Hydraulic changes due to pipe upgrade

My pool operates with a Sta-Rite Dura-Glas (P2RA5F-182L) pump and a Hayward Star Clear cartridge filter (C1200). The pump is fed by three (3) 1 - 1/2 inch pipes, two skimmers and a main drain. The pool return was a single 1 - 1/2 inch pipe routed to a Master Pools Turbo Clean water valve. The valve selects one of five in-floor circuits, in rotation, which are also 1 - 1/2 inch lines that are branched at some point to feed two pop-up heads per circuit. The pipe from the pump discharge to the filter is also 1 - 1/2 inch - about 2 feet in length.

The water valve is not working right and two of the circuits are leaking so I currently have two circuits open to the return flow at the same time and the others are plugged. This effectively gives me two 1 - 1/2 inch return lines after the water valve, but a single 1 - 1/2 inch lines from the filter to the water valve - about 60 feet in length.

Putting in a patio and anticipating adding solar prompted me to upgrade the plumbing. I replaced the pipe between the pump and filter with 2 inch (1 - 1/2 inch pump discharge fitting), and from the filter to the water valve with 2 inch. Eventually, I'll eliminate the in-floor fittings.

So when I fired it back up, the filter pressure was half it's previous value. It had been about 20 psi and now it's about 10 psi. I expected some reduction in output pressure, but that's more than I thought. (I can help you move water with a fire pump, but this is a different bag)

Is this unexpected? Did my flow rate go up as well? Is this a satisfactory operating condition for the pump and filter? I suspect that the system is more efficient. Hoping Mark can chime in.

The change in head loss for pipe is proportional to the fifth power of diameter. For fittings it is the forth power. So going from a 1.5" pipe to a 2" pipe is a 1.284 ratio in I.D. so pipe head loss would change by a ratio of 3.48:1 and fittings by a ratio of 2.72:1. Not all of the pressure that is measured by a filter gauge is reflective of the pipe and/or fittings. The filter also contributes to the pressure and that would not change. Plus the interaction with the pump head curve complicates things. But yes, it is possible that the return head loss is halved by what you did. That is why I always tell people to forget about using swept 90s. The pipe diameter is a much more impactful change.

But also keep in mind that reducing the return side head loss increases pump flow rate and while you might think that is a good thing, the pump will actually draw more power because of that. So your energy costs will go up unless you reduce pump run time.

[EDIT] - I neglected the in-floor system. Those tend to generate a lot of pressure so when you took these two pressure measurements, was the in-floor engaged or bypassed in some way? 10 PSI seems low when using an in-floor system.

Thanks Mark.

It's more like a 7 psi reduction rather than the 10 psi I originally reported. The in floor system is effectively bypassed. The pop-up heads are removed, but the return still goes through the water valve into two of the circuits. The other circuits are plugged. The two open circuits are both open at the same time, all the time. So from my perspective, they are just pipes that happen to return water to the floor and using the water valve as a manifold. This is the same configuration before and after the pipe upgrade. I haven't even cleaned the filter. I imagine the high pressure from in-floor systems is due to the springs that need to be overcome to get the heads to pop up. Since these are removed, this would not be a factor.

The in-floor fittings will eventually get eliminated and return water will go to side outlets - eventually.

It makes sense that flow and power consumption would increase. I'm already on board with reduced run times from the paper you posted in another thread (and attributed to Doc). I've been running about 3 hours all winter. My water is at 80 degrees now and looks good so far. I may try to run the pump even less with this change.

But the addition of 288 sq. ft. of solar panels (2 inch headers) should increase flow loss back to nearly what it was before, I suspect. That was my thinking anyway. So after solar, run time will be dictated by solar heating or cooling demand during those months that it's needed which will no doubt exceed the run time needed for water clarity.

7 PSI is reasonable for the changes you made and since you are bypassing the in-floor that doesn't really come into play.