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HEATING SYSTEMS ANTI SCALD VALVES BOILERS, HEATING 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 |
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The warm air furnace fan limit switch is a control which determines when the hot air furnace blower assembly turns on and off. The switch prevents the furnace blower from sending chilly air into the building if the oil or gas burner has not sufficiently heated up the furnace heat exchanger and supply air plenum. The fan limit switch is also a safety control which protects the furnace heat exchanger from damage by turning the burner off on the furnace gas or oil burner if the temperature inside the warm air supply plenum (just above or just next to the heat exchanger) gets too high. This would be an unusual condition but might occur if air ducts were blocked or if someone has fouled up the system controls. The fan limit control switch is usually found on both oil and gas fired heating furnaces.
In the photographs above we show to illustrations of a hot air furnace fan limit switch as you're likely to find one at a typical furnace.
A fan limit switch on a gas fired furnace is shown in the left hand photo. You can see the black switch body just above the gas regulator valve. The dial at the right of the switch contains the fan on, off, and limit temperature settings. The white button you can see at the lower right corner of the furnace fan limit switch is a manual override which can cause the blower fan to turn on and run continuously regardless of furnace temperature. On systems where we have installed high quality air filtration to address an indoor air quality issue, and where the fan is rated for continuous duty, we may pull this switch out to keep the fan on continuously. Usually the fan limit switch of this type has a silver cover hiding the switch details but with the white "fan override" button projecting through the cover and visible.
A fan limit switch on an oil fired furnace is shown the right hand photo where we can see about 3/4 of the silver colored dial where the fan limit switch settings are made. The switch is about in the center of the photo, and is partially hidden by a low voltage transformer and a metal electrical junction box which are mounted at the right side of the furnace cabinet. (The soot and foil tape above the oil burner assembly also tell us that this system has been operating improperly with back pressure in the combustion chamber.)
Furnace Oil-burners use either a cad cell or stack relay to confirm that the furnace oil burner is operating properly and to avoid flooding the combustion chamber with un-burned oil.
Flame sensing devices on oil-fired heating appliances: modern oil-fired heating furnaces, boilers, and water heaters use a Cadmium Cell sensor, usually located inside the oil burner tube, to "see" the presence of flame and thus to assure that the oil burner assembly stops pumping oil into the combustion chamber if flame ignition is unsuccessful. The cad cell itself, that is the little cell that "sees" the oil burner flame, is wired (often by a yellow wire) to the cad cell relay switch (see photo) which is usually a gray box with a red "reset" button located on top of or alongside the the oil burner assembly. The cad cell causes the relay to switch the oil burner off when a flame is not established or if flame is lost at the oil burner (or inside the furnace or boiler's combustion chamber). Our photograph shows a modern Honeywell(R) R8184G 4009 cad cell relay for use on (typical) intermittent ignition oil burner equipment. This device is likely to be found on oil burners less tan 15 years old, and on both hot air furnaces and hot water boiler heating systems provided that they are heated by an oil burner. |
How to Reset the Oil Burner Cad Cell Relay: if the red button on the cad cell relay is sticking up and the oil burner has shut down, the homeowner is permitted to try ONCE to "reset" the system by pressing the red reset button. If the oil burner does not turn on and run normally and continuously (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. The photograph shows an older Honeywell cad cell relay switch - the gray box in the right in this illustration. If the cad cell has told the relay that it can't see any flame, the cad cell relay switch will have turned off electrical power to the oil burner and the red button will have popped up. |
Testing cad cell relays: is possible by simply pressing the red button down when the oil burner is operating. If the oil burner is operating normally and you press the cad cell relay reset button down it should turn off the oil burner. When you release the button the oil burner should re-start. This test checks the interrupt circuit in the relay. We recommend not trying this test unless the oil burner has been on for a few minutes or more (and has had time to warm up) so that we don't produce unnecessary sooting in the combustion chamber during stop and start of the burner.
