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
http://www.natenergy.org.uk/myths.htm (http://tinyurl.com/6nol7 ) 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.
poke around here
Thanks, I'll take a look!
It is an urban myth, with no basis in fact.
(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  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.
 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.
Knowledge may be power, but communications is the key
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