSIS OIL BURNERS OIL BURNER NOISE SMOKE ODORS OIL FUEL TYPES & CHARACTERISTICS OIL TANK LEAKS OIL TANK SLUDGE OIL TANKS, BURIED PLASTIC HEATER VENT RADIANT HEAT Floor Mistakes to Avoid RELIEF VALVES - TP Valves Safety Recalls BLUERAY Recall CHIMNEYS & Flues - Asbestos Transite Goodman HTPV RECALL Lennox WARNING Weil McLain RECALL SAFETY DURING HEATING INSPECTION SPILL SWITCHES - Flue Gas Detection STAIN DIAGNOSIS on Indoor Surfaces STEAM HEATING SYSTEMS Cad Cell Relay Switch Flame Sensors Mixing Valves Relief Valves - TP Valves Sight Glass, Steam Boiler Pressure Switch, Steam Boiler Spill Switches Stack Relay Switch Steam Vents LOW WATER CUTOFF CONTROLS WATER FEEDER VALVES TANKLESS COILS THERMAL TRACKING THERMOSTATS Transite Pipe Chimneys & Flues ZONE VALVES More Information InspectAPedia TM Home & Site Map Air Conditioning InspectAPedia Bookstore Electrical Environment Exteriors Heating Home Inspection Insulate Ventilate Interiors Mold Inspect/Test Plumbing Water Septic Roofing Structure Accuracy & Bias Pledge Contact Us |
 Guide to Stack Relay Switches on Oil Fired Heating Equipment
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Oil Burner Stack Relay Switches Explained Older oil burners used to heat a warm air furnace, a heating boiler, or a water heater may use a Stack Relay to prevent sending fuel into the heater if it has not been successfully ignited. Stack relays are an older type of flame sensor than the Cad Cell Relays which we discussed above, but they accomplish the same purpose (turn off the oil burner if the flame is not established). The "stack relay" is a bimetallic spring inserted into the flue vent connector located usually quite close to the heating boiler between the boiler top and the chimney. The bimetallic spring warms in response to hot oil burner exhaust, confirming that combustion is taking place. If combustion is not occurring a timer inside the stack relay turns off the oil burner to prevent flooding of the combustion chamber with un-burned oil. |
How Stack Relay Switches Work: Older oil burners may use a Stack Relay to accomplish the same purpose (turn off the oil burner if the flame is not established). The "stack relay" is a bimetallic spring inserted into the flue vent connector located usually quite close to the heating boiler between the boiler top and the chimney. The bimetallic spring warms in response to hot oil burner exhaust, confirming that combustion is taking place. If combustion is not occurring a timer inside the stack relay turns off the oil burner to prevent flooding of the combustion chamber with un-burned oil.
The stack relay switch shown in this photograph is attached at the flue vent connector which in turn passes flue gases from an old "octopus" type furnace to the chimney. (See the black box with its red reset button.) This same switch might also be found on some oil-fired heating boilers (hot water, hydronic, baseboard, or radiator heat) not just on furnaces (hot air heat).
Knowing how to reset the stack relay can avoid a costly "no heat" service call or it can keep the heating boiler running sufficiently to keep the building warm while waiting for the heating service technician. There are actually two resets that can be performed on an oil burner stack relay. A gray box with a red reset button housing the stack relay and its reset switch will be found mounted on the flue vent connector if this control is in use. The first is simply to press the red reset button that protrudes through the cover of the stack relay box itself. As with cad cell relays and other heating system "reset" buttons, if the oil burner does not turn on and run continuously and normally (no smoke, no loud noises, etc.) for at least 5 or 10 minutes after resetting the relay or pressing the reset button, DO NOT keep resetting the system since doing so can flood the combustion chamber with un-burned heating oil - a dangerous condition.
Why might you need to remove the cover and look for a second, "internal" reset lever inside of this control? Sometimes simply pressing the reset button through the cover of the stack relay switch will not turn on the oil burner, but the problem could be in the switch itself, not the burner.
The second and more thorough reset of the stack relay can be accomplished by removing the cover on this control box (SAFETY WARNING LIVE VOLTAGE electrical connections are inside this box - a shock hazard). The stack relay bimetallic spring assembly can be "reset" by removing the cover of the assembly and pulling the (visually obvious) reset lever out away from the flue vent connector, and then carefully and gently releasing it again. Watch out for sensitive stack relays: sometimes these switches trip off during oil burner testing or operation and they can be trick to re-set.
If you're able to get the system running again by using this internal reset, but if the relay switch tends to trip off if you simply tap on the outside of the relay switch when its cover has been replaced, then there is a problem needing further investigation, either with an electrical connection in the switch, or with the switch itself. In this case the stack relay switch may need replacement, but before replacing it see our note below on cleaning the stack relay switch.
Since this oil burner relay switch depends on sensing the heat of the flue gases as they pass through the flue vent connector (or "stack pipe") from the oil burner/combustion chamber in a furnace or boiler up to the chimney, the sensor, a bimetallic spring, is exposed to soot in the oil burner exhaust. If the oil burner has been running poorly, perhaps itself needing cleaning or adjustment, the level of soot in the oil burner exhaust can be quite highb.
Soot produced by oil combustion (or by poor or incomplete heating oil combustion) is itself an insulator. Not only do we need to remove this soot from inside of a furnace or boiler in order to get the most heat from our heating oil into the building's heating distribution system (hot air or hot water), but this same soot can collect on, clog, or insulate the oil burner stack relay switch.
Soot collected on the stack relay switch bimetallic sensor spring, acting as an insulator, can "fool" the swtich into thinking that there is not good combustion (temperatures look too low at the sensor) and thus it might switch off on safety and need to be frequently "reset" to keep the heating system running. It may be necessary to clean the stack relay switch sensor.
During oil burner service for a boiler or furnace, if the heating system uses a stack relay switch (which in fact is rather obsolete), the switch should be removed during system cleaning (when you remove the flue vent connector piping), and its external parts or sensor should be cleaned (normally by light brushing).
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