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Thread: Heat Pumps

  1. #1
    Joe Cannarozzi Guest

    Default Heat Pumps

    I have been told that the reason the heat pumps are set not to function below 40 degrees is due to the fact that freon does not flow very good below that point.

    If this is true then could someone explain why it is that every car with factory A/C is designed for the A/C to cycle in the winter with the defrost on with evidently no adverse effect from cycling the freon below 40 degrees??????????

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

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    Joe,

    I don't think it is a freon problem, I think it is a thermodynamics problem in that there isn't enough heat to draw out of the air relative to the delta T needed to efficiently operate the refrigeration cycle. No liquid expansion to gas means no heat.

  3. #3
    dalej Guest

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    Jerry, I'm impressed!

  4. #4
    Joe Cannarozzi Guest

    Default

    Thank you Jerry. That leads me to my next question.

    We have OTR A/C and the switch for the control is a 3 position rocker switch. Off is in the middle, up is A/C, down is heat.

    When we purchased our bus the fellow that ran through things with me suggested that in the winter when needing heat not to put the switch in heat but always in the A/C position and with the thermostat set warm it will in fact give heat and the windows will stay defrosted better in this position.

    I never practiced this because I was concerned of doing harm to the A/C compressor but I guess from what I now know this is probably a better way to go.?????

  5. #5
    Jim_Scoggins Guest

    Default

    Joe:
    Thanks for the info.

    Quote
    When we purchased our bus the fellow that ran through things with me suggested that in the winter when needing heat not to put the switch in heat but always in the A/C position and with the thermostat set warm it will in fact give heat and the windows will stay defrosted better in this position.
    End Quote

    I haven't really thought about it but that sounds like a good to know item that could be used selectively. I do believe that the OTR compressor running just to heat the coach would result in more, and in most cases, unecessarily increased fuel consumption. I will use it for those early launches then switch back to heat only.

    Good tip.

    A thinky. The other morning I was running a cruise air just puttering around trying to figure some things out about it. It was around 35 degrees out side. The cruise air was putting out warm air. Do some of these units have an additional heat element in them?

  6. #6
    Just Plain Jeff Guest

    Default

    A 'thinky?' From a Marine? Hmmm.

    Anyway, here's 2 cents worth. In a camper, you have two types of heat from your A/C units: Heat sticks (very rare in a Prevert conversion) and heat pumps.

    Heat pumps actually run the A/C units 'backwards,' and are designed to do just that. If you have a cruise air with SMX, you can go to the front of the POG site and download or look at the SMX control manual for heat pump operation. If you have roof airs, usually there is a setting on the thermostat which will invoke the heat exchanger.

    Depending upon your altitute and ambient air temperature/humidity, a heat pump is good to go at between 28-35F, assuming a fully-charged and in good condition unit (clean those A/C filters troops).

    Air conditioners with heat pumps like to blow hot air. Wait a minute....ah....anyway, you get it.

    The alternative to using heat pumps is, well, y'know.

  7. #7
    Join Date
    Jan 2006
    Location
    Nichols Hills
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    Not to dispute Sock Boy, but if you have a heat pump and are trying to get heat out of it at 28-35 degrees, you will freeze your cajones off. They are piss poor heaters at those temps.

  8. #8
    Join Date
    Apr 2006
    Location
    Lake Forest
    Posts
    2,486

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    I believe that Marathon also sets up the SMX controllers to not try to run the heat pumps (at least on roof airs) below 40 degrees. My Marathon manual specifically states that they will be disabled below 40 degrees.

    A walk down the lane at ORA last week in the evenings would bring the sound of many webastos running in a row.

    Ray

  9. #9
    Join Date
    Jun 2006
    Location
    Indian Hills
    Posts
    1,129

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    We have what Marathon calls, The "Canadian Heat Pkg" our our our last and present coach. Our cruise airs work down into the teens. One of our four isn't working correctly, and it used to blow hot, now its cool. Haven't yet addressed it since we are comfortable at present. Canadian Heat option pkg gives you also extra underbelly heaters both Webasto fired and electric. We also able to heat the coach's motor with the Webasto by a separate switch on the side dash.
    Jim and Chris
    2001 Featherlite Vogue XLV 2 slide with Rivets-current coach, 1999 shell
    Previous 22 years,
    We have owned every kind of Prevost shell but an H3-40

  10. #10
    Petervs Guest

    Default Heat Pumps and Cruisaire

    Ok, thermodynamically there is no reason the cruisaire heat pump operates any different than a regular house heat pump. They routinely use the heat pump system on houses to provide heating down to outside temperatures of 25 degrees before the heat pump shuts off and the auxiliary electric resistance heaters take over. Remember, to suck heat out of outside air only requires the evaporator to be cooler than the ambient air. A freon evaporator gets much colder than 40 degrees. More like 0 or -10 degrees F. There may be some problem with coil freezup in humid conditions, but we have not experienced any.

    The SMX controllers on Cruisare units on our busses are factory programmed to switch to straight electric heat at 40 degrees F, but only if you have electric resistance heaters attached to your cruisaire system. Mine does not have that.
    You can easily program the SMX controller to swith off the heat pump at a lower temp that 40 F. This will allow them to run effectively when the nightime air is cooler.

    While at ORA Aguanga last weekend out cruisaires ran all night just dandy, we never fired the webasto.

    And by the way, that was a very fine gathering of POGers, and I would like to thank Ray again for instigating the shindig. Well done Ray!

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