CH pump flow resistance

[CS]

When a CH pump is stopped, what resistance does the pump rotor offer to water flow??

The reason I ask is that I have an oil central heating system in a boat and want to use heat from the generator cooling to heat the CH system via a tubular heat exchanger. The system layout is such that I can easily do it on existing CH system pipes, with a second pump that runs when the generator is running, but I do not want the post heat exchanger hot water taking the route of least resistance round the boiler. I am hoping that the stationary CH system pump rotor will offer a high resistance. This is fitted at the supply end of the CH boiler. I could fit a solenoid valve to stop the water going where I do not want it to go, but would prefer not to.

[EricP]

Very little at all.

[Roger Mills]

Central heating pumps are centrifugal in nature rather than positive displacement - which means that they will happily run stalled without getting damaged. But it also means that water will flow freely through them when they are not operating. If you want to stop the flow, you'll need some other means such as a motorised valve.

[John Stumbles]

or a gravity/swing check valve which has very little resistance to flow in the 'correct' direction due to its flap action.