The vertical static head loss on the cold water riser is immediately offset by and equal and opposite gain in the drop on the hot water side, therefore the actual gain in vertical head pressure loss is zero. With an 18" increase in elevation the combined total additional pipe length is 18"x2=36" or 3'. For 3/4" copper pipe the additional friction loss at a flow rate of 1gpm would then be 0.003psi and in a worst case scenario, at 10gpm the additional friction loss would be 0.024psi which is hardly enough to even be perceivable on a standard pressure gage. It is far more likely that some sediments were stirred up during the change out and have now obstructed the screens in the Kitchen faucet & lavatory faucet aerators, washing machine line screens, the shower head flow restrictor or water ports and possibly in the toilet fill valve orifices. The solution would be to remove the aerators and washing machine feed lines and clean the screens. then disconnect the shower head and clean the flow restrictor or water ports and finally, if necessary disassble the toilet fill valves and either clean the orifices or change the diaphrams.

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.

I have the water heater completely disconnected and hoses running from the pipes sticking out of the wall. The hoses run down into the washing machine. When I turn the main water line on water comes out both pipes at the same pressure. This has nothing to do with a water heater. Since the pipes are inside the wall I cannot check them for a crossover pipe. The last water heater was in 2012 and I don't remember having any problems with that tank type heater. In 2012 I added a tankless water heater and it never worked right. So I decided that I wanted to change back to a tank water heater. As a test and trying to remember which line coming out of the wall was the inlet, I put hoses on it and put them down in the washing machine and turn the water on to see which one produce the Water. Surprise, they both shooting water at the same high pressure. There is the problem.

You are getting closer to the 150 psi temporarily. That's really, really hard on all of your plumbing - especially the water heater. The P&T valve is the last line of defense against catastrophic tank failure. Do not allow that to continue. Install the expansion tank and set the pressure to 60 psi. If you want to up the size from the 2 gal to the 4 gal that's fine too. It will give you just a bit more cushion. Be sure that you support the tank - particularly the larger one - properly. Thanks to oneandonlybobjones for the sizing chart - I like it when people give references. weedmeister is correct about what the higher pressure does to things: It's hard on solenoid valves in washers, ice makers and dishwashers. It also exaggerates water hammer and stresses things like flex risers and flex hoses. Coming home to a burst washing machine hose that has been running for hours and having your furniture float out the front door when you open it is no fun. If you don't have enough flow at 60 psi you might look at cleaning aerators and such... but in general you shouldn't expect to pressure wash the dishes from the kitchen faucet.