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Stack relay guide: Here we explain how to inspect, test, reset, or clean the stack relay switch used as a flame detection/safety device for primary control on oil fired heating systems (boilers and furnaces). This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.
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A "stack relay" is a primary heating system safety control mounted on the flue vent connector close to an oil fired heating boiler, furnace, or water heater.
This switch includes a bimetallic spring that responds to the presence of heat in the flue vent - indicating that the oil burner has achieved and is maintaining proper flame ignition.
If the stack relay does not sense successful ignition within a short period (typically 75 seconds), the switch "opens" to cut off line voltage to the oil burner, effectively shutting off the system to avoid a harmful or dangerous situation. This switch handles loss of oil burner flame either at start-up or during ongoing operation of the heating appliance.
Following a safety shut-down, most stack relay switches must be re-set manually by pushing a reset button, illustrated and explained below.
The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. We include product safety recall and other heating system hazards. Also see GAS PIPING, VALVES, CONTROLS for more details on how to inspect and test LP and natural gas piping, controls, valves, and tanks. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Oil Burner Stack Relay Switches (Protectorelay® type controls), Detailed Explanation
The stack relay switch shown in the photograph at page top and illustrated in Carson Dunlop's sketch, above, 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)
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.
Stack relays are an older type of flame sensor than the CAD CELL RELAY SWITCH, but they accomplish the same purpose (turn off the oil burner if the flame is not established).
The "stack relay" uses a bimetallic spring (also called a bimetallic helix) inserted into the flue vent connector located usually quite close to the heating boiler between the boiler top and the chimney.
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 Safety Switches Work on Oil Fired Heating Equipment
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 explain 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.
How many times can I press the reset button on the oil burner primary controller?
Watch out: 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.
Watch out: sometimes the red plastic button on older heating equipment primary controls can break off. At Reset Switch Broken - Quick Repair we show how to reset the equipment safely if this happens to you.
How do I Find the Reset Button on the Stack Relay?
You can see the "reset button" illustrated in sketch earlier on this page. Note: the "reset button" on some primary safety controls is actually a round red button protruding through the control cover. Usually when the control has locked out the oil burner, this button "pops out" to project further through the control cover.
Using a cad cell relay as a model we illustrate that the safety control red reset button may be a small (5/16" diameter) round plastic button protruding up through the control cover (below left) or it may be a larger round red rubber button such as the cad cell reset switch (below right).
Where the Heck Are The Various Reset Buttons on Heating Equipment?
If you are looking for the main reset button on heating equipment you'll want to see:
CAD CELL RELAY SWITCH (hot water boilers and some water heaters),
Stack Relay Switch on older oil fired boilers and furnaces,
SPILL SWITCHES (gas fired equipment), and
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.
Finding the Primary Control Reset Switch
Watch out: on many primary controls, the red plastic reset button is actually a plastic button and spring-assembly mounted to the control cover itself.
Just to make things tricky, on some controls the reset switch is black and even recessed, such as on the Energy Kinetics cover shown at left.
But other manufacturers know we're seeing red if we've lost heat - and most of them make the reset button red.
Below we discuss other reset functions peculiar to the Stack Relay Switch, also called the Primary Controller on older oil fired heating equipment. But first we make a brief warning about puffbacks.
Oil Burner puffback Problems & Warning
Watch out: There is a reason that instructions for a homeowner tell you to push the primary heating control reset button just "once". If you keep pushing it the oil burner will indeed keep trying to start, spraying more heating oil into the combustion chamber at each "try".
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.
See OIL BURNER SOOT & PUFFBACKS for details.
How to Reset the Oil Burner Stack Relay - Using the Internal Relay Stepping Lever to Re-Step the control
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 detector contacts are actuated by a friction clutch that is mounted on a rod connected directly to the heat actuated element. Occasionally, this clutch becomes out of step after a long period of idleness. To place the clutch and the contacts in step [... you will need to pull and release the lever shown at left and explained below] 
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.
This second and more thorough reset of the stack relay (after tying just pressing the reset button found on the switch cover) can be accomplished by removing the cover on this control box.
Watch out: (SAFETY WARNING LIVE VOLTAGE electrical connections are inside this box - a shock hazard
Simple Procedure to Re-step the Pyrostat Detector Relay
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 (it will move about 1/4 of an inch), and then carefully and gently releasing it again. Put the cover back on and push the red safety reset button.
If electrical power is on to the system and if there is an oil supply and the oil valve is open, and if the thermostat is calling for heat, the oil burner should start.
Watch out for sensitive stack relays: sometimes these switches trip off during oil burner testing or operation and they can be tricky 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.
Watch out: Honeywell warns to be careful not to bend any of the switch parts such as contact arms or stops on the "pyrostat" detector mechanism (the bimetallic spring, relays, sensors) and also warns not to try making any adjustments other than those given in the installation instructions for the equipment.
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).
Tips for Servicing and Testing the Oil Burner Primary Control: the Type RA116A Protectorelay Stack Relay
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.
How to check the safety lockout function of the oil burner primary control or stack relay or Protectorelay®:
Pressing the red "reset" button while the oil burner is running normally will also interrupt or stop the oil burner.
Turning off electrical power to the oil burner, with the burner running, will of course stop the burner. But loss of power should not trip the stack relay switch. This means that when electrical power is turned back on, provided the thermostat is calling for heat, the oil burner should start.
Check the Scavenger Timing cycle of the primary control, using the procedure at Scavenger Cycle Restart Delay Time.