Cleaning or replacing an oil burner cad cell is pretty easy, but this is a job for the service technician since it's necessary to turn off power to the oil burner and partly disassemble the oil burner to access the cad cell sensor. Often by removing a lock screw or two, the voltage transformer (the black box atop[ the oil burner in this photo) is simply hinged back and one can see the yellow wire entering the oil burner tube and leading to the cad cell sensor. The cad cell relay sensor will be mounted in a little bracket and positioned where it can "see" the flame when the oil burner is operating.
If the oil burner has not been operating properly, say has been producing too much smoke and soot, even after it has been adjusted it may be necessary to inspect and clean soot from the cad cell sensor so that it has a non-sooty clear view of the flame. Or the technician, if s/he has any doubt about the sensor, will simply replace it. It's a simple plug-in part (two pins on the sensor plug into a receptacle that is secured by a bracket inside of the oil burner tube).
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. |
Oil Burner Stack Relay Switches Explained 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 high.
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 switch 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).
This photo of a York gas fired furnace displays a conventional draft hood opening - the large horizontal opening space shown in the middle of the furnace. The purpose of this opening is to permit additional air to flow into the flue vent connector (stack pipe) and chimney when the gas burner is operating. This additional air flow avoids excessive draft at the gas burner. Too much draft at the gas burner could result in improper gas combustion. (The gas burner will be below this opening and behind the cover with the louvered openings. The louvers provide combustion air to the gas burner.) |
The draft hood or draft regulating device we are discussing here is normally used only on gas-fired heating equipment, not on oil-fired equipment. The equivalent draft control on oil fired heating systems is discussed at Draft Regulators barometric dampers on oil fired equipment.
Dome type gas appliance draft hood clearances:: Dome type draft hoods are commonly installed on gas fired heating boilers. For dome type draft hoods such as the funnel-shaped device shown at the center of this photo (air enters at the under-side of the dome) the manufacturer of the boiler specifies the required distance from the bottom edge of the hood to the top surface of the boiler. Usually this clearance, in inches, is embossed right into the lower edge of the draft hood itself. Look for it. If the draft hood is installed too close to the boiler top, or too high, too far above the boiler top, it will not work properly and the system may be unsafe. |
Modification or removal of a draft hood can be very dangerous, and also the presence of rust or debris on top of the boiler below the draft hood may indicate a dangerous condition such as a blocked chimney - risking dangerous combustion gas or carbon monoxide spilling in the building. We describe a case history where this occurred at DANGEROUS CHIMNEY CASE STUDY.
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These photos show an unusual accumulation of debris at a gas fired furnace draft hood opening. Rust at this location could indicate an unsafe condition. If the chimney draft is inadequate or if the chimney is blocked, or if the heater has been damaged by flooding or other wet conditions, you may observe rust and debris on and around the gas operated furnace, boiler, or water heater draft hood. Further inspection by an expert is needed.
Stains or debris at a water heater draft hood:: The black stains around the draft hood on this water heater were an indication of something seriously wrong with the installation. Tracing the flue vent connector from the draft hood atop the water heater (shown in this photo) to its connection at a chimney (not shown) we found that while the water heater was a gas fired appliance it was sharing a flue with an oil fired heating boiler whose chimney was blocked. Back pressure at the chimney was sending the oil burner's exhaust back down the gas-fired water heater's flue and into the building. |
Flue gas spillage at a water heater draft hood:: Our client is pointing to the draft hood on the gas fired water heater in the home she was buying. We could see two things: first, foam insulation on the hot water pipe was melting and second, we suspected that there was excessive flue gas spillage from this appliance. This is an unsafe condition that needs investigation and repair. |
Blockages at a water heater draft hood:: this photo of a draft hood on the gas fired water heater in the home lets us see two concerns: first, the gas fired water heater's draft hood was partly blocked with hair - so the water heater may not be burning its fuel nor venting its combustion gases safely. Second, the previous owner of the home must have kept a very hairy and shed-prone dog in the basement where it's hair was so thick in the air that the draft hood was blocked. If this home were going to be occupied by someone with dog allergies, extensive cleaning would also be in order. |
Combustion gas or flue gas spill switches are usually installed at the edge of the gas fired appliance draft hood. Some appliances may also have a spill switch installed at the gas burner opening itself. This photo shows a spill switch at a gas fired water heater draft hood. In the somewhat blurry photo of dog hair blocking a heater draft hood (above) you can also make out the spill switch and its wire at the right edge of the draft hood in that photo. In the photo at left, a spill switch was not installed but had been simply left loose, disconnected, atop the water heater. |
Missing flue gas spill switches: we've seen these switches removed from modern water heaters, gas boilers, or gas furnaces when they were originally installed. If you see holes drilled into the edge of a draft hood or other marks indicating that a device has been removed, or if you see the devices themselves lying loose, perhaps on or near the equipment, an expert service technician should examine the heater promptly as it may be unsafe.
