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 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 switch as a primary controller to prevent sending fuel into the oil burner if the oil burner flame has not been successfully ignited. This sketch of a stack relay used as primary controller on oil fired heating equipment (heating boilers or water heaters) is provided courtesy of Carson Dunlop. 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). Mounting Location for the Primary Control Protecto Relay
The round tube containing the bimetallic temperature sensing spring is inserted into the heating appliance flue stack so that it is located in the center of the path of flue gases. (Audel sketch at left, edited). Taking advantage of the fact that two different metals laminated together will expand at different rates, the bimetallic spring moves in response to the presence or absence of heat inside of the flue vent connector ("smoke pipe" or "stack") of an oil fired boiler or furnace. Sensing heat in the stack is a means of assuring that the burner flame is present. |
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.
If the oil burner fails to ignite or if the flame is intermittent, too small an cool, or lost entirely, this safety control will sense the loss of oil burner flame by sensing the drop of temperature in the flue. As the bimetallic spring changes shape in response to cooling, it mechanically operates a primary control safety-off switch to turn off the oil burner motor (so that we don't keep pumping un-burned heating oil into the combustion chamber).
Similarly to failure to ignite at boiler or furnace start-up, if the flame is lost during oil burner operation this control will also turn off the oil burner motor.
The Type RA 116A or Type RA117A primary control is also designed to automatically re-start the oil burner after a cool-down cycle of a minute to a minute and a half after an abnormal shutdown. If multiple re-start attempts fail to get the oil burner operating satisfactorily the system will shut down entirely in "SAFETY OFF" condition.
This control includes a red safety reset button (#8 in the drawing) that pops out if the control has caused a safety shut down of the oil burner. Here we discuss when, how, and how-often to press the red reset button to try re-starting the oil burner.
Our sketches (above) of a Type RA 116A / RA 117A Primary Control, also called a Stack Relay or a Protectorelay, shows the cover removed and identifies the principal components of this heating system control. Sketch from Audel. The Type RA 116A primary control was used on constant-ignition oil burners that fired heating boilers, warm air furnaces, and water heaters. You will still find this old control on older oil fired heating equipment, though not on newer oil-fired equipment which is more likely to use a Cad Cell Relay Switch Flame Sensor to detect the successful ignition of the oil burner.
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 the page top photograph and in the sketch above is attached at the flue vent connector which in turn passes flue gases from an old "octopus" type furnace (in our photograph) 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).
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.
The primary control reset button on a Delco master control is labeled at the center right of the illustration at left. A primary control stack relay reset button is also pointed out by arrow #8 in our sketch above.
This red button should project through a small hole in the primary control's cover. If you don't see the red button but you see the hole in the control cover, it's possible that a plastic reset button extension has broken off and been lost, but the actual working reset button and switch can still be found inside the control cover and it can still be pressed.
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 second stack control reset might be necessary - we discuss it at How to Reset the Oil Burner Stack Relay below.
Most heating technicians and inspectors will tell you to only press the reset button once. That's because we worry that a homeowner will keep pressing the button even though the oil burner flame is never igniting.
Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: Aquastat Functions and Cad Cell Relay Switch Flame Sensors (hot water boilers and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.
Below we discuss other reset functions peculiar to the Stack Relay Switch, also called the Primary Controller on older oil fired heating equipment.
If you flood the heater's combustion chamber with un-burned oil which later does ignite, the result can be a dangerous puffback which blows soot and smoke into the building, can damage the equipment, is unsafe, and could even start a fire.
So 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 as we just explained.
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.
This procedure is called "re-stepping" the Protectorelay or Type RA 116A Primary Control (which we call the "stack relay" because its bimetallic spring sensor is inserted into the oil-fired boiler or furnace "stack pipe" (flue vent connector) where it senses the heat from a successful oil burner ignition and flame.
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).
Do not lubricate the primary control parts with oil.
Keep the cover on the primary control - this protects the control from dust and from mechanical damage, either of which could make it stop working or make it unsafe.
Check the safety lockout function of the oil burner primary control: with the oil burner running, close the manual oil valve at the oil burner - this simulates a flame failure of the oil burner. When the oil burner flame goes out, "hot contacts" (#6) should open after a the stack temperature begins to drop (and the bimetallic spring of the A 116A contracts). This should turn off the oil burner motor itself.
The burner should stay off until the stack relay RA116A cools down enough to re-make the cold contacts (#7) which should re-start the oil burner automatically. But as you have left the oil flow turned off, after 90 seconds or less the burner motor should stop again.
Pressing the red "reset" button while the oil burner is running normally will also interrupt or stop the oil burner.
Check the Scavenger Timing cycle of the primary control, using the procedure at Scavenger Cycle Restart Delay Time.
-- the above is paraphrased from Audel
The following procedure, recommended by Frank Graham of Audel, tests the scavenger timing cycle of an oil burner that is controlled by a Type RA 116A Protectorelay or "stack relay" primary control.
When an oil burner is in proper adjustment and is working but has been shut off using the electrical power switch (line switch) to the oil burner, the scavenger timing is checked by turning on the oil burner at the line switch. (See ELECTRICAL POWER SWITCH FOR HEAT). The oil burner should start and operate normally.
Let the oil burner keep running, but watch the outer clutch finger (#3 in our sketch from Audel) in the contr9ol. You can see these parts easily when the control cover has been removed. (Watch out: Live Voltage!).
As the oil burner runs from the moment you've switched on its electrical power, you should see the clutch outer finger move outward until it engages the stop arm (#2). The drive shaft (#4) should continue to move outwards after the clutch finger reaches the stop arm. Only a small amount of movement is needed, but it's very important.
After letting the oil burner continue to run for about two to three minutes longer, open (turn off) and then immediately re-close (turn on) the electrical power to the oil burner (the line switch).
Look at the second hand on your watch when you flip the electrical switch on and off, and begin timing the number of seconds until the oil burner re-starts.
If the oil burner restarts in less than one minute it is re-starting too soon. Turn off the electrical power again, let the heating system cool down. Then adjust the scavenger time period as follows:
Move the recycle stop lever (#1) outwards one notch. (You'll see detent notches on the steel rim against which the stop lever arm slides).
Watch the clutch finger (#3) as it moves outwards.
The clutch finger (#3) should move outwards until it engages the stop arm. The drive shaft (#4) should continue to move outward after the clutch finger reaches the stop. This continued movement, as we said before, is a small but important amount.
Now again switch electrical power to the oil burner on, let the system run as above, and then switch the electrical power quickly off and back on as before.
Again time the length of time until the burner re-starts. The oil burner should stop immediately and should not re-start until a minute or more has passed. If the scavenger period (the "down time" before the oil burner re-starts) is too short repeat this process, moving the stop lever one notch at a time until you've achieved more than a minute of scavenger time.
-- the above is paraphrased from Audel
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02/25/2010 - 09/05/2006 - InspectAPedia.com/heat/StackRelaySwitch.htm - © 2010 - 1988 Copyright Daniel Friedman All Rights Reserved - InspectAPedia® is a Registered U.S. Trademark
