Poster has 1.5" plumbing to the softener and is considering a 1.25" control valve on his new softener.

The 1.25" control valve has more than enough SFR both constant and peak and the softener's resin volume will allow 18gpm SFR.

What would be the loss in pressure or flow using the 1.25" control valve on the 1.5" service line?

FHL (Friction Head Loss) is the actual loss in pressure incurred from the friction of the liquid rubbing the inner wall of the pipe as it flows. The problem here is that while the flow in GPM might be increased proportionally the FHL will increase exponentially. By example, an 1-1/4" line at 15gpm will have a FHL of 0.020psi/ft (2psi/100ft) but if we double the flow to 30gpm the FHL jumps over 3 times to 0.073psi/ft (7.3psi /100ft).

On the "Fitting Insertion Loss" charts a the actual loss introduced by a fitting is expressed as a equivalent length of straight pipe. By example, while a 90deg elbow may only add a physical length of 1/2" the fitting insertion loss is 3ft, which means the elbow introduces the same amount of friction loss as 3ft of pipe.

Now let us put it all together...

1-1/4" type L copper @15gpm has a FHL of 0.073psi/ft
1-1/2" type L copper @15gpm has a FHL of 0.020psi/ft

The difference is 0.053psi per foot, but the fitting insertion loss for a Ball Valve is .5ft so if he installs a ball valve the velocity of flow will remain the same and the pressure will drop 0.053psi / 2 = 0.0265psi.

The bottom line, the pressure drop would not be enough to give it a second thought.

Control valve has a twin valve bypass mounted on the rear of the valve.

"The bottom line, the pressure drop would not be enough to give it a second though"

That's what I thought... thanks

Thanks so much for a very professional and thorough answer!