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Running heat pump 24/7?

  1. Jun 21, 2007 #1
    Hi,

    Is it more efficient to run a heat pump in a house 24/7 or turn it off at night time and during work. All the pros seem to say its more efficient running them 24/7 I cant understand that as there is more heat loss with the greater heat differential running it 24/7

    Thanks
    Darren
     
  2. jcsd
  3. Jun 21, 2007 #2
    Can someone work out the power required to maintain 20degrees c in a house that has 40% heat loss and how much power required if the heat pump is turned off for 6 hours twice a day (12 hours all up)
    That would be much appreciated.

    Thanks
    Darren
     
  4. Jun 21, 2007 #3
    Wow Darren,

    That is a bold request. Perhaps you should try working it out yourself, and just ask for pointers or help on something you get stuck on.

    I would think this would be easy to do with a simple computer program that can very the outside temperature with time.

    CraigD, AMInstP
    www.cymek.com
     
  5. Jun 21, 2007 #4

    russ_watters

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    Residential heat pumps and air conditioners are all-or-nothing devices. They are either on or off. So if you run it all the time, you'll just end up with a cold house (in the summer) and that certainly wastes energy.

    I don't know who "all the pros" are or what their logic is, but what you are saying doesn't make any sense.
     
  6. Jun 21, 2007 #5
    What you should invest in is a programmable thermostat, which would allow you to set the temps lower at night and during working hours, and keep the house where you like it when you're actually in the house.

    We have one that is set to come on and off as people are scheduled to be in/out of the house.

    Get a two-cycle furnace so that you can keep the temps from peaking back and forth and maintain a more constant temp in the house in the winter.

    Brien
     
  7. Jun 21, 2007 #6

    NoTime

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    The logic seems to be that you size the heat pump to the heat input rather than use a unit with excessive reserve capacity.

    Obviously, geting a constant input for a house is not going to happen, but in some cases like cooling a server closet this could be doable.

    I never got around to finding out if it really makes a significant difference.
     
  8. Jun 21, 2007 #7
    Maybe "all the pros" are referring to the effect whereby it is more efficient to run the heat pump at less usual times (eg. pre-cooling the building before sunrise instead of starting it up during the peak heat of day, or pre-heating your home in the early afternoon rather than waiting until you get home in the evening).
     
  9. Jun 22, 2007 #8

    russ_watters

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    Yes....
    It does not. Whether you use a 1.5kW unit (1 Ton or 12,000 btu) all the time or a 3 kW unit half the time, you get the same amount of cooling for the same input. The bigger issue (for a server room) is that no self-respecting IT director is ever happy with the current status of his server room, so you need to install at least 30% excess capacity for when he adds more equipment to it. We often do multiple units for partial redundancy giving 2/3+2/3 = 4/3 capacity.

    For houses, far and away the biggest issue is that upwards of 80% of all houses have bad ductwork (from a DOE study I'll find for you if I can...). Insufficient airflow will kill the efficiency of a unit - and may just kill the unit itself. If your "pros" are contractors - they are not to be trusted.
     
    Last edited: Jun 22, 2007
  10. Jun 22, 2007 #9

    NoTime

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    Interesting.
    That makes a lot of sense.
    Restricted duct work would require more power to push air thru it. If you don't get enough air going thru then it seems likely to me that the evaporator could just freeze up.
    Not a good thing for the unit, as I understand it, this can destroy a compressor.

    I don't :smile:
     
  11. Jun 23, 2007 #10

    russ_watters

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    Yes, all of that is correct, but the bigger efficiency problem than fan energy is the loss of heat transfer efficiency at the coil. The unit will put out fewer btus and more of those will go to dehumidification because of the lower air temperature.
     
  12. Jun 25, 2007 #11

    NoTime

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    Thanks - That part never crossed my mind.
    A useful bit of information. :smile:

    My practical experience with this is AC in cars.
    And washing out the condenser coils on some window units and refrigerators.
    Dirty coils also make a huge drop in efficiency and can kill a compressor.
     
  13. Aug 10, 2008 #12
    The question is: is it more efficient to leave the heater on 24/7. The question is NOT: is it more convenient or cosy to leave the heater on 24/7. It is a distinction which MUST be kept in mind.
    When in doubt, think in extremes!
    Is it more efficient leaving your heater on 24/7!
    Leave your heater on while walking to the garage and back – certainly!
    Leave your heater on while going on holiday – certainly NOT!
    So, leaving your heater on while you asleep 7 hrs or at work 10 hrs? the short answer is NO!
    It's a myth that leaving the HEATER on while you're away at work uses less energy than turning it on when you get home. Here's why:
    Heat goes to where it's not. That's why heat from inside of your house goes (leaks) to the outside, provided the outside temperature is lower than the inside temperature. With the heater off, at some point your house will be as cold as the outside, but no more. When you come home and turn the heater on, the heater removes/heats all that cold air in your house by heating it.
    But if the heater is on when you're gone, then you've turned your house into a cold magnet by keeping it artificially warm. There's no limit to the amount of heat which will leak through the windows, doors, ceiling and walls to the outside. Your heater has to replace lost heat constantly. Your heater kicks in and heats some of that cold, then, because the house is warmer than the outside, it leaks more heat to the outside, so your heater kicks in again and heats that cold air again, and so on.
    This means that throughout the day, your house has leaked way more than one houseful of cold and your HEATER had to reheat it all. By contrast, with the HEATER off all day, then it has to remove just one houseful of cold air when you come home and turn it on.
    Let's say you leave the HEATER off, and your house looses 200kJ or (BTU's if you living in a none-metric country) of heat and then stops, because that's all it can leak to be equal with the outside air temperature. You come home and switch your heater on; all you pay for is 200kJ of energy to heat up your house.
    Now let's say that you have the HEATER running instead. The house leaks 50kJ/hr (BTU's) of heat, so the HEATER kicks in and heats the cold air. Then it leaks another 50kJ, and your HEATER kicks in and heats that. Repeat that process say once an hour for 10hrs during the day and 7hrs during the night, you pay for 17x50=850kJ, or 425% more energy than by switching the heater off while you asleep and at work. My heat pump kicks in every 20min during the day to keep my room at 220C and every 30min during the night to keep the temperature at 160C. Go figure, go metric!
    The heater start numbers will vary with the daily outside temperature and day and night temperature and the “leakage factor” or insulation quality of your ceiling, walls, doors and windows, Do you have your curtain closed or open, and many other factors. I haven't tested this to see exactly how much the penalty for leaving the HEATER on during the day is, but there is zero question that running the HEATER all the time uses more energy than turning it on when you get home. This is not a gray area, its simple physics, and no person with any knowledge of this subject disputes it. Running the HEATER when you're not home wastes energy, period.
     
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