The PVC pipe from the meter to the house location is not a problem other than it is probably undersized, but we cannot determine the correct size without having a list of all plumbing fixtures in the house and the number of irrigation heads and the GPM rate for each head. Schedule 40 PVC pipe is rated for a maximum working temperature of 73degF and a maximum pressure of 450psi. Schedule 80 has a max temp of 73 and a max pressure of 630psi so the line pressure from the municipal main down to the house is okay. The house is 25' lower than the meter box at the street so there is a 10.85psi increase in pressure at the house from VSH (vertical static head). The TEE to the irrigation system is also ok, but irrigation systems are not governed by code and the code requires that there MUST BE a backflow preventer immediately after the TEE to isolate the irrigation system from the house potable water supply. The irrigation "main water shutoff" valve may be placed at any convenient point between the backflow preventer and the air gap. The supply line to the irrigation system must also rise vertically until it reaches an elevation no less than 6" higher than the elevation of the highest irrigation head (when the head is extended to operating position) and there MUST BE an "Air Gap" on the top of the riser. Female threaded PVC fittings may be used on the irrigation system however both the IRC (International Residential Code) & the UPC (Uniform Plumbing Code) prohibit any female threaded PVC fittings. For the house water supply when using IPS male & female threads to transition from PVC to another type of pipe you must use Male PVC adapters. The red handled valve below the pressure regulator is a "Gate valve". That is the house "Main Water shutoff valve". Code requires that to be a "Full Bore Valve" so a gate valve or ball valve would be the correct valve. The PRV (pressure reducing valve)is intended to protect the pipe & fixtures in your house. With your vertical drop from the meter and the irrigation system on the same line that is the correct location for your PRV however I don't think your valve is installed correctly. They do make some prv's for vertical mount and there are even some that are made for "All Position" but the PRV shown there appears to be one that is intended for a horizontal mount. Also note the two round discs on the right hand side of your PRV. Those are caps that cover internal sediment screens. When a PRV is mounted horizontally it must be mounted high enough to allow removing those caps to clean the screens. The large nut on the bottom end of your PRV body is a compression nut, and there is a flange connection under that nut. The purpose of that nut is to serve as a union to permit removing the valve for service. Note the yellow handle on the hose bibb. That should not be yellow. You may use any other color, but the plumbing codes specifically say that yellow indicates "Non-Potable water". Now in regards to your concerns about soldering. 1. Begin by soldering all copper pipes & fittings, then after they have cooled to touch you can make the final connections with the PVC. 2.Your hose bibb is probably mounted on the low end of your water system. Close the main water shutoff valve (red handled gate valve)and open the hose bibb. That will allow the water in the house to drain out. You should also open the cold water faucetts in the kitchen and bathrooms to vent air into the lines to insure they drain correctly. I would then disconnect the nut on the bottom end of your PRV and cut the pipe between the top of the PRV and the hose bibb so you can remove the PRV for now. Make sure the hose bibb is open and heat the TEE where it connects to the stub from the house. While heating the tee grip the hose bibb with a pair of channel locks and you should be able to pull it off the stub from the house. Once you have the tee removed, heat the end of the stub and both quickly & carefully wipe the end of the stub to remove as much excess solder as you can. What your looking for is a light film of solder on the copper as if it is copper plated, but no beads of solder that will make attaching your new fitting difficult. (The light film of solder plating on the pipe will actually make soldering easier later). I looked at the second photo you have on your photobucket and it appears that your existing hose bibb is soldered on. When you re-assemble this I would suggest you put a copper female thread adapter on the tee. You could then easily screw in a new hose bibb, and in future if you have need to change it, it would again be easy to screw one off and screw another on. Now regarding your reluctance to solder pipes that might have water in them. The reason most ppl have problems soldering pipe with water in it is because they simply do not understand the physical characteristis of water. When water flashes to steam it expands to 12 to the third power or in simple terms, one cubic inch of water will expand to one cubic foot of steam and both water & steam are not compressible, so when the water flashes to steam it fills the pipe and the excess steam pushes out through the solder joint preventing the solder from entering. Don't underestimate even a few drops of water in the pipe. When it flashes to steam one teaspoon of water produces 2.5gallons of steam and 1 gallon will of steam would fill 98' of 1/2" pipe or 43' of 3/4". As you can see, it would only take a few drops to flash to steam and make soldering impossible. The secret it so first drain the line as much as possible, then vent the line from both ends if possible by opening faucetts or valves. In this case the line will be open to the male thread adapter so soldering should not be a problem. See annotated copy of your photo below:

What is the vertical rise from the meter box to the house? What is the diameter of your water supply line from the meter to the house? What is the physical length of your water supply line from the meter to the house? how many bathrooms do you have?

If you have excessive presure coming into your house a reducing valve should help, in the USA most codes have a maximum of 80 lbs. you should be running your system at approx. 60 lbs. So-called air chambers are a waste of time, they have to be serviced often, your best bet is to use the self-contained ones, they have to be installed as close to the fixture as possible. Hope this helps. Lots of luck "If all else fails, read the directions"

Thank you both so much for sharing your knowledge. Randy427: I would have thought our units were enough to run only two hot water outlets at the same time but the Rinnai RL94 significantly reduces hot water flow from the sink when the HE washing machine is running. The Noritz significantly reduces hot water flow from the shower when the bathroom sink faucet is turned on. Aliceinwonderland: We are in Southern California and I have wondered about scale deposits but it seems odd that it would begin affecting both water heaters at exactly the same time. The 3 year old Rinnai manual says that it does not need to be flushed if using a scale inhibitor. The 10 year old Noritz was installed before we owned the home and doesn't have a valve to flush (not sure why not). We had a plumber come to look at it about a year ago and he said his maintenance procedure was just to clean out the dust and dirt that might have accumulated under the cover.