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Thread: Heat Pumps

  1. #11
    Join Date
    Apr 2006
    Location
    Lake Forest
    Posts
    2,486

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    Thanks Peter. It was a pleasure to spend time with you and Katherine last week!

    So, this raises the question, as to why the SMX controllers are programmed to do "something" at 40 degrees. I have only roof airs, and my Marathon manual specifically states that the ac heat pumps are disabled below 40 degrees.

    If it's simply a matter of changing the programming on the controller, why did they limit things at 40? Surely there must be some reason?

    I spoke to someone (I think it was Kevin) who indicated his cruiseair heat did indeed turn-off in the middle of the night when things got cold enough.

    Ray

  2. #12
    Join Date
    Jan 2006
    Location
    Anaheim
    Posts
    566

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    Yes Ray, that was our bus, it was 53 inside the bus at about 5AM, I had the heat side of the cruise airs set to 68 when we went to bed. The outside air temp was 41 on my SMX when I looked. The next night I used the base board electric heat but I think the Webasco is the way to go.
    I am going to see if there is a reason to let it go down to about 35, that will take care of all our needs. I will let you know what I find out.

  3. #13
    Join Date
    Nov 2006
    Location
    Wilsonville, OR 97070
    Posts
    852

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    Check the manual for the roof air in it's heat pump section to see if the reasons for this 40 degree shut off is a A/C issue and not a arbitrary decision by Marathon.

  4. #14
    Join Date
    Jan 2006
    Location
    Nichols Hills
    Posts
    2,465

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    Peter,

    I agree that there should be no difference between a residental heat pump and the Cruisair unit. I do however disagree with the temp at which they provide much (if any) heat.

    The chart shows the balance point at near 30 deg. but the reality is the discharge temperature of the air may only be at or near the temp you set the thermostat for, so the unit is A) drafty and B) runs all the time, so the efficiency is greatly reduced.

    fig16e.gif

    The only heat pump that I have seen successfully combat this curve is a ground source unit that circulates water thru a loop in the ground and thus the effective temperature at the condensor is in the 50's so it continues to work regardless of the outside temp.

    The Cruisair in our Marathon is also set to not function below 40 deg, so the resistance heat comes on or if you select the 12v Heat, the Webasto fires up. Mike and I had the Cruisair in his H running in El Paso when we went to bed and the temp dropped to 30 degrees during the night. The "heat pump" part was working when we turned the lights out, but it was a frosty morning when we got up. We did not know how to select the alternate heat function and learned first hand about the heat pump operating range.

  5. #15
    Join Date
    Nov 2006
    Location
    Wilsonville, OR 97070
    Posts
    852

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    I found this response to this question on one of the RV.net sites.
    GregM


    "Nick Badame Refrig.
    Cape May Court House, N.J.
    New Member
    Joined: 08/23/2006
    View Profile

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    Posted: 01/27/08 08:31am Link | Quote | Print | Notify Moderator

    --------------------------------------------------------------------------------
    Hi John,

    Your basement HP does not have an electric resistance back-up like regular Home type heat pumps. So, below 35 deg. it will freeze up and have no means to defrost. That is why the controls cut it out around 40 deg. and usually will have a redundant system take over. I run 2 basement heat pumps with a 45,000btu diesel fired ProHeat as secondary.

    Good Luck
    Nick-


    --------------------------------------------------------------------------------
    MCI 102C3 40' 2006' Bus Conversion by Atlantic Custom Coach
    Detroit 475hp 8V92t w/Allison Ht750 auto
    Master Mason, Cannon Lodge #104 S. Seaville N.J.
    FMCA# F-27317-S
    ""

  6. #16
    wrongagain Guest

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    Since I am coming into this thread late,...
    Heat pumps compressors "in residential applications" have an electric band around them to keep the freon heated and moving.
    There has always been a concern that if it is too cold and all the freon is sitting in the compressor it will pop its internal non resetable overload switch, and it does happen.
    Up here if we "have" to install a heat pump we always install electric back ups in the air handlers.
    And most all manufacturers of residential a/c units say do not turn on below 50 degrees.
    You can get around that by installing the electric band around the compressor.

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