Shurflo question

Stalled might just be the word. Does this happen with the battery on charge? A steady electrical humming or buzzing noise would suggest that the motor is struggling to start the pump, which could indeed be a faulty pump but might possibly be due to insufficient volts at the pump.

Is there an external non-return valve in the system? If so where is it, before or after the pump?

As I have said elsewhere in my opinion a lot of the wiring in these vans is undersize, causing excessive voltage drop. Shurflos need a good jolt to start since the swash plates are under some degree of back pressure from the system, unlike the in-tank submersibles which are just a freely rotating centrifugal impeller. If the pressure doesn’t drop promptly as an outlet is opened then the motor might be constrained by the back pressure in the system. Measure the voltage at the pump as somebody else tries to flush the loo. If the voltage dives the pump is drawing power but cannot start.

The Bargee wrote:...
Is there an external non-return valve in the system? If so where is it, before or after the pump?

...

If there is, the Shurflo hasn't been installed correctly.  The Whale NRV that is integrated with the Whale pressure switch should have been removed when the Shurflo was fitted.  But even if it remains connected to the Shurflo's water inflow, it shouldn't stop the pump from working.  Some installations have a second Truma NRV that should stop water from the hot water tank mixing with the cold water but again this should have no impact on the pump.

The OP reported trouble when a tap is only partially open i.e. it is restricting the flow.  We tend not to do this as it makes the pump go on and off quickly just as the Whale used to.  So for us its taps fully open or shut.

Some have suggested fitting an accumulator.  We haven't done this and haven't found the need to but maybe if we did it would make the pump cycle on and off more slowly when a tap is only partially open.

The OP reports a hum but no water flow from the pump so I expect the normal cycling of the pump on and off when a tap is only partially open is not the problem.

The reason that I mentioned the NRV is that some installers, including myself, fitted an external NRV just downstream of the pump (on the outlet) to overcome the problem of tiny particles in the water fouling the built-in NRV and allowing back flow to the tank. If that (optional) NRV is installed upstream (before) the pump then the system pressure is lying against the swash plates. You wouldn’t think that this would cause a problem but going back some time (seventies) I recall a lot of problems with failed micro switches (the actual pressure switch) where the micro switch would fail “on” and the pump would run until it physically stalled and burnt out (or the fuse blew). Hence again some installers (including myself) also fitted an external pressure switch to improve reliability.

Personally I don’t think the NRV hypothesis is the problem, but it is a possibility if one has been fitted inadvertently, or remains 8n place, upstream of the pump.

The OP perhaps needs to address again the question of is this reported “hum” indeed continuous as he says, which would suggest a stalled motor, or intermittent in the sense of “on-off-on-off” which would be “cycling.”

I would still suspect voltage drop but it is easy to test this either by meter or by simply connecting the pump temporarily to the battery with some heavier cable.

Anyway, I now see that the OP has apparently cured the problem by reducing the cut-off pressure, which will effectively de-rate the pump and reduce the starting load on the pump motor, so all would seem well!

The main issue now is the vibrating noise of the swing wall makes as the system pressures back up.

Try removing the access panel on the swing wall marking which is the top which makes it easier to replace.  Then fit foam pipe insulation to the hot and cold water pipes.  This helps reduce the throbbing noise in the bathroom.  Outside the bathroom we found the Shurflo to be quieter than the rather whiny Whale.  However I think this depends on how the Shurflo has been mounted i.e. it’s feet aren’t screwed too tightly or the water pipes are not too short and pressed against the internal fittings.