Demand Water Pump - dumb question

Pump draws from the onboard tank only. You don't use it when hooked up to shore water source.

When the shore water pressure is low and our water pump is turned on, the water pump will try to raise the pressure by pumping more water. The water comes from the on board tank. If there is no water in the tank, the water pump will still run to try to improve the pressure. We always keep water in our on board tank so the pump increases the water pressure in low pressure situations (like the one we had yesterday).

Bipeflier wrote:
Pump draws from the onboard tank only. You don't use it when hooked up to shore water source.

Best correct answer.

Pump draws from the onboard tank only. You don't use it when hooked up to shore water source. It is not intended to be a pressure booster for low pressure shore water. If you use it as such, it will attempt to pump water back into the shore water supply, which should lock up the back flow preventer valve required to be in your shore water inlet and you will wind up ONLY using water from the fresh water holding tank. If the back flow preventer does not do its job, you risk contaminating the shore side water system with whatever possibly untreated water is in YOUR fresh water tank, which is why there is required to be such a preventer valve.

Bipeflier wrote:
Pump draws from the onboard tank only. You don't use it when hooked up to shore water source.

Which also means it's best to turn the switch off when connected to a shore water source so that the other mentioned booster mode doesn't happen all the time.

Simple terms

Pump pressure switch turns pump on if system pressure goes low enough (35# or 45# depending on pump)

City water inlet has a check valve to prevent pump discharge from leaking out thru connection
Pump has check valve to prevent city water from back-flowing thru pump filling/over flowing fresh water tank

Pump and city water supply same system

Generic RV water System (some minor differences like tank fill, water panel etc) But plumbing Hot/Cold same