4JawChuck said:

I just re-read the OP and thought I should mention, if your returns are 1" orifice size...how would a 2" pipe offer less restriction over a 1.5"? Have you done the flow restriction calculation for 2" versus 1.5" at the typical flow rates a pool centrifugal pump delivers?

Its pretty miniscule...

I seriously doubt the added cost of the larger pipe will ever pay for itself over the life of the pool in reduced pumping losses when you consider the orifice is the restriction...not the pipe. 2" pipe to the return split off junction is common, thats how my pool is plumbed and it only travels 20ft...seems to be SOP in my country.

The resistance of fluid to flow is not based on the narrowest restriction, per se. There is resistance to flow in pipe runs and there is more resistance in narrower pipe. This is in addition to the resistance from a narrow return which simply represents some equivalent length of pipe. If the flow rates are low, then the resistance from the pipe may be fairly low, but at higher flow rates this can make a difference.

My own pool is piped the way you describe with 2" from the pump to a split near the pool into three separate 1.5" lines, one to each return. There are two separate 1.5" suction lines all the way to valves near the pump. I very much wish I had 2" pipe where 1.5" was used (especially for the suction lines) and 2.5" where 2" was used. Here's the difference it would have made with 90 GPM (around what my older 1 HP single-speed pump did when solar was off) making some simplifying assumptions below:

[EDIT] I've added the suction lines I didn't account for the first time and separated out solar on vs. off at different flow rates (60 GPM on, 90 GPM off) and changed from using my spreadsheet to using standard tables

here for head loss in pipe (my spreadsheet is more optimistic). [END-EDIT]

[EDIT] I corrected the head loss for the split lines only counting once -- I originally correctly split the flow, but the head loss is that from only one line assuming they are of roughly the same length so the same loss in each line. [END-EDIT]

SOLAR OFF @ 90 GPM

3 returns 1.5" pipe 30 feet (avg.) 30 GPM each ---> 30*(5.5/100) = 1.6 feet of head

1 combo line 2" pipe 40 feet 90 GPM ---> 1*40*(12.4/100) = 5.0 feet of head

2 separate lines from floor drains and skimmer 1.5" pipe 70 feet 45 GPM each ---> 70*(11.7/100) = 8.2 feet of head

TOTAL = 14.8 feet of head from straight piping alone

3 returns 2" pipe 30 feet (avg.) 30 GPM each ---> 30*(1.6/100) = 0.5 feet of head

NOTE: an abrupt contraction 2" to 1.5" reducer would add only 0.06 feet of head at 30 GPM

1 combo line 2.5" pipe 40 feet 90 GPM ---> 1*40*(5.2/100) = 2.1 feet of head

2 separate lines from floor drains and skimmer 2.0" pipe 70 feet 45 GPM each ---> 70*(3.4/100) = 2.4 feet of head

TOTAL = 5 feet of head from straight piping alone

SOLAR ON @ 60 GPM

3 returns 1.5" pipe 30 feet (avg.) 20 GPM each ---> 30*(2.6/100) = 0.8 feet of head

1 combo line 2" pipe 40 feet 60 GPM ---> 1*40*(5.8/100) = 2.3 feet of head

2 lines to/from solar 2" pipe 50 feet 60 GPM each ---> 50*(5.8/100) = 2.9 feet of head

2 separate lines from floor drains and skimmer 1.5" pipe 70 feet 30 GPM each ---> 70*(5.5/100) = 3.9 feet of head

TOTAL = 9.9 feet of head from straight piping alone (not counting roof piping, solar panels, and their 2" headers)

3 returns 2" pipe 30 feet (avg.) 20 GPM each ---> 30*(0.8/100) = 0.2 feet of head

1 combo line 2.5" pipe 40 feet 60 GPM ---> 1*40*(2.5/100) = 1.0 feet of head

2 lines to/from solar 2.5" pipe 50 feet 60 GPM each ---> 50*(2.5/100) = 1.3 feet of head

2 separate lines from floor drains and skimmer 2.0" pipe 70 feet 30 GPM each ---> 70*(1.6/100) = 1.1 feet of head

TOTAL = 3.6 feet of head from straight piping alone (not counting roof piping, solar panels and their 2" headers)

In reality, my solar runs are longer and the piping on the roof for the solar panels as well as the 12 panels themselves (with 2" headers) is substantial so is much more than shown above.

You can see how the resistance is cut by around 1/3rd by going from 1.5" to 2" pipe for each return but in absolute terms there is greater savings going from 2" to 2.5" for the combo line. Now in reality the above gain would only be achieved at constant flow rates so from a variable speed/flow pump. With a fixed speed pump, the lower head would have the flow rate rise, but with enough savings from piping one could downsize the pump to get down to a similar flow rate but at much greater energy savings. I could very likely have been able to go down to a 3/4 HP pump, for example.

So yes, if one just uses larger piping without also getting a lower HP pump (or lowering the RPM of their variable speed pump), then one won't get much benefit.

Heckpools said:

guys built pools in the 70s and 80's with 1.5" black poly pipe. They had 1 skimmer and 1 return. The pools were crystal clear and the customers enjoyed it... the end

No one is talking about using larger pipe diameters to get better circulation. It's mostly to get better energy savings by not having as large a pump or as fast a speed on a variable speed pump. People didn't care that much about energy saving in the 70s and 80s at least with regard to pool pumps. Why waste energy on friction in pipe? Use the energy as much as possible for circulation -- not to improve it, but to not use as much energy for the same flow rate.