🚀 go-pugleaf

mcd@out2.nntp.min.net  writes:
> 
> It seems to me that it would be more work to cool down a house that was 95
> degrees to 76 degrees than it would be to maintain 76 throughout the day
> much the same as you get better mileage in your car on the highway than
> you do in the city.
> 
> I could be wrong, but has anyone actually tested the theory?
> 

Ok, couple of thoughts on this based on what I know from all
those Chemical Engineering classes...

If the house is at 95 degrees and the outside air is 95
degrees, then no additional heat is flowing
into the house (save solar heating that could raise the temp
above ambient). If it is at 76 degrees all day, and the outside
temp is at 95 all day, then there is a constant heat flow into
the space.  

Heat conduction (through the walls) is a linear function of
temperature difference. Roughly (at high school physics level):

rate of heat in = k * (T2-T1)

where k is a constant, T2 and T1 are the two temperatures.  

In case one, as the temperature of the house gets closer to the
outside temp, the rate of heat flow in decreases and goes to
zero.  There is a fixed amount of heat that will need to be
removed to lower the temperature back down.

In case two, since the heat is flowing in at a higher rate all
day, more heat needs to be removed to maintain a 76 degree
house than to just pull it back down when you get home.

Now my question is, why do heat pumps become less efficient in
a situation like case one?  The two temperatures they are
operating between become closer, so the Carnot efficiency
increases, what happens physically to make the actual system
lose efficiency?  I can understand on an AC unit if icing
occurs due to continuous use, creating heat transfer problems,
but other than those sort of conditions?

-Brian

-- 
Brian K. Sward
bks4q@virginia.edu

"Time flies like an arrow; Fruit flies like a banana."