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  1. #1
    Join Date
    Nov 2008
    Location
    Northern California, foothills.
    Posts
    212

    Fan/Limit switch heater?

    York furnace PCMD LD 16N095B, EMAM 409779

    This is one of those old-style limit switches that also
    turns on and off the fan via temperature. Fan on/off
    temps are adjustable. Limit switch is anchored, I think
    at about 150. Supply air just on the other side of the furnace.
    I didn't have much choice about where to take it as the air
    handler was right up against the floor. Anyway, when
    the supply air was about 107, the limit switch opened
    once or twice. Other times, it ran for a long while without
    opening.

    My BIG question is the limit switch heater. That's what
    it says. There are two terminals in the middle of things
    just to the side of the dial, and they are run to the gas
    valve. In the schematic, it says the heater is only on some
    models. York says the particular one is obsolete and
    wanted to know the lenghth. Well, I didn't remove it
    to measure because I thought York would have a replacement
    so I didn't know the length. But I have a couple of sizes of
    Honeywell ones on my van but they don't have heaters.
    So what is the purpose of the heater and can I likely
    use a fan/limit withOUT a heater with no ill effects?
    (The Honeywells also allow adjustment of the limit temp.)

    Thank you.

  2. #2
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,723
    purpose of the heater is to speed up the fan coming on, they are still available as far as i know.
    did you check temperature at limit location? plugged up a/c coil or any restriction in airflow could cause the problem.

  3. #3
    Join Date
    Nov 2008
    Location
    Northern California, foothills.
    Posts
    212
    Quote Originally Posted by t527ed View Post
    purpose of the heater is to speed up the fan coming on, they are still available as far as i know.
    did you check temperature at limit location? plugged up a/c coil or any restriction in airflow could cause the problem.
    No, I didn't check at the limit location because it didn't seem consistent.
    I didn't think about being able to leave the limit switch installed AND
    put a thermocouple in there. Maybe....

    I will do further checking.

    But do you think the heater is important, that IF I replace the fan/limit,
    I need to get a switch with a heater? As I said, the fan on/off temps
    are adjustable.

  4. #4
    Join Date
    Jan 2010
    Posts
    6,206
    [QUOTE=t527ed;15051551]purpose of the heater is to speed up the fan coming on, they are still available as far as i know.


    t527ed,
    I have never seen or heard of these types of limits. I'm confused as to why one would not just set the fan limit at a lower temperature to achieve the same result. What am I missing ? Can you explain the reasoning ? Thanks

  5. #5
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    4,564
    There are 4 wires going into the limit. 2 are for your control circuit and 2 are the fan delay. Those limits are available aftermarket
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  6. #6
    Join Date
    Nov 2008
    Location
    Northern California, foothills.
    Posts
    212
    Quote Originally Posted by 2sac View Post
    There are 4 wires going into the limit. 2 are for your control circuit and 2 are the fan delay. Those limits are available aftermarket
    Yes, the Honeywells in my truck are like that. But this one has two more for the heater.

  7. #7
    Join Date
    Dec 2007
    Posts
    4,685
    the heater is there to bring on the fan with a call for heat.

    it is wired off the "W" and "C" and when a heat call is initiated the heater begins and then brings on the fan- kind of like sequencer in an electric furnace.

    it is a safety to assure the fan comes on in case the mechanical portion fails.

    I do not install replacements with this, only use the HW without it. never had an issue without it and do get the comment about it blowing cold at first when it is wired to work. they usually comment after you disconnect it as to how it feels better without it.
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  8. #8
    Join Date
    Oct 2011
    Location
    Chicagoland Area
    Posts
    4,564
    I'm not familiar with the heater in the limit. It sounds like when the gas valve is energized, the heater in the limit also energizes? and according to the schematic this was an option? If that's the case, it's my guess it was used for efficency purposes. I don't believe there would be any ill affect to using your truckstock limit
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  9. #9
    Join Date
    Nov 2008
    Location
    Northern California, foothills.
    Posts
    212
    When you say it is a safety to assure the fan comes on in case the mechanical
    fails, do you mean it is like a heads-up (as I think of it) to tell folks that something
    is amiss? So it is more of an alert than a safety feature? Or am I confused
    about why furnaces typically run their ibm when something faults?

    Thank you for the encouragement to install one of my Honeywells, if they are
    the same length.

  10. #10
    Join Date
    Jan 2010
    Posts
    6,206
    Wow.... just when you think you have seen them all. The only difference I have found with them is the manual fan button option.

  11. #11
    Join Date
    Jan 2010
    Posts
    6,206
    Quote Originally Posted by georgelass View Post
    When you say it is a safety to assure the fan comes on in case the mechanical
    fails, do you mean it is like a heads-up (as I think of it) to tell folks that something
    is amiss? So it is more of an alert than a safety feature? Or am I confused
    about why furnaces typically run their ibm when something faults?

    Thank you for the encouragement to install one of my Honeywells, if they are
    the same length.
    Here's my take on what was said. If the cork screw was to break lets say. You could lose the high limit safety. Yes,No or Maybe ?

  12. #12
    Join Date
    Oct 2011
    Location
    Chicagoland Area
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    4,564
    Quote Originally Posted by VTP99 View Post
    Here's my take on what was said. If the cork screw was to break lets say. You could lose the high limit safety. Yes,No or Maybe ?
    It's sounds like a redundant delay
    Officially, Down for the count

    YOU HAVE TO GET OFF YOUR ASS TO GET ON YOUR FEET

    I know enough to know, I don't know enough
    Liberalism-Ideas so good they mandate them

  13. #13
    Join Date
    Nov 2008
    Location
    Northern California, foothills.
    Posts
    212
    Quote Originally Posted by 2sac View Post
    I'm not familiar with the heater in the limit. It sounds like when the gas valve is energized, the heater in the limit also energizes? and according to the schematic this was an option? If that's the case, it's my guess it was used for efficency purposes. I don't believe there would be any ill affect to using your truckstock limit
    The schematic said the heater was not on all units. But it didn't say it was optional on
    the units that had it. Because in my experience my assumptions have often coincided
    with screw-ups, I did not want to assume that the heater was optional for the furnace
    models that had them installed at the factory. Often what sounds logical to me
    usually has some loophole that I didn't consider. I try to be more cautious about
    winging it these days.

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