Justin,

On Wed, 17 Jan 2007 12:16:57 +1030, Justin Richards wrote:

> > >
> >  Not quite true. A heat pump can run up to about 300% efficient in heating mode.
> >
> > RP
> > --
> 
> I have often wondered about heat pumps and would like to clarify.  I
> assume a reverse cycle aircon (or even a single cycle) uses or is a
> heat pump.
> 
> I also assume that I can *move* more heat energy into a room than the
> energy I use to move the energy.  Is this correct.  If so then I will
> think of it like getting a large container of hot coals and moving
> them into a room.  I use less energy to move them than the coals are
> giving off in heat.
> 
> Is the analogy anywhere near correct.

Yes, it's not bad.  :-)  The critical feature of the "300%" efficiency is that the heat outside the house is free.  So economically it is correct, but 
physically it's obviously not.  If it costs me X to lift a weight from the ground to the top of a mountain but I charge you 3X for doing it, I have made 
300% profit, but that doesn't make me a 300% efficient machine!  :-)
 
> Another assumption I will make is that I can not say the same for
> removing heat from a room using a heat pump.  Ie I must use as much
> energy to cool a room as the energy I am removing.  Is this correct.

No, there's nothing inherently less efficient in running a heat pump one way or the other (although its design and the relative temperatures may/will 
introduce a difference), so you should be able to remove 3X Whrs of energy for the expenditure of X Whr (or whatever unit of energy you like).  
There's nothing magic about 300%, either - over here units are rated in COP (Coefficient of Performance, I think) which is the ration of energy input : 
energy moved, and I've seen them with this figure slightly over 4 ("400% efficient" in the parlance we've been using).

Cheers,


Howard Winter
St.Albans, England


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