Joe Sandlin wrote:
...I'm trying to envision some kind of on demand pump system that would let
her fill her watering cans from a rain barrel that is located one story
below her balcony. She opens the faucet on the balcony, the pump kicks in
and fills the can. She turns off the faucet, the pump shuts off...



You're describing a full demand water system. You need a pump (a sump
pump will probably work), a pressure tank (to store water under
pressure), a pressure switch, and a check valve. The pressure tank
doesn't have to be a full-sized home tank, just an empty tank attached
to the line from the pump (past the check valve) to the faucet. The
pressure switch will turn on the pump when the pressure in the system
drops below a predetermined level. When it hits another predetermined
level the pump will shut off, leaving the system pressurized. The sump
pump has to be capable of providing enough pressure to turn off the
switch, otherwise it will run continuously. Pressure switches are
adjustable, so you will need the appropriate wrench to do the adjustment.

Cost estimate:

Sump pump: $25-75
Check valve: $10-15
Pressure switch: $15-20
Pressure tank: $50-100

All this stuff is available at my Home Depot and by implication, at
yours also.

You could probably construct your own pressure tank by adding a large
diameter vertical pipe (closed at the top end) to the system. The volume
of the tank determines the frequency with which the pump will cycle.

The system has to have provision for draining during the winter
(assuming your area has winter). Copper or pvc pipe will split if the
water inside it freezes. Polyethylene pipe is more forgiving, but there
are fewer options for valves etc. Pvc pipe is probably the easiest to
assemble, since it's glued rather than soldered or threaded. It's
flexible enough to take slight bends when mounting to your wall or balcony.