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  1. #1
    Join Date
    Jul 2005
    Posts
    717
    We have an oil fired (1998 Brock) furnace. The return air temp can be obtained with no temp (approx 68 ) variation at any point along the return duct path to the fan compartment.
    Where is the heated supply air best measured? A probe thermometer inserted any where in the supply plenum near the heat exchanger can vary many ,many degrees from a probe that is inserted 24" higher.
    Within 2" of exchanger it is 160
    at 12" above exchanger it is 150
    at 24" it is 140
    And downstream from this plenum a few feet it is approx 120
    The furnace's manual says a 55-85 rise is ok.
    but it does not say from WHERE these temps are taken taken.
    IMO,the probe location can make or break the safe "rise"
    Any out there got any ideas?



    [Edited by deejoe on 10-23-2005 at 03:56 PM]

  2. #2
    Join Date
    Nov 2004
    Posts
    2,927
    measure away from radient heat.
    in other word,place the probe in a spot where you cannot "see" the flame,like around a corner.the nearest outlet should do.

  3. #3
    Join Date
    Jul 2005
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    717
    it still does not tell me where to take this reading.

  4. #4
    Join Date
    Aug 2002
    Location
    Westlake, Ohio
    Posts
    2,470
    24" up from the heat exchanger should be adequate but you need to take multiple readings from all for sides to comes up with an average. 75-85 degrees would be the more acceptable range assuming proper airflow.
    captain CO

  5. #5
    Join Date
    Jul 2005
    Posts
    717
    Thanks, Jim for your reply.
    ok, When I take the plenum temp at 24" above exchanger, the temp is approx 140.... return air coming into fan compartment is 65, so then the heat rise is 75. (140-65= 75)
    this means the rise is ok, so this also means that the fan's heating speed is adequate, right?

  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Yes.

    Now if that was with a clean filter ,remember that the air flow will be less as it gets dirty,so being 75 with a clean filter hopefully allows it to stay within range(85),with the dirty one,just before you change it.

  7. #7
    Join Date
    Jul 2005
    Posts
    717
    thanks, Dash for your reply. Yes, we always make sure the filters are replaced before getting too dirty.

  8. #8
    Join Date
    Oct 2005
    Location
    Minnesota
    Posts
    12
    I have a couple of question on this as well. Is it 24 inches above the "heat exchanger" or 24 inches above the outlet of the unit? Because there is a few inches of difference there.

    Also, how many inches into the plenum does the temp prob go? I could take readings on all four sides and put the probe at the exact same location?

  9. #9
    Join Date
    Mar 2004
    Location
    CHICAGO SUBURBS
    Posts
    464
    just went to a heating seminar and was told,its best to take readings a couple of feet from all take offs from the plenum and average your readings.in plenum anywhere is subject to false reading do to radiation.
    Some people swear by me and some at me

  10. #10
    Join Date
    Apr 2004
    Posts
    7,680
    If you have an airconditioning coil, that eliminates radiant heat and generally mixes the air well. So above the coil is fine.

  11. #11
    Join Date
    Jul 2005
    Posts
    717
    thanks, Docholiday for your reply.we have no a/c.

    With all the input on where to take this temp reading,now I'm really confused.
    If i take it 6" above exchanger, 24" above exchanger, 48" from exchanger, or at the first main duct branch takeoff, we can get readings anywheres from a high of 160 to a low of 120.
    I guess there is no definite location as the location point of this measure. If a unit is above the
    "temp rise" range, then all one has to do is to take the reading further downstream (lol) to make it lower.

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