Misplaced flue spillage switches: the spill switch needs to be installed in the proper location so that if a chimney blockage or some other operating problem causes combustion gases to spill out of the appliance into the building, the flow of flue gases, while still warm, will pass over the switch sensor. (There may be other flue spillage switches which sense carbon monoxide (CO) or other gases directly and without depending on the gas temperature.
Older heaters with no flue spill switch: On older heating systems these safety switches may not be installed at all. A spill switch or a set of them can be added to almost any gas fired appliance, but it is likely that the gas control valve/regulator will need to be replaced too, since the old regulator may not have a point to which the spill switch's sensor wire can be connected to tell the valve to close.
Flue gas spill switch operating failure: While a spill switch could simply fail to sense passing hot gases and thus not perform its safety function of turning off the heater, or while such as switch might simply fail internally, forcing the heater to turn off when it should not, in our experience these are rare events. We do not have at hand industry failure rates for this device but we suspect that installation errors or omissions are far more common.
Barometric dampers are devices used to regulate the draft on oil-fired heating equipment such as furnaces, boilers, or water heaters. On oil fired equipment the barometric damper, or draft regulator is typically a round Tee inserted in the flue vent connector between the heating appliance and the chimney. The face of the tee contains a round "door" with an adjustable weight. The service technician adjusts the weight to control the swing or opening of this door which in turn controls the amount of excess air that can enter the flue and chimney when the oil burner is operating. The barometric damper or draft regulating device we are discussing here is normally used only on oil-fired heating equipment, not on gas-fired equipment. The equivalent draft control on gas fired heating systems is discussed at Furnace Draft Hood on gas fired equipment. |
During oil burner operation, air moves into the oil burner and combustion chamber (as combustion air) and continues onwards as combustion gases moving out of the combustion chamber, up through the boiler or furnace heat exchanger and on to the chimney where these gases are vented outside. The force with which this air or combustion gas moves is the "draft" inside of the heating appliance. Normally we measure draft at two locations: "over the fire" or in the combustion chamber where typically we may see -0.02" of water column pressure, and "in the breech" or at the stack pipe (properly, the "flue vent connector") measured just a few inches above the boiler or furnace top, and before the barometric damper itself. In the breech we want to see about -0.5" WC pressure. If the breech draft is too low the combustion process and venting process may be inadequate, and if the draft measured in the breech is lower than the draft measured over the fire, the oil burner and combustion chamber are operating under pressure - which is often a problem on residential heating systems since few of them are designed to work this way.
But the "draft" that the oil burner and furnace or boiler experience are not constant. For example wind blowing over a chimney top can increase draft, as can a second appliance using the same chimney as the heater. Since the force of draft is not normally constant, and since we want the draft to be constant for optimum oil burner operation, the barometric damper is installed.
The service technician adjusts the barometric damper to maintain a continuous draft in the range we described above. Then if local conditions change, the barometric damper can open or close to let in more or less additional air into the flue and chimney, keeping the draft constant.
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