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[schoppy]

OK all you plumbing gurus I have a question for you. How much head would you calculate for a 1" pex system with the pump located about 175 feet from the boiler just before my 40 plate hx? My boiler is located 10 to 12 feet above my pump which should provide some supply side pressure to my pump, much like a water tower to a city water supply (.43 psi per foot of elevation) since this is an open, non-pressurized? Would I just need to account for the 40 plate hx and the 175 feet back to the boiler to size the pump? I know you can only get so much flow through each certain size lines also. Just wondering?     

[slimjim]

There are more things to consider, 90s, tees, ball valves if they aren't full port, lots of things in the system will affect head, temp gauges seem to work far better for me with 3 speed circs when setting up the flow on a sytem and they will tell you if you are having an issue later as well.

[BoilerHouse]

Any suction head would be lost as discharge head.  The two cancel out each other in a recirculating system.  Figure out head loss from pipe length, fittings etc.

This may help as a rough guide;

http://www.taco-hvac.com/uploads/FileLibrary/SelectingCirculators.pdf

[RSI]

Another thing also is the pressure at the inlet of the pump. 12' above the pump would get you 5.2 psi but the pressure drop in 175' of 1? pex is 5 psi at 7gpm. That means you will have close to 0 psi at the pump inlet if you run at that flow rate (or if you only do have 10' of elevation, it will be closer to 6gpm)

Low or negative pressure at the pump inlet will shorten the pump life considerably.