Hi what is ment by neg or pos pump? I also am not sure what you mean by saying ten will give me five that the take off is from the vent pipe. What and where would the gravity loop be. The drawing down of hot water is a strong pos. thanks.
Not quite sure what all your questions mean, but a negative head is where the shower head is close to or above the water level in the header tank.
Normal pumps which require a positive head need a small natural flow of water to trigger the flow switch and start the pump. Negative head pumps get round this by pressurising the feed and the pump is activated by the fall in pressure when the shower is switched on.
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