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  1. #1

    How low can a heat pump go?

    Just dropped in my shop expecting it to be freezing, but the Fujitsu mini split heat pump is still churning out warmth. It is minus 5f outside . and the shop is set in the low 60's. At what temp will a heat pump quit delivering warmer air? Very pleased with this purchase. I just installed a couple of
    5KW unit heaters as back ups and am wondering if I really will ever be using them.

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I believe the output capacity just keeps going lower. Most of the charts only show capacity down to -10F. I don't see why it can't go lower, but again, with the same air flow, the air will get cooler as the unit abilit to transfer heat reduces.

    Maybe someone else can give the theoretical limit. I believe it has to do with the maximum pressure drop across the TXV valve. There may also be a limitation to the ability to defrost effectively and the diminishing returns of the defrost cycle vs. heat output at colder temperatures. At some point, the unit may use more BTU's defrosting than it provides to heat the space.

    By any chance does this unit have supplemental resistance heat? Most do. It might be that the heat pump isn't actually running and you're jsut feeling the supplemental heat or both are running together.

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,752
    Varies with the mini.

    Some stop at 14, some at 0.

    Your, apparently, is lower yet.
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  4. #4
    I don't believe it has any supplemental resistance, plus the whole thing runs on one 20 amp 220V circuit.

  5. #5
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,680
    No supplemental heat on them, I believe they will do low ambient cooling to -17 on some Fujitsu's, the HP probably the same, If you have it blocked from the wind they will work down real low, of course it's probably oversized so that helps. We have one that started losing ground around 0 but it is exposed to the wind. They have infloor heat so below 20 I would have had that cranked anyway, I think he was just testing it.
    You can't fix stupid

  6. #6
    Correction, it was 5 F when I posted, not minus 5F. Hope it works at -5.

  7. #7
    Join Date
    Jul 2006
    Posts
    613

    Hmm

    Quote Originally Posted by cmajerus View Post
    No supplemental heat on them, I believe they will do low ambient cooling to -17 on some Fujitsu's, the HP probably the same, If you have it blocked from the wind they will work down real low, of course it's probably oversized so that helps. We have one that started losing ground around 0 but it is exposed to the wind. They have infloor heat so below 20 I would have had that cranked anyway, I think he was just testing it.
    I know, responding to and old post. Is this true? I thought windchill was a mammal\human feel thing. Temp is temp right? Why doesnt the fan shut off on a condensing unit if wind is causing an issue for operating below a certain temperature? My heat pump condensing fan only shuts down during defrost.

    I cannot see how more or lack of wind flow affects a properly installed condensing unit\heatpump to enable it to operate at a lower OD ambiant temp.

    Can someone explain this?

  8. #8
    Join Date
    Jul 2007
    Location
    Jacksonville, Fl
    Posts
    219
    Freon Temp relates to Freon Pressure
    Freon pressure/temp is guided by the manufacture design for proper heat transfer to maximize efficiency of heat transfer.

    It's a Balance of this Surface Area, Ambient Temperature, and Air Velocity for proper heat transfer. Too much or too little of air velocity will hinder heat transfer.

    It's like trying to catch a bus at the bus stop but the bus never stops.

    heat gain/loss and stagnant air or air moving at to great of velocities will cause pressure problems in normal operating ambient temperatures and these problems will cause other problems where heat transfer can not take place efficiently.

    reverse what you said.
    If it is a 100 degrees outside and you have no fan why does the compressor go off on thermal overload? Since the compressor will over heat with out the fan why doesn't the compressor overheat when the fan is running?
    Its all about surface area and heat transfer and creating air to move through the surface area at the right velocity so it will either pick up heat or absorb heat to keep the freon condensing pressure stable.

    In some cases we put pressure sensor controls that control the condensing fan for heat pumps and why this isn't more thought out by manufactures with residential heat pumps in cold climates is puzzling.
    this is done in commercial refrigeration as the norm.

  9. #9
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,680
    can't explain why it helps to block them from the wind, just do what i am told, wind increases the flow through the condenser, so it would affect heat transfer to a point I suppose. I think the condenser fan on a fujitsu does ramp up and down as needed also
    You can't fix stupid

  10. #10
    Join Date
    Dec 2003
    Posts
    709
    I put in Lg minis on the 2nd floor of my house this summer, no backup heat and they have kept it at 74* all winter even in 0* ambient. I love these units! Got a standard dual fuel on the first floor lennox xp13 and it cant keep up below 25* at 0* ambient. The floors are about the same and the lg is 27,000 btus, and the lennox is 18,000 so obviously the output has something to do with it.

  11. #11
    Join Date
    Jul 2006
    Posts
    613
    Shoot, I cannot "disprove" any of your responces. Where is Been There when you need him.

  12. #12
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,752
    I sitting back reading the replies.
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  13. #13
    Join Date
    Aug 2008
    Posts
    6

    My results and findings

    Well I installed a Friedrich Tri-Zone heat pump this past fall and the Friedrich is the same as the Fujitsu Tri-Zone. I expected moderate oil savings, but this thing has blown me away with the results. Our lowest temperature so far this year is - 27.5 degrees celcius and the heat pump continued to run. At minus 20 celcius the output temperature of the indoor units is about 96 degrees fahrenheit. We have supplemented oil for heating only 3 days this winter and those days the heat pump was still chugging along. The colder it gets though the more it defrosts too.

    So I emailed the company, and emailed thecompany, and a few more times to see if this was a fault in the equipment. They said there are two factors to consider. If the ouside temperature is below 14 degrees fahrenheit and the air temperature coming out of the indoor units is less than 85 degrees fahrenheit the unit should shutdown, or the unit should be shutdown manually (they weren't really clear on that part). If the outside is colder and the inside is still above 85 degrees it will keep going. I'm not really happy with their support regarding clear and concise answers to support questions, but the units work well.

    For comparison purposes, understanding our electricity is probably the most costly on the continent at almost 16 cents a kilowatt, our electricity costs last month were 390 dollars for the month. Compare that to a similar person here that installed a geothermal system this past fall and his electricity bill was 310 dollars for the month. His house is newer and better insulated according to the person that just did an energy audit for us. So I hope this helps.

    Barry

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