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Urban Myth - Central Heating?

Discussion in 'Home Power and Microgeneration' started by Ben, Jan 1, 2005.

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  1. Ben

    Ben Guest


    My girlfriend has been told that if you leave the central heating on low all
    day, it uses less energy than turning it on and off just when your home (we
    currently have it on for 2 hours in the morning and 4 hours in the evening).
    I think this is an urban myth, but I was wondering if anyone had any
    websites that I could check? I found some 'myths or truths' on ( and it has
    one myth for immersion heaters, but not central heating (gas), I would have
    thought the same principle applies, but I just want to check to be sure.


  2. SQLit

    SQLit Guest

    poke around here
  3. It is an urban myth, with no basis in fact.
  4. (OP asked about whether turning central heat off when unoccupied
    saved or wasted money)
    In general, with one very key and one less frequent exception, turning
    down the thermostat when you're gone and turning the heat back on when you
    walk in saves money.

    Think about it for a second. Take teh extreme position that you're gone
    all winter and you've turned off the heat completely. (Draining the water
    pipes first, of course).

    No matter how much oil or gas or even electricity you use when you come
    back home, there's no way you're going to come anywhere near the hundreds
    of gallons you saved.

    Now think about just a day. As the tempreature inside the building drops
    and the differential between inside and outside becomes less, there's less
    seepage of heat to the outside. So when the outside is at, say, 30F, you
    use 1/4 as much fuel keeping the inside at 40F than you do at 70F.

    That ten hours (which, natch, won't usually drop all the way to 40F
    inside - if it does you've got piss poor insulation) has saved you plenty
    of fuel. Brining the inside back up to 70F will use some extra oil or gas
    or electricity, to be sure, but it'll be less than what you'd have needed
    to keep it that way all day long.

    NOW for the exceptions:

    a) electrical heat pumps only have a limited time/gradient
    BTU output curve. If you try to get more heat out of it than the
    heat pump can provide through the reverse-refrigeration cycle
    (which could be the case when you're trying to raise the
    room temperature quickly), that'll kick the unit [1] into
    "auxilary (or "extra" or "emergency") heat mode.

    What that is is simple resistance heat elements, which
    gobble up PLENTY of power.

    In rough general, with lots and lots and lots of variation
    depending on design and the local temperatures, a heat pump
    is three times as efficient as straight electrical heating.

    If you're using the auxiliary strips, you're going to
    pay through the nose.

    [1] with any intelligently installed controller, you
    can tell it NOT to go into reistance mode. But you'll
    find yourself waiting a _long_ time for the indoor
    temperature to rise.

    b) some areas use "time of day" electrical rates, so
    if you're pulling more power during "peak" periods
    you're paying extra. In those cases it's worth raising
    the temperature a bit (or maintaining it..) during
    lower cost times.
  5. daestrom

    daestrom Guest

    The key is having the house temperature drop while you are away. The heat
    losses from the home are directly proportional to the temperature difference
    between inside and outside. So if you're away long enough for the house to
    cool down appreciably, then the house stops loosing as much heat.

    Yes, it will take some amount of energy to heat the house back up to a
    comfortable temperature when you return, but there is a net savings. The
    amount varies depending on a lot of factors. If your house has so much
    thermal capacitance that it doesn't cool down very much while you're gone,
    then you don't save as much.

  6. JeroenH

    JeroenH Guest

    Depends on how long you're gone. If you turn the heating all the way
    off, your house and everything in it will eventually cool off until
    eventually ambient (outside) temperature is reached. When you get home,
    all that mass has to warmed up to the temperature you want. This costs
    al lot of energy.

    So, I do it like this: when I know I'll be gone a short time (say, 1 - 4
    hours) I'll just lower the temperature on the thermostat. When I'll be
    gone for longer than that I'll turn it all the way off.
  7. Guest

    No. Turning the heat off saves energy, no matter how briefly.

  8. Ben

    Ben Guest

  9. Ben

    Ben Guest

    Hi Duane,
    I thought so, (I usually am }:) ) I just wanted some facts I could show her
    to prove the point.


  10. Ben

    Ben Guest

    Hi Daestrom,

    Thanks for the reply.

    We live in a 2 bed semi detached, and when we first moved in I made sure I
    added a 2nd layer of insulation in the loft (its so cheap to do now a days
    its silly not to), so it doesn't cool down very fast. In fact I usually try
    to leave the heating off unless its going to be colder than usual! I'm also
    looking at replacing our 15yo+ boiler with a new combi boiler, and putting
    some TRVs on the radiators, which should save some more money!


  11. Gymy Bob

    Gymy Bob Guest

    Turning it completely off and living outside saves energy also.

    That is sometimes ridiculous.

    On a mild day your furnace may not run for a few hours. Turning it off for
    that few hours saves nothing. The recovery can cost just as much as the
    amount saved and you have to walk around on cold floors and deal with cold
    furniture for hours, depending on how far the temp falls.
  12. Willcox

    Willcox Guest

    Not good getting in the habit of turning off your furnace where the
    climate can freeze your pipes :eek:/ "Gee honey, did you turn the furnace
    down or off before we left on vacation??" :O/
  13. Gymy Bob

    Gymy Bob Guest

    I always turn my furnace down to about 60 degress and then put the back up
    heat on (gas fireplace) at a low temp to give a redundancy to help protect
    against that kind of severe damage. It ain't pretty and not worth any power
    saving ever.
  14. Guest

    That seems incorrect, statistically-speaking. BTW, what's a TRV?

  15. Ben

    Ben Guest

  16. Ben

    Ben Guest

    Here's another I'm not sure about: My mother always told me to turn the
    central heating off when it wasn't needed because it breeds germs, and you
    get ill more often.
    I never knew if she was telling the truth or just trying to get me to save

    True or Not?

  17. Gymy Bob

    Gymy Bob Guest

  18. Gymy Bob

    Gymy Bob Guest

    Did you find out what a TRV was yet?
  19. daestrom

    daestrom Guest

    But LESS than if you leave the heat on all the time!!! This whole argument
    about it 'costing a lot of energy' to heat the home back up again is bogus.
    It costs a lot of energy to keep a house warm too. And in almost every
    case, it takes more energy to keep a house warm than it does to re-heat the
    house after it's cooled off.

  20. Ben

    Ben Guest

    Yeah, see my post above, below [email protected] question

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