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

    Why auxillary limit switches on downflows?

    I was googling about and some site said ALL downflows
    have auxillary limits switches? Why?

    (I would think that downflows would have warmer return
    air than upflows, and so a high return air temp would be
    more likely to occur and thus less likely to be a problem.)

    What do auxillary limit switches cover that high limit ones
    don't?

    Thank you.

  2. #2
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,106
    If the blower fails, heat rises and you need aux limits at the top of the furnace as the lower limits are not too effective.
    ...

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,886
    Sometimes with a blower motor failure, for example, the main limit on a downflow doesn't open fast enough so they use the aux limits in the blower compartment as extra protection.

  4. #4
    Join Date
    Nov 2008
    Location
    Northern California, foothills.
    Posts
    212
    Crap. That sounds so logical. Now I feel like an idiot.
    Thank you.

  5. #5
    Join Date
    Jul 2012
    Location
    Cincinnati ohio
    Posts
    252
    Quote Originally Posted by georgelass View Post
    Crap. That sounds so logical. Now I feel like an idiot.
    Thank you.
    Don't feel bad, I've always wondered why there was a limit on the blower housing.

  6. #6
    Join Date
    May 2004
    Location
    Salt Lake City/Tooele
    Posts
    2,418
    Many downflows use the same board as an upflow. The default blower OFF delay maybe set for too short of a duration. I will usually set the Blower OFF Delay to the longest duration available. Reason being, if the blower on a downflow does not run long enough to purge the residual heat from the heat exchangers, this heat will rise and cause nuisance trips of those auxiliary switches. Not to much of an issue with the auto-rest type, but a sure PIA if you have the manual reset types.
    Just an FYI

  7. #7
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,145
    IMHO all heat blower off delays should be set at maximum unless you get complaints for end of cycle cold drafts.

  8. #8
    Join Date
    Feb 2004
    Posts
    2,458
    One problem that occurs is a power outage. If the furnace is in operation at the time of the outage the limit will open.

  9. #9
    Join Date
    Nov 2005
    Location
    Around Nash Vegas
    Posts
    133
    Yep and some manufactuers put manual resets on them which is terrible for the homeowner during power failure.
    Give a man a fish, and he eats for a day. Teach a man to fish, and he eats for a lifetime.

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