-- the above is paraphrased from Audel with some details added from Honeywell's Protectorelay® control installation literature.
How to Test the Stack Relay using the Safety Switch
At CAD CELL RELAY SWITCH we describe using the safety switch, a small black plastic or metallic lever found next to the reset button on some primary heating system controls including stack relays and cad cell sensors.
When to Adjust the Oil Burner Scavenger Cycle Restart Delay Time on the RA 116A Protectorelay Switch
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.
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.
-- the above is paraphrased from Audel
Cross Reference & Substitution Chart for Stack Switches and ProtectoRelay's
If you need to replace a stack relay switch and cannot find the same brand and model control, there are often several controls that can do the job. Your heating supplier can make sure you choose the proper safety control replacement, but here are some examples of cross referencing.
Honeywell's Protectorelay® and other stack relay switch installation instructions include a table of cross-references showing which Honey controls (or Penn Controls (Pen 664, Penn670, Penn 680)) should be used to perform the equivalent functions as controls made by other stack relay manufacturers including the following, courtesy of Honeywell Corporation
Frequently Asked Questions (FAQs) about Stack Relays on Heating Equipment
Question: Penn Oil Burner Primary Control (Stack Relay Switch) operation: how long should it take for the stack relay to turn off the oil burner?
I have two really old used stack relays that I'm tinkering with to learn about them and want to know the seconds at when they are set to trip at usually?
One of mine trips at 100 seconds and I have not tested the other but was wondering if that is not the right time or too long or if there is a way to lower that number so it trip at less seconds or is it fried past fixing cause I love to fix or rebuild and tinker with oil burner stuff??
At left is the Penn oil burner primary control stack relay. The one that is working in my workshop
Thanks Ben. Please reply. - Ben Bourdreau, Maine
Typically safety switches cut off the burner at a "nominal" delay of 75 seconds (RA 116A and RA 117A Protectorelays) according to Honeywell. 100 is a bit long but probably within tolerance in my opinion. Keep in mind that these are not lab grade instruments.
It is also my OPINION that in comparison with a cad cell relay switch, a stack relay type switch may have to let the oil burner run a bit longer before deciding to shut the system down. That's because the cad cell is right in the combustion chamber where it can "see" the flame (or not), while a stack relay is located in the flue vent connector - a location at which more time might be needed for the flue gases to reach a sufficient temperature to keep the switch closed (that is, to avoid turning off the burner).
The issue is the longer the oil burner gun assembly shoots unburned oil into the combustion chamber the greater the risk of a puffback when ignition finally occurs.
Exactly how long the stack relay takes to respond to the temperature in the flue vent connector depends on some variables besides the instrument itself including
When I worked on heating systems I found these switches reliable and very durable, and I note that at these Stack relays or Honeywell Protectorelay® devices have been in use for a very long time and are still produced and installed and in use on some heating equipment.
Watch out: while we applaud your interest in understanding and restoring antique oil burner controls such as the Penn Oil Burner Primary Control or the Honeywell Stack Relay, just cleaning up contacts and removing rust may not assure that the device will work properly and safely. In particular, a bent, modified, corroded, or obstructed helix, that bimetallic spring that responds to temperature and operates the control may appear to work but may not respond properly at the proper temperature range.
Modern versions of these Protectorelay® switches are the #RA117A1047/U (three wire control) and the #RA116A1055 (stack relay).
For a wiring schematic and installation instructions you can contact Honeywell Corporation at the location listed in our references below.
Shown below are photos of the front and rear of the Honeywell Stack Relay primary control. Here [below left] is the outside of the Honeywell The small slot is the slide reset lever and the large one must have been where the Honeywell tag was. The thing looks so new because I replaced some parts like the cover and wiring knockouts and the back of it from a bad relay but good parts ( is it always workout that way)
If I ever get time or when I can I will send info I know or have learned about stuff I tinker with!! I'm only 22 so I got life ahead of me, Thanks again for replying and Thanks so much for one of the best web pages I have ever come across about the stuff I like!!! I am always on your page looking at stuff.
Also I heard back from a guy I know that knows about heating stuff and he said honeywell use to make a 90 sec stack relay in the 60's and stopped making them but some are still in use today!! Feel free to use what I said if you would like about the stack relay!! You can also use my name thanks again Ben BOUDREAU P. s. I will send you a photo of the working stack relay but not installed yet and I also have a picture of a working and installed one made by PENN controls in one of my work shops on the furnace. - Ben
Below left Mr. Bourdreau's photo shows the (corroded) stepping relay inside the Honeywell primary control stack relay.
At below right Bourdreau illustrates the reset lever for the stack relay. The notched red lever protrudes through the control cover when the cover has been replaced on the unit. The stepping relay (below left) is accessible only by removing the cover. The corrosion on these parts would lead a heating service tech to trash this control as unreliable and possibly unsafe. - Ed.
Mr. Bourdreau's photo shows the interior of the Honeywell stack relay looking down on the controls, relays, and wiring terminals.
Question: stack relay replacements
Can the stack relay on an old boiler be replaced by a newer protector relay?
Yes. The replacement involves installing a properly located bracket to mount the cad cell sensor inside the burner tube where it can see the flame, wiring from the sensor to the protector relay or cad cell sensor control, and wiring that control to the burner motor.
Question: is there a smoke measuring device that will give alarm if the oil burner is operating improperly?
Does anyone make a smoke measuring device, if stack smoke is out of a calibrated range it will alarm? To prevent puff backs. - Gerald 4/30/12
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