On 6/18/2016 10:44 AM, Frank wrote:
replying to Don Y, Frank wrote:
I have no hot water heater to expand. I have a Rennai on demand system.

You have a closed system -- the valve at the well preventing any backflow
and the stops/faucets preventing any "forward flow".

If pressure is increasing after the pump shuts down, then either the
volume of that system is DEcreasing (pipes magically getting smaller),
the supply valve is leaking pressure into the system (pump having
pressurized the lines upstream from the inlet valve, *or* the water
is getting "bigger".

Pipes can't magically shrink.
Water will only get bigger if heated.
Pump is pushing water through inlet check valve even though the
actual mechanism is off.

Take your pick.

Is there a MANUAL shutoff to isolate the pump from your system?
If so, when pump shuts off, manually close that valve and monitor
pressure. If it doesn't change, then pump is pushing water through
the check valve.

If it *does* change, water is expanding because of the increase
in temperature (whether it is because of residual heat from
your water heater *or* the difference between water temperature
*in* the aquifer vs. "room temperature".