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Stack temp switch in 1958 era oil furnace, FHW--- expected life?

Leonard Member Posts: 903
edited February 2020 in Oil Heating
Working fine , but since it's so old I started to wonder how long these things last.
And if they typically fail in a way that they don't shut down burner if no-fire.

It's original equipment, so no newer than 1958. Old sunbeam oil furnace, FHW and domstic hot water. Currently firing at ~ 1.1 GPH, IIRC think boiler is rated for ~ 180kBTU ball park

It's a Honeywell Type R986A ,Minneapolis-Honeywell Regulator Co
About 1 foot long spiral bi-metal? spring in exhaust flow after heat exchanger.
Contains low voltage transformer. power relay, time delay( likely thermal)

Was considering designing in a cadium cell flame sensor for no-fire safty redundency.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,127
    edited February 2020
    Expected life? 30 years ago.
    It's basically bullet proof, back from an era when things were made to last, as you obviously have proven.

    Failure can be catastrophic as it could pump oil into the chamber until you empty the tank-same is true really of any primary that fails. However, these controls typically have times of 60, 90, even 120 seconds of failure to sense heat before shutting down.
    90 seconds of oil pumping, no flame. Then it would recycle for another 90. The the homeowner would reset the control and give it 2 more 90 second cycles. They were the cause of many oil soaked equipment back in the day, that would require hours (and usually the fire department for back up) to burn off.
    This control senses the lack of heat to shut it off, using a bi-metal helix design that expands and contracts at different temperature rates to open/close contacts on a relay.
    They are prone to getting jammed with debris preventing movement.

    Switching to a modern, 15 second primary control is rather easy.
    Beckett electric box, modern primary control (I like the Carlin ProMaxx, cad cell wire with the various mounting brackets, and cad cell.
    You'll most likely have to use the clip that mounts the cad cell to the nozzle line and point it at the fire in the air tube.
    Then with a modern primary, you can read the ohms to make sure you have it positioned optimally for the lowest ohm reading you can get.
    Don't do anything like paint the inside of the air tube silver or any other goofy ideas you read on the internet or hear from oil burner hacks.
    EBEBRATT-Ed Member Posts: 13,424
    Stack switches are pretty safe they will shut the burner down on a flame failure.....

    But They have a very long trial for ignitiion.

    Consider this. The boiler is running and shuts down normally on the thermostat.

    The boiler later tries to start but the ignition transformer has failed so the stack switch allows the burner to throw oil into the combustion chamber for 90 seconds the chamber is hot and the burner lights off....eventually.

    It could easily blow the fire door open, blow the smoke pipe down or worse.

    Cad cell controls have been around since the early 60s.

    Time for an upgrade

    I don't know about other states but in MA. stack switches were/are legal up to a 3gph firing rate

    But I saw an old burner installed in the 40s that fired 30gph and it had a stack switch.

    When ignition failed it moved a cinder block wall 2"
  • Leonard
    Leonard Member Posts: 903
    edited February 2020
    45 sec was just a guess, been a long while since I timed it, could have been ~2 minutes.

    If burner made oil mist for ~ 2minutes then lit I can see how mist in chimney might ignite and pop/explode. Puts my ~ 4 ft of exhaust sheet metal pipe at risk. Do they make room CO2 detectors with HIGH limits to shut off burner? I suppose would be good idea to mount a high temp 120VAC shut off switch on ceiling.

    I watched oil tech clean stack switch when he cleaned furnace. Said have to brush them off to keep them from getting locked up from soot/ash deposits.

    I can likely figure out how/where to place cadium celI .....between it's instructions and being a mechanical engineer.

    Just need name/numbers of a good model. I figure easiest to have one that has relay to cut 120VAC power to furnace. I don't want to bother digging into and switching power to relay coil of aquastat's burner power relay.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,127
    I gave you one.
    Wiring is straightforward.
    Power into your fan/limit. Wire from limit to primary control.
    Primary control to transformer, burner motor, and oil valve (if equipped), following wiring instructions with the primary control.
    Thermostat wired to primary.
    EBEBRATT-Ed Member Posts: 13,424

    What should the ohm reading be? If I remember right you have to be under 1500-1600 to avoid lockouts but it should be in the 700-800 range?
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,127
    Older primaries should lock out at that range. Newer ones can run higher, especially with an NX burner.
    If he can get that older burner in the 700-800 range, with a trace of smoke, he'd probably be fine.
  • Steamhead
    Steamhead Member Posts: 15,986
    edited February 2020
    @Leonard , what burner is on that Sunbeam? Post some pics if you can. A newer burner will already have a mounting for a cad cell.
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • mikeg2015
    mikeg2015 Member Posts: 1,187
    Might consider what company/boiler you might eventually replace this with, then use the same brand burner or controls, then when the boiler fails some day, you have spares.

    But thast the former industrial maintenance supervisor in me taking.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,127
    @mikeg2015 In all honesty, these parts rarely break/fail. And unless you are somewhere very remote, all parts are easily obtained, and cross referenced. Any oil burner tech has these on their trucks.
    All 3 major brands of primary work on any burner, except Riello. And Riello only has 2 different styles.
    HEATON Member Posts: 117
    I have a Mercoid oil primary stack control made in 1920 & was still working when I tore out furn. to replace . New primary will be lucky to make 5 years
  • SuperTech
    SuperTech Member Posts: 1,919
    edited February 2020
    > @HEATON said:
    > I have a Mercoid oil primary stack control made in 1920 & was still working when I tore out furn. to replace . New primary will be lucky to make 5 years

    Five years for a new primary? No, I don't think so. The only time I have to replace a modern microprocessor based primary control is if has got wet due to a flood or some other non operational based cause of failure. I've never had any problems with a Carlin pro Maxx or Honeywell R7284U, I've been working on oil equipment a lot longer than five years. I replace a lot of R8184 controls and in favor of these controls to provide the customers with better safety shutdown and interrupted ignition.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,127
    Agree with @SuperTech
    Checking my customer records, I've replaced 8 primary controls in the last 10 years for about a 450 customer base.
    Don't forget heat and power surges could kill them too.