Am a DIY’er that just moved into an existing house that’s presently on a well – am hooking up to city (town) water (and before someone asks – yes, I am completely disconnecting the well from the house plumbing) 8^). I believe the proper order of the main shut-off valve and backflow preventer is (moving from street to house) valve *then* BF preventer (or in “plumber-talk”, valve upstream, BF preventer donwstream). I would call the town inspector to ask, but am trying to finish this up this week-end and want to get it right the first time.
Thanks in advance for an answer (*really* thankful if it’s the *correct* answer). 8^)
Main shut off valve closest to the incoming wall THEN the water meter if required THEN a test tee to make sure the meter is accurate THEN the RPV THEN another valve so you can service this device. Some codes also require a pressure reducing valve and in this case it would go right after the main shut off valve prior to the meter DEPENDING on local codes. There is also some codes that require a shock absorber on the in coming main to prevent water hammering and a means of draining this device when it becomes water logged. Your very best bet is not only to get a copy of your local building code BUT a copy of your department of environmental supply rules with regard to water supply inside the property line.
A lot of times the building (plumbing codes) are in conflict with the DEP codes and the more stringent one shall prevail GOOD LUCK