Originally Posted by seahag
One would be to take a new line right off the thru hull
by teeing the existing plastic stuff, and starting the pex right after placing the pump right there, then running laterally about six feet and going straight up through a locker to the deck
. Option two would be to put this pump, which would be the third pump under the galley
sink, which is about a ten foot run with some bends in after the thru hull, and then about the same situation afterward as plan A-six feet laterally then six feet up. This might be easier to route
, but having an even bigger cluster of plumbing under the sink than is already there. Should I just use 1/2" pex (which I have the most supplies on hand for) to supply the pump and wash down hose? Seems adequate for 4 gpm to me.
4 gpm through 1/2" PEX will give you a velocity of about 6ft/min.
I think recommended for PEX piping is somewhere around 10ft/min so you should be good there.
The next is working out the head
pressure required for the pump.
A simplified way to calculate this is;
Head = Vertical Lift
+ Friction loss of total length of pipe (+ Fixture Losses if not already included in the friction loss)
is simply difference in inlet to outlet height.
Friction loss needs to be looked up on a table (as it's a function of GPM, velocity and pipe material). In this case it's about 0.014 ft per foot of pipe.
Fixture losses can just be assumed to be a percentage of the total pipe length, assume 20% for PEX.
You can use this online tool;
Total Dynamic Head Calculator (TDH)
Option A is about 10.5 ft (6ft lift, 12ft length)
Option B is about 12.7 ft (6ft lift, 18ft length)
I don't know if this tool allows for fixture losses, so you could add 20% to be safe.
Option A -> 12.6ft
Option B -> 15.2ft
So you need a pump capable of pumping 4 gpm @ 12.6 (or 15.2) ft of head.
Btw, "feet" in this case is actually a measurement a pressure (thanks imperial system!) and 1ft = 0.433 PSI
So if you need PSI it's 4 gpm @ 5.5 (or 6.6) PSI
Believe it or not this is actually the "simplified" approach.
Lookup the model number of your pump and you should find what head or PSI it is capable of producing.