Question about Pressure reducing valve

[Dj2]

A house has 1-1/4" main water line, from the street into the house, but no PRV.
Asked me to install one, as his pressure is 110.
Can I cut the main line just passed the main shut off, install a reducer from 1-1/4" to 1", then use 1" PRV, then install another reducer from 1" to 1-1/4" and connect into the pipe that goes into the house?

The house is a 2 story 1700 sf, with 2 and a half baths, with fire sprinklers (fed off the main before the shut off valve).

I've done something similar before, from 1-1/4" to 3/4" (but didn't increase it back to 1-1/4" after the PRV) and had no issues.

 

[wwhitney]

Presumably the 1-1/4" water lateral was sized for the fire sprinklers, and as you are installing the PRV after the fire sprinklers come off, the domestic demand doesn't really require a 1-1/4" lateral. So you would expect a 1" PRV to be sufficient.

The complete answer depends on the demand through the PRV you want to design for. For example, looking at the Zurn NR3XL spec sheet (a model I picked for no particular reason), the pressure loss curve shows the following:

10 gpm flow, 1" model 5 psi pressure loss, 1-1/4" model 4 psi pressure loss
20 gpm flow, 1" model 12 psi pressure loss, 1-1/4" model 7 psi pressure loss

Thus if you set the PRV to, say, 60 psi, that will be the downstream pressure when there's no flow (static pressure). With the 1" valve, you'd see 55 psi just after the PRV when 10 gpm is flowing, and 48 psi when 20 gpm is flowing. While with the 1-1/4" valve you'd see 56 psi and 53 psi respectively. So the larger PRV provides a more consistent pressure for a given range of flows.

Cheers, Wayne