Larger suction pipe to pump - better?

With the tank sitting on the ground part of the water in the tank is going to have to be lifted higher than the water that would normally be in the onboard tank. The pump will have to work harder then it normally would to lift it that distance. The Shurflo pumps are designed to lift water about 12 feet using 1/2 inch ID tubing so your tank on the ground is well within the operating parameters. However, keep in mind when switching to three quarter inch pipe (1.066 OD/.824 ID for standard schedule 40) your internal diameter will be significantly larger and your volume and weight of water in that pipe that the pump is lifting will more than double and the pump will be working harder especially while self-priming to lift that amount of water.

I don’t think the “weight” of the water has any effect. Pumps don’t actually “suck”, they create a low pressure which allows atmospheric pressure to push the water to the low pressure area. A larger pipe hs a larger cross section for the 14.7 PSI pressure to act against.

That is the exact reason that most pumps cannot lift water more than approx 33 feet.

mobeewan wrote:
With the tank sitting on the ground part of the water in the tank is going to have to be lifted higher than the water that would normally be in the onboard tank. The pump will have to work harder then it normally would to lift it that distance. The Shurflo pumps are designed to lift water about 12 feet using 1/2 inch ID tubing so your tank on the ground is well within the operating parameters. However, keep in mind when switching to three quarter inch pipe (1.066 OD/.824 ID for standard schedule 40) your internal diameter will be significantly larger and your volume and weight of water in that pipe that the pump is lifting will more than double and the pump will be working harder especially while self-priming to lift that amount of water.

The weight of the water in the pipe is larger, but the pressure at any given height is the same because the weight is spread out over a larger area (ignoring the frictional resistance from flowing along the pipe wall). The pump doesn't "see" how big the feed pipe is, only what the pressures are, be they positive or negative (with respect to air pressure).

You can demonstrate this by comparing how hard it is to drink a beverage through a skinny straw and a fat straw. You don't need to suck more strongly with the larger straw. If you try to slurp rapidly, the skinny straw is harder to use because of the frictional losses.

Ok, so 3/4 will be the same or not much better than 1/2.

Another question: will it make any difference to the pump if the intake tube is 1) up-lift tube inside from the bottom of the aux tank, through the opening near top of the tank, then 3ft down to the ground, then 5ft up to the faucet (let's ignore shower for now),
or 2) from another opening near bottom of the aux tank, few inches to the ground - and then again 5ft up to the faucet.

#1 adds one more 90-bend, this should add more friction - but I have a feeling that pump won't even notice this.

Almot wrote:
Ok, so 3/4 will be the same or not much better than 1/2.

Another question: will it make any difference to the pump if the intake tube is 1) up-lift tube inside from the bottom of the aux tank, through the opening near top of the tank, then 3ft down to the ground, then 5ft up to the faucet (let's ignore shower for now),
or 2) from another opening near bottom of the aux tank, few inches to the ground - and then again 5ft up to the faucet.

#1 adds one more 90-bend, this should add more friction - but I have a feeling that pump won't even notice this.

Your pump has a specification for height (head) & tube diameter of the suction line. Generally speaking these pumps have a limited amount of head they will pull fluid reliably. Adding one-way check valves can extend the suction abilities somewhat but add failure points. On the discharge side -you are not as limited in regards to size and distance.