Let's say that we have a return pump with a rated head of one foot and a output diameter of 0.5 inches. According to the laws of whoever (Pascal, maybe?) the pump output can empty upwards into a 0.5" tube or an olympic sized swimming pool; in either case, the vessel will be filled to one foot.
What I wonder about though, is what happens when the diameter of the collection vessel above the pump is decreased beyond that of the diameter of the pump's output pipe? If you have a 1' piece of tubing pointing straight up from the output pipe with a diameter of 0.5", then you have a volume of water of 2.4 inches cubed. This is 39 grams worth of water that is being kept aloft by the impeller. If the diameter of the upright tubing is decreased to 0.25", then 1' of this tubing will only contain 0.6 inches cubed of water, which will have a mass of 10 grams.
So, if the impeller is perfectly capable of keeping 39 grams aloft, shouldn't it be able to have a greater head if the diameter of the output tubing is decreased below the diameter of the pump's output?
What I wonder about though, is what happens when the diameter of the collection vessel above the pump is decreased beyond that of the diameter of the pump's output pipe? If you have a 1' piece of tubing pointing straight up from the output pipe with a diameter of 0.5", then you have a volume of water of 2.4 inches cubed. This is 39 grams worth of water that is being kept aloft by the impeller. If the diameter of the upright tubing is decreased to 0.25", then 1' of this tubing will only contain 0.6 inches cubed of water, which will have a mass of 10 grams.
So, if the impeller is perfectly capable of keeping 39 grams aloft, shouldn't it be able to have a greater head if the diameter of the output tubing is decreased below the diameter of the pump's output?