>    Assuming a constant pump speed, when the thermostat closes and head
> pressure goes up, power required to drive the waterpump does NOT go up.  It
> actually goes DOWN.  Reason: There is less mass being accelerated (energy)
> at lower flow rates.  In the extreme example (zero flow) the same water in
> the pump housing is being spun around at a constant velocity which requires
> no energy.  Of course there are losses in the pump so the energy consumed
is
> not zero.
>  
>    This argument applies ONLY to centrifugal pumps (of which automotive
> water pumps are an example) and not positive displacement types (like oil
> pumps).
>  
>    OK, somebody shoot this argument down and educate me.
>  
>    Tracy
>  

Accurate to a fault. But if I may:

The pump is a device designed to move water. Power (energy) is supplied via
"V" belt from the prime mover and spins the pump impeller at some speed. The
impeller introduces some of this energy into the water in the form of increased
pressure at the outer edge of the impeller and by increasing the heat of the
water slightly. The difference in pressure between the center of the impeller
(the suction side of the pump) and the outer edge of the impeller (the pressure
outlet) moves the water.

A mass of water moving a distance in a period of time can be distilled
mathematically and reported as horse power. Or 33,000 pounds lifted one foot in one
minute. So long as the water is moving, work is being performed, and work is
reported as horse power.

Restricting the output of the pump reduces the amount of fluid being moved,
but increases the fluid pressure by some amount. Along this plot, the increase
in pressure represents increased work, even when total flow is decreased.
Continued restriction will slow flow to the point where the impeller starts to
become inefficient, thus the amount of work being performed (both flow rate and
pressure) drops off sharply.

Blocking the output of the pump with a thermostat or overly small restrictor
stops flow through the pump, and the pump can do no work. Thus the pump can
absorb energy from the prime mover only by adding heat to the water. This method
of heating water is dreadfully inefficient and only a small amount of energy
can be converted to work in this fashion.

So the energy converted to work by the pump drops to near zero, and the prime
mover will use the (now) excess energy elsewhere. Perhaps in more propeller
RPM.

What Tracy said, but with a bunch more words.

Lynn E. Hanover