I'm planning two pumped circuits in my central heating plan which is incorporating a boiler, an air source heat pump, and a solar heater. I'd like to know if each pump, when in the off mode, will act as a closed valve, or if there will be leakage through the impeller.

I assume you mean gravity circulation.

Are you using a store or buffer tank set-up or other.

Yes you could get gravity circulation through the pump, but good design should limit/stop it.

Need information.